<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <link rel="stylesheet" href="style.css" type="text/css"> <meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type"> <link rel="Start" href="index.html"> <link rel="previous" href="Ratio.html"> <link rel="next" href="Set.html"> <link rel="Up" href="index.html"> <link title="Index of types" rel=Appendix href="index_types.html"> <link title="Index of exceptions" rel=Appendix href="index_exceptions.html"> <link title="Index of values" rel=Appendix href="index_values.html"> <link title="Index of modules" rel=Appendix href="index_modules.html"> <link title="Index of module types" rel=Appendix href="index_module_types.html"> <link title="Arg" rel="Chapter" href="Arg.html"> <link title="Arith_status" rel="Chapter" href="Arith_status.html"> <link title="Array" rel="Chapter" href="Array.html"> <link title="ArrayLabels" rel="Chapter" href="ArrayLabels.html"> <link title="Big_int" rel="Chapter" href="Big_int.html"> <link title="Bigarray" rel="Chapter" href="Bigarray.html"> <link title="Buffer" rel="Chapter" href="Buffer.html"> <link title="Callback" rel="Chapter" href="Callback.html"> <link title="CamlinternalLazy" rel="Chapter" href="CamlinternalLazy.html"> <link title="CamlinternalMod" rel="Chapter" href="CamlinternalMod.html"> <link title="CamlinternalOO" rel="Chapter" href="CamlinternalOO.html"> <link title="Char" rel="Chapter" href="Char.html"> <link title="Complex" rel="Chapter" href="Complex.html"> <link title="Condition" rel="Chapter" href="Condition.html"> <link title="Digest" rel="Chapter" href="Digest.html"> <link title="Dynlink" rel="Chapter" href="Dynlink.html"> <link title="Event" rel="Chapter" href="Event.html"> <link title="Filename" rel="Chapter" href="Filename.html"> <link title="Format" rel="Chapter" href="Format.html"> <link title="Gc" rel="Chapter" href="Gc.html"> <link title="Genlex" rel="Chapter" href="Genlex.html"> <link title="Graphics" rel="Chapter" href="Graphics.html"> <link title="GraphicsX11" rel="Chapter" href="GraphicsX11.html"> <link title="Hashtbl" rel="Chapter" href="Hashtbl.html"> <link title="Int32" rel="Chapter" href="Int32.html"> <link title="Int64" rel="Chapter" href="Int64.html"> <link title="Lazy" rel="Chapter" href="Lazy.html"> <link title="Lexing" rel="Chapter" href="Lexing.html"> <link title="List" rel="Chapter" href="List.html"> <link title="ListLabels" rel="Chapter" href="ListLabels.html"> <link title="Map" rel="Chapter" href="Map.html"> <link title="Marshal" rel="Chapter" href="Marshal.html"> <link title="MoreLabels" rel="Chapter" href="MoreLabels.html"> <link title="Mutex" rel="Chapter" href="Mutex.html"> <link title="Nativeint" rel="Chapter" href="Nativeint.html"> <link title="Num" rel="Chapter" href="Num.html"> <link title="Obj" rel="Chapter" href="Obj.html"> <link title="Oo" rel="Chapter" href="Oo.html"> <link title="Parsing" rel="Chapter" href="Parsing.html"> <link title="Pervasives" rel="Chapter" href="Pervasives.html"> <link title="Printexc" rel="Chapter" href="Printexc.html"> <link title="Printf" rel="Chapter" href="Printf.html"> <link title="Queue" rel="Chapter" href="Queue.html"> <link title="Random" rel="Chapter" href="Random.html"> <link title="Ratio" rel="Chapter" href="Ratio.html"> <link title="Scanf" rel="Chapter" href="Scanf.html"> <link title="Set" rel="Chapter" href="Set.html"> <link title="Sort" rel="Chapter" href="Sort.html"> <link title="Stack" rel="Chapter" href="Stack.html"> <link title="StdLabels" rel="Chapter" href="StdLabels.html"> <link title="Str" rel="Chapter" href="Str.html"> <link title="Stream" rel="Chapter" href="Stream.html"> <link title="String" rel="Chapter" href="String.html"> <link title="StringLabels" rel="Chapter" href="StringLabels.html"> <link title="Sys" rel="Chapter" href="Sys.html"> <link title="Thread" rel="Chapter" href="Thread.html"> <link title="ThreadUnix" rel="Chapter" href="ThreadUnix.html"> <link title="Tk" rel="Chapter" href="Tk.html"> <link title="Unix" rel="Chapter" href="Unix.html"> <link title="UnixLabels" rel="Chapter" href="UnixLabels.html"> <link title="Weak" rel="Chapter" href="Weak.html"><link title="Introduction" rel="Section" href="#6_Introduction"> <link title="Formatted input channel" rel="Section" href="#6_Formattedinputchannel"> <link title="Type of formatted input functions" rel="Section" href="#6_Typeofformattedinputfunctions"> <link title="The general formatted input function" rel="Section" href="#6_Thegeneralformattedinputfunction"> <link title="Format string description" rel="Section" href="#6_Formatstringdescription"> <link title="Specialised formatted input functions" rel="Section" href="#6_Specialisedformattedinputfunctions"> <link title="Reading format strings from input" rel="Section" href="#6_Readingformatstringsfrominput"> <link title="Functional input with format strings" rel="Subsection" href="#7_Functionalinputwithformatstrings"> <link title="A simple example" rel="Subsection" href="#7_Asimpleexample"> <link title="Formatted input as a functional feature" rel="Subsection" href="#7_Formattedinputasafunctionalfeature"> <link title="The space character in format strings" rel="Subsection" href="#space"> <link title="Conversion specifications in format strings" rel="Subsection" href="#conversion"> <link title="Scanning indications in format strings" rel="Subsection" href="#indication"> <link title="Exceptions during scanning" rel="Subsection" href="#7_Exceptionsduringscanning"> <title>Scanf</title> </head> <body> <div class="navbar"><a class="pre" href="Ratio.html" title="Ratio">Previous</a> <a class="up" href="index.html" title="Index">Up</a> <a class="post" href="Set.html" title="Set">Next</a> </div> <h1>Module <a href="type_Scanf.html">Scanf</a></h1> <pre><span class="keyword">module</span> Scanf: <code class="code"><span class="keyword">sig</span></code> <a href="Scanf.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info module top"> Formatted input functions.<br> </div> <hr width="100%"> <br> <h6 id="6_Introduction">Introduction</h6><br> <br> <div class="h7" id="7_Functionalinputwithformatstrings">Functional input with format strings</div><br> <br> The module <code class="code"><span class="constructor">Scanf</span></code> provides formatted input functions or <em>scanners</em>. <p> The formatted input functions can read from any kind of input, including strings, files, or anything that can return characters. The more general source of characters is named a <em>formatted input channel</em> (or <em> scanning buffer</em>) and has type <a href="Scanf.Scanning.html#TYPEin_channel"><code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scanning</span>.in_channel</code></a>. The more general formatted input function reads from any scanning buffer and is named <code class="code">bscanf</code>. <p> Generally speaking, the formatted input functions have 3 arguments:<ul> <li>the first argument is a source of characters for the input,</li> <li>the second argument is a format string that specifies the values to read,</li> <li>the third argument is a <em>receiver function</em> that is applied to the values read.</li> </ul> Hence, a typical call to the formatted input function <a href="Scanf.html#VALbscanf"><code class="code"><span class="constructor">Scanf</span>.bscanf</code></a> is <code class="code">bscanf ic fmt f</code>, where: <p> <ul> <li><code class="code">ic</code> is a source of characters (typically a <em> formatted input channel</em> with type <a href="Scanf.Scanning.html#TYPEin_channel"><code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scanning</span>.in_channel</code></a>),</li> </ul> <ul> <li><code class="code">fmt</code> is a format string (the same format strings as those used to print material with module <a href="Printf.html"><code class="code"><span class="constructor">Printf</span></code></a> or <a href="Format.html"><code class="code"><span class="constructor">Format</span></code></a>),</li> </ul> <ul> <li><code class="code">f</code> is a function that has as many arguments as the number of values to read in the input.</li> </ul> <br> <br> <div class="h7" id="7_Asimpleexample">A simple example</div><br> <br> As suggested above, the expression <code class="code">bscanf ic <span class="string">"%d"</span> f</code> reads a decimal integer <code class="code">n</code> from the source of characters <code class="code">ic</code> and returns <code class="code">f n</code>. <p> For instance, <p> <ul> <li>if we use <code class="code">stdin</code> as the source of characters (<a href="Scanf.Scanning.html#VALstdin"><code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scanning</span>.stdin</code></a> is the predefined formatted input channel that reads from standard input),</li> </ul> <ul> <li>if we define the receiver <code class="code">f</code> as <code class="code"><span class="keyword">let</span> f x = x + 1</code>,</li> </ul> then <code class="code">bscanf <span class="constructor">Scanning</span>.stdin <span class="string">"%d"</span> f</code> reads an integer <code class="code">n</code> from the standard input and returns <code class="code">f n</code> (that is <code class="code">n + 1</code>). Thus, if we evaluate <code class="code">bscanf stdin <span class="string">"%d"</span> f</code>, and then enter <code class="code">41</code> at the keyboard, we get <code class="code">42</code> as the final result.<br> <br> <div class="h7" id="7_Formattedinputasafunctionalfeature">Formatted input as a functional feature</div><br> <br> The OCaml scanning facility is reminiscent of the corresponding C feature. However, it is also largely different, simpler, and yet more powerful: the formatted input functions are higher-order functionals and the parameter passing mechanism is just the regular function application not the variable assignment based mechanism which is typical for formatted input in imperative languages; the OCaml format strings also feature useful additions to easily define complex tokens; as expected within a functional programming language, the formatted input functions also support polymorphism, in particular arbitrary interaction with polymorphic user-defined scanners. Furthermore, the OCaml formatted input facility is fully type-checked at compile time.<br> <br> <h6 id="6_Formattedinputchannel">Formatted input channel</h6><br> <pre><span class="keyword">module</span> <a href="Scanf.Scanning.html">Scanning</a>: <code class="code"><span class="keyword">sig</span></code> <a href="Scanf.Scanning.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><br> <h6 id="6_Typeofformattedinputfunctions">Type of formatted input functions</h6><br> <pre><span id="TYPEscanner"><span class="keyword">type</span> <code class="type">('a, 'b, 'c, 'd)</code> scanner</span> = <code class="type">('a, <a href="Scanf.Scanning.html#TYPEin_channel">Scanning.in_channel</a>, 'b, 'c, 'a -> 'd, 'd) format6 -> 'c</code> </pre> <div class="info "> The type of formatted input scanners: <code class="code">(<span class="keywordsign">'</span>a, <span class="keywordsign">'</span>b, <span class="keywordsign">'</span>c, <span class="keywordsign">'</span>d) scanner</code> is the type of a formatted input function that reads from some formatted input channel according to some format string; more precisely, if <code class="code">scan</code> is some formatted input function, then <code class="code">scan ic fmt f</code> applies <code class="code">f</code> to the arguments specified by the format string <code class="code">fmt</code>, when <code class="code">scan</code> has read those arguments from the formatted input channel <code class="code">ic</code>. <p> For instance, the <code class="code">scanf</code> function below has type <code class="code">(<span class="keywordsign">'</span>a, <span class="keywordsign">'</span>b, <span class="keywordsign">'</span>c, <span class="keywordsign">'</span>d) scanner</code>, since it is a formatted input function that reads from <code class="code"><span class="constructor">Scanning</span>.stdin</code>: <code class="code">scanf fmt f</code> applies <code class="code">f</code> to the arguments specified by <code class="code">fmt</code>, reading those arguments from <code class="code"><span class="constructor">Pervasives</span>.stdin</code> as expected. <p> If the format <code class="code">fmt</code> has some <code class="code">%r</code> indications, the corresponding input functions must be provided before the receiver <code class="code">f</code> argument. For instance, if <code class="code">read_elem</code> is an input function for values of type <code class="code">t</code>, then <code class="code">bscanf ic <span class="string">"%r;"</span> read_elem f</code> reads a value <code class="code">v</code> of type <code class="code">t</code> followed by a <code class="code"><span class="string">';'</span></code> character, and returns <code class="code">f v</code>.<br> <b>Since</b> 3.10.0<br> </div> <pre><span id="EXCEPTIONScan_failure"><span class="keyword">exception</span> Scan_failure</span> <span class="keyword">of</span> <code class="type">string</code></pre> <div class="info "> The exception that formatted input functions raise when the input cannot be read according to the given format.<br> </div> <br> <h6 id="6_Thegeneralformattedinputfunction">The general formatted input function</h6><br> <pre><span id="VALbscanf"><span class="keyword">val</span> bscanf</span> : <code class="type"><a href="Scanf.Scanning.html#TYPEin_channel">Scanning.in_channel</a> -> ('a, 'b, 'c, 'd) <a href="Scanf.html#TYPEscanner">scanner</a></code></pre><div class="info "> <code class="code">bscanf ic fmt r1 ... rN f</code> reads arguments for the function <code class="code">f</code>, from the formatted input channel <code class="code">ic</code>, according to the format string <code class="code">fmt</code>, and applies <code class="code">f</code> to these values. The result of this call to <code class="code">f</code> is returned as the result of the entire <code class="code">bscanf</code> call. For instance, if <code class="code">f</code> is the function <code class="code"><span class="keyword">fun</span> s i <span class="keywordsign">-></span> i + 1</code>, then <code class="code"><span class="constructor">Scanf</span>.sscanf <span class="string">"x= 1"</span> <span class="string">"%s = %i"</span> f</code> returns <code class="code">2</code>. <p> Arguments <code class="code">r1</code> to <code class="code">rN</code> are user-defined input functions that read the argument corresponding to the <code class="code">%r</code> conversions specified in the format string.<br> </div> <br> <h6 id="6_Formatstringdescription">Format string description</h6><br> <br> The format string is a character string which contains three types of objects:<ul> <li>plain characters, which are simply matched with the characters of the input (with a special case for space and line feed, see <a href="Scanf.html#space"><i>The space character in format strings</i></a>),</li> <li>conversion specifications, each of which causes reading and conversion of one argument for the function <code class="code">f</code> (see <a href="Scanf.html#conversion"><i>Conversion specifications in format strings</i></a>),</li> <li>scanning indications to specify boundaries of tokens (see scanning <a href="Scanf.html#indication"><i>Scanning indications in format strings</i></a>).</li> </ul> <br> <br> <div class="h7" id="space">The space character in format strings</div><br> <br> As mentioned above, a plain character in the format string is just matched with the next character of the input; however, two characters are special exceptions to this rule: the space character (<code class="code"><span class="string">' '</span></code> or ASCII code 32) and the line feed character (<code class="code"><span class="string">'\n'</span></code> or ASCII code 10). A space does not match a single space character, but any amount of 'whitespace' in the input. More precisely, a space inside the format string matches <em>any number</em> of tab, space, line feed and carriage return characters. Similarly, a line feed character in the format string matches either a single line feed or a carriage return followed by a line feed. <p> Matching <em>any</em> amount of whitespace, a space in the format string also matches no amount of whitespace at all; hence, the call <code class="code">bscanf ib <span class="string">"Price = %d $"</span> (<span class="keyword">fun</span> p <span class="keywordsign">-></span> p)</code> succeeds and returns <code class="code">1</code> when reading an input with various whitespace in it, such as <code class="code"><span class="constructor">Price</span> = 1 $</code>, <code class="code"><span class="constructor">Price</span> = 1 $</code>, or even <code class="code"><span class="constructor">Price</span>=1$</code>.<br> <br> <div class="h7" id="conversion">Conversion specifications in format strings</div><br> <br> Conversion specifications consist in the <code class="code">%</code> character, followed by an optional flag, an optional field width, and followed by one or two conversion characters. The conversion characters and their meanings are: <p> <ul> <li><code class="code">d</code>: reads an optionally signed decimal integer.</li> <li><code class="code">i</code>: reads an optionally signed integer (usual input conventions for decimal (<code class="code">0-9</code>+), hexadecimal (<code class="code">0x[0-9a-f]+</code> and <code class="code">0<span class="constructor">X</span>[0-9<span class="constructor">A</span>-<span class="constructor">F</span>]+</code>), octal (<code class="code">0o[0-7]+</code>), and binary (<code class="code">0b[0-1]+</code>) notations are understood).</li> <li><code class="code">u</code>: reads an unsigned decimal integer.</li> <li><code class="code">x</code> or <code class="code"><span class="constructor">X</span></code>: reads an unsigned hexadecimal integer (<code class="code">[0-9a-fA-<span class="constructor">F</span>]+</code>).</li> <li><code class="code">o</code>: reads an unsigned octal integer (<code class="code">[0-7]+</code>).</li> <li><code class="code">s</code>: reads a string argument that spreads as much as possible, until the following bounding condition holds: <ul> <li>a whitespace has been found (see <a href="Scanf.html#space"><i>The space character in format strings</i></a>),</li> <li>a scanning indication (see scanning <a href="Scanf.html#indication"><i>Scanning indications in format strings</i></a>) has been encountered,</li> <li>the end-of-input has been reached.</li> </ul> Hence, this conversion always succeeds: it returns an empty string if the bounding condition holds when the scan begins.</li> <li><code class="code"><span class="constructor">S</span></code>: reads a delimited string argument (delimiters and special escaped characters follow the lexical conventions of OCaml).</li> <li><code class="code">c</code>: reads a single character. To test the current input character without reading it, specify a null field width, i.e. use specification <code class="code">%0c</code>. Raise <code class="code"><span class="constructor">Invalid_argument</span></code>, if the field width specification is greater than 1.</li> <li><code class="code"><span class="constructor">C</span></code>: reads a single delimited character (delimiters and special escaped characters follow the lexical conventions of OCaml).</li> <li><code class="code">f</code>, <code class="code">e</code>, <code class="code"><span class="constructor">E</span></code>, <code class="code">g</code>, <code class="code"><span class="constructor">G</span></code>: reads an optionally signed floating-point number in decimal notation, in the style <code class="code">dddd.ddd e/<span class="constructor">E</span>+-dd</code>.</li> <li><code class="code"><span class="constructor">F</span></code>: reads a floating point number according to the lexical conventions of OCaml (hence the decimal point is mandatory if the exponent part is not mentioned).</li> <li><code class="code"><span class="constructor">B</span></code>: reads a boolean argument (<code class="code"><span class="keyword">true</span></code> or <code class="code"><span class="keyword">false</span></code>).</li> <li><code class="code">b</code>: reads a boolean argument (for backward compatibility; do not use in new programs).</li> <li><code class="code">ld</code>, <code class="code">li</code>, <code class="code">lu</code>, <code class="code">lx</code>, <code class="code">lX</code>, <code class="code">lo</code>: reads an <code class="code">int32</code> argument to the format specified by the second letter for regular integers.</li> <li><code class="code">nd</code>, <code class="code">ni</code>, <code class="code">nu</code>, <code class="code">nx</code>, <code class="code">nX</code>, <code class="code">no</code>: reads a <code class="code">nativeint</code> argument to the format specified by the second letter for regular integers.</li> <li><code class="code"><span class="constructor">Ld</span></code>, <code class="code"><span class="constructor">Li</span></code>, <code class="code"><span class="constructor">Lu</span></code>, <code class="code"><span class="constructor">Lx</span></code>, <code class="code"><span class="constructor">LX</span></code>, <code class="code"><span class="constructor">Lo</span></code>: reads an <code class="code">int64</code> argument to the format specified by the second letter for regular integers.</li> <li><code class="code">[ range ]</code>: reads characters that matches one of the characters mentioned in the range of characters <code class="code">range</code> (or not mentioned in it, if the range starts with <code class="code">^</code>). Reads a <code class="code">string</code> that can be empty, if the next input character does not match the range. The set of characters from <code class="code">c1</code> to <code class="code">c2</code> (inclusively) is denoted by <code class="code">c1-c2</code>. Hence, <code class="code">%[0-9]</code> returns a string representing a decimal number or an empty string if no decimal digit is found; similarly, <code class="code">%[\\048-\\057\\065-\\070]</code> returns a string of hexadecimal digits. If a closing bracket appears in a range, it must occur as the first character of the range (or just after the <code class="code">^</code> in case of range negation); hence <code class="code">[]]</code> matches a <code class="code">]</code> character and <code class="code">[^]]</code> matches any character that is not <code class="code">]</code>. Use <code class="code">%%</code> and <code class="code">%@</code> to include a <code class="code">%</code> or a <code class="code">@</code> in a range.</li> <li><code class="code">r</code>: user-defined reader. Takes the next <code class="code">ri</code> formatted input function and applies it to the scanning buffer <code class="code">ib</code> to read the next argument. The input function <code class="code">ri</code> must therefore have type <code class="code"><span class="constructor">Scanning</span>.in_channel <span class="keywordsign">-></span> <span class="keywordsign">'</span>a</code> and the argument read has type <code class="code"><span class="keywordsign">'</span>a</code>.</li> <li><code class="code">{ fmt %}</code>: reads a format string argument. The format string read must have the same type as the format string specification <code class="code">fmt</code>. For instance, <code class="code"><span class="string">"%{ %i %}"</span></code> reads any format string that can read a value of type <code class="code">int</code>; hence, if <code class="code">s</code> is the string <code class="code"><span class="string">"fmt:\"number is %u\""</span></code>, then <code class="code"><span class="constructor">Scanf</span>.sscanf s <span class="string">"fmt: %{%i%}"</span></code> succeeds and returns the format string <code class="code"><span class="string">"number is %u"</span></code>.</li> <li><code class="code">\( fmt %\)</code>: scanning sub-format substitution. Reads a format string <code class="code">rf</code> in the input, then goes on scanning with <code class="code">rf</code> instead of scanning with <code class="code">fmt</code>. The format string <code class="code">rf</code> must have the same type as the format string specification <code class="code">fmt</code> that it replaces. For instance, <code class="code"><span class="string">"%( %i %)"</span></code> reads any format string that can read a value of type <code class="code">int</code>. The conversion returns the format string read <code class="code">rf</code>, and then a value read using <code class="code">rf</code>. Hence, if <code class="code">s</code> is the string <code class="code"><span class="string">"\"%4d\"1234.00"</span></code>, then <code class="code"><span class="constructor">Scanf</span>.sscanf s <span class="string">"%(%i%)"</span> (<span class="keyword">fun</span> fmt i <span class="keywordsign">-></span> fmt, i)</code> evaluates to <code class="code">(<span class="string">"%4d"</span>, 1234)</code>.</li> </ul> This behaviour is not mere format substitution, since the conversion returns the format string read as additional argument. If you need pure format substitution, use special flag <code class="code">_</code> to discard the extraneous argument: conversion <code class="code">%_\( fmt %\)</code> reads a format string <code class="code">rf</code> and then behaves the same as format string <code class="code">rf</code>. Hence, if <code class="code">s</code> is the string <code class="code"><span class="string">"\"%4d\"1234.00"</span></code>, then <code class="code"><span class="constructor">Scanf</span>.sscanf s <span class="string">"%_(%i%)"</span></code> is simply equivalent to <code class="code"><span class="constructor">Scanf</span>.sscanf <span class="string">"1234.00"</span> <span class="string">"%4d"</span></code>. <p> <ul> <li><code class="code">l</code>: returns the number of lines read so far.</li> <li><code class="code">n</code>: returns the number of characters read so far.</li> <li><code class="code"><span class="constructor">N</span></code> or <code class="code"><span class="constructor">L</span></code>: returns the number of tokens read so far.</li> <li><code class="code">!</code>: matches the end of input condition.</li> <li><code class="code">%</code>: matches one <code class="code">%</code> character in the input.</li> <li><code class="code">@</code>: matches one <code class="code">@</code> character in the input.</li> <li><code class="code">,</code>: does nothing.</li> </ul> Following the <code class="code">%</code> character that introduces a conversion, there may be the special flag <code class="code">_</code>: the conversion that follows occurs as usual, but the resulting value is discarded. For instance, if <code class="code">f</code> is the function <code class="code"><span class="keyword">fun</span> i <span class="keywordsign">-></span> i + 1</code>, and <code class="code">s</code> is the string <code class="code"><span class="string">"x = 1"</span></code>, then <code class="code"><span class="constructor">Scanf</span>.sscanf s <span class="string">"%_s = %i"</span> f</code> returns <code class="code">2</code>. <p> The field width is composed of an optional integer literal indicating the maximal width of the token to read. For instance, <code class="code">%6d</code> reads an integer, having at most 6 decimal digits; <code class="code">%4f</code> reads a float with at most 4 characters; and <code class="code">%8[\\000-\\255]</code> returns the next 8 characters (or all the characters still available, if fewer than 8 characters are available in the input). <p> Notes: <p> <ul> <li>as mentioned above, a <code class="code">%s</code> conversion always succeeds, even if there is nothing to read in the input: in this case, it simply returns <code class="code"><span class="string">""</span></code>.</li> </ul> <ul> <li>in addition to the relevant digits, <code class="code"><span class="string">'_'</span></code> characters may appear inside numbers (this is reminiscent to the usual OCaml lexical conventions). If stricter scanning is desired, use the range conversion facility instead of the number conversions.</li> </ul> <ul> <li>the <code class="code">scanf</code> facility is not intended for heavy duty lexical analysis and parsing. If it appears not expressive enough for your needs, several alternative exists: regular expressions (module <code class="code"><span class="constructor">Str</span></code>), stream parsers, <code class="code">ocamllex</code>-generated lexers, <code class="code">ocamlyacc</code>-generated parsers.</li> </ul> <br> <br> <div class="h7" id="indication">Scanning indications in format strings</div><br> <br> Scanning indications appear just after the string conversions <code class="code">%s</code> and <code class="code">%[ range ]</code> to delimit the end of the token. A scanning indication is introduced by a <code class="code">@</code> character, followed by some plain character <code class="code">c</code>. It means that the string token should end just before the next matching <code class="code">c</code> (which is skipped). If no <code class="code">c</code> character is encountered, the string token spreads as much as possible. For instance, <code class="code"><span class="string">"%s@\t"</span></code> reads a string up to the next tab character or to the end of input. If a <code class="code">@</code> character appears anywhere else in the format string, it is treated as a plain character. <p> Note: <p> <ul> <li>As usual in format strings, <code class="code">%</code> and <code class="code">@</code> characters must be escaped using <code class="code">%%</code> and <code class="code">%@</code>; this rule still holds within range specifications and scanning indications. For instance, <code class="code"><span class="string">"%s@%%"</span></code> reads a string up to the next <code class="code">%</code> character.</li> <li>The scanning indications introduce slight differences in the syntax of <code class="code"><span class="constructor">Scanf</span></code> format strings, compared to those used for the <code class="code"><span class="constructor">Printf</span></code> module. However, the scanning indications are similar to those used in the <code class="code"><span class="constructor">Format</span></code> module; hence, when producing formatted text to be scanned by <code class="code">!<span class="constructor">Scanf</span>.bscanf</code>, it is wise to use printing functions from the <code class="code"><span class="constructor">Format</span></code> module (or, if you need to use functions from <code class="code"><span class="constructor">Printf</span></code>, banish or carefully double check the format strings that contain <code class="code"><span class="string">'@'</span></code> characters).</li> </ul> <br> <br> <div class="h7" id="7_Exceptionsduringscanning">Exceptions during scanning</div><br> <br> Scanners may raise the following exceptions when the input cannot be read according to the format string: <p> <ul> <li>Raise <code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scan_failure</span></code> if the input does not match the format.</li> </ul> <ul> <li>Raise <code class="code"><span class="constructor">Failure</span></code> if a conversion to a number is not possible.</li> </ul> <ul> <li>Raise <code class="code"><span class="constructor">End_of_file</span></code> if the end of input is encountered while some more characters are needed to read the current conversion specification.</li> </ul> <ul> <li>Raise <code class="code"><span class="constructor">Invalid_argument</span></code> if the format string is invalid.</li> </ul> Note: <p> <ul> <li>as a consequence, scanning a <code class="code">%s</code> conversion never raises exception <code class="code"><span class="constructor">End_of_file</span></code>: if the end of input is reached the conversion succeeds and simply returns the characters read so far, or <code class="code"><span class="string">""</span></code> if none were ever read.</li> </ul> <br> <br> <h6 id="6_Specialisedformattedinputfunctions">Specialised formatted input functions</h6><br> <pre><span id="VALfscanf"><span class="keyword">val</span> fscanf</span> : <code class="type"><a href="Pervasives.html#TYPEin_channel">in_channel</a> -> ('a, 'b, 'c, 'd) <a href="Scanf.html#TYPEscanner">scanner</a></code></pre><div class="info "> Same as <a href="Scanf.html#VALbscanf"><code class="code"><span class="constructor">Scanf</span>.bscanf</code></a>, but reads from the given regular input channel. <p> Warning: since all formatted input functions operate from a <em>formatted input channel</em>, be aware that each <code class="code">fscanf</code> invocation will operate with a formatted input channel reading from the given channel. This extra level of bufferization can lead to a strange scanning behaviour if you use low level primitives on the channel (reading characters, seeking the reading position, and so on). <p> As a consequence, never mix direct low level reading and high level scanning from the same regular input channel.<br> </div> <pre><span id="VALsscanf"><span class="keyword">val</span> sscanf</span> : <code class="type">string -> ('a, 'b, 'c, 'd) <a href="Scanf.html#TYPEscanner">scanner</a></code></pre><div class="info "> Same as <a href="Scanf.html#VALbscanf"><code class="code"><span class="constructor">Scanf</span>.bscanf</code></a>, but reads from the given string.<br> </div> <pre><span id="VALscanf"><span class="keyword">val</span> scanf</span> : <code class="type">('a, 'b, 'c, 'd) <a href="Scanf.html#TYPEscanner">scanner</a></code></pre><div class="info "> Same as <a href="Scanf.html#VALbscanf"><code class="code"><span class="constructor">Scanf</span>.bscanf</code></a>, but reads from the predefined formatted input channel <a href="Scanf.Scanning.html#VALstdin"><code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scanning</span>.stdin</code></a> that is connected to <code class="code"><span class="constructor">Pervasives</span>.stdin</code>.<br> </div> <pre><span id="VALkscanf"><span class="keyword">val</span> kscanf</span> : <code class="type"><a href="Scanf.Scanning.html#TYPEin_channel">Scanning.in_channel</a> -><br> (<a href="Scanf.Scanning.html#TYPEin_channel">Scanning.in_channel</a> -> exn -> 'd) -> ('a, 'b, 'c, 'd) <a href="Scanf.html#TYPEscanner">scanner</a></code></pre><div class="info "> Same as <a href="Scanf.html#VALbscanf"><code class="code"><span class="constructor">Scanf</span>.bscanf</code></a>, but takes an additional function argument <code class="code">ef</code> that is called in case of error: if the scanning process or some conversion fails, the scanning function aborts and calls the error handling function <code class="code">ef</code> with the formatted input channel and the exception that aborted the scanning process as arguments.<br> </div> <br> <h6 id="6_Readingformatstringsfrominput">Reading format strings from input</h6><br> <pre><span id="VALbscanf_format"><span class="keyword">val</span> bscanf_format</span> : <code class="type"><a href="Scanf.Scanning.html#TYPEin_channel">Scanning.in_channel</a> -><br> ('a, 'b, 'c, 'd, 'e, 'f) format6 -><br> (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g</code></pre><div class="info "> <code class="code">bscanf_format ic fmt f</code> reads a format string token from the formatted input channel <code class="code">ic</code>, according to the given format string <code class="code">fmt</code>, and applies <code class="code">f</code> to the resulting format string value. Raise <code class="code"><span class="constructor">Scan_failure</span></code> if the format string value read does not have the same type as <code class="code">fmt</code>.<br> <b>Since</b> 3.09.0<br> </div> <pre><span id="VALsscanf_format"><span class="keyword">val</span> sscanf_format</span> : <code class="type">string -><br> ('a, 'b, 'c, 'd, 'e, 'f) format6 -><br> (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g</code></pre><div class="info "> Same as <a href="Scanf.html#VALbscanf_format"><code class="code"><span class="constructor">Scanf</span>.bscanf_format</code></a>, but reads from the given string.<br> <b>Since</b> 3.09.0<br> </div> <pre><span id="VALformat_from_string"><span class="keyword">val</span> format_from_string</span> : <code class="type">string -><br> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> ('a, 'b, 'c, 'd, 'e, 'f) format6</code></pre><div class="info "> <code class="code">format_from_string s fmt</code> converts a string argument to a format string, according to the given format string <code class="code">fmt</code>. Raise <code class="code"><span class="constructor">Scan_failure</span></code> if <code class="code">s</code>, considered as a format string, does not have the same type as <code class="code">fmt</code>.<br> <b>Since</b> 3.10.0<br> </div> <pre><span id="VALunescaped"><span class="keyword">val</span> unescaped</span> : <code class="type">string -> string</code></pre><div class="info "> Return a copy of the argument with escape sequences, following the lexical conventions of OCaml, replaced by their corresponding special characters. If there is no escape sequence in the argument, still return a copy, contrary to String.escaped.<br> <b>Since</b> 4.00.0<br> </div> </body></html>