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title="UnixLabels" rel="Chapter" href="UnixLabels.html"> <link title="Weak" rel="Chapter" href="Weak.html"><link title="Functional input with format strings." rel="Section" href="#6_Functionalinputwithformatstrings"> <link title="Formatted input functions" rel="Section" href="#6_Formattedinputfunctions"> <title>Scanf</title> </head> <body> <div class="navbar"><a href="Random.html">Previous</a> <a href="index.html">Up</a> <a href="Set.html">Next</a> </div> <center><h1>Module <a href="type_Scanf.html">Scanf</a></h1></center> <br> <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>Formatted input functions.<br> <hr width="100%"> <br> <a name="6_Functionalinputwithformatstrings"></a> <h6>Functional input with format strings.</h6><br> <br> The formatted input functions provided by module <code class="code"><span class="constructor">Scanf</span></code> are functionals that apply their function argument to the values they read in the input. The specification of the values to read is simply given by a format string (the same format strings as those used to print material using module <a href="Printf.html"><code class="code"><span class="constructor">Printf</span></code></a> or module <a href="Format.html"><code class="code"><span class="constructor">Format</span></code></a>). <p> As an example, consider the formatted input function <code class="code">scanf</code> that reads from standard input; a typical call to <code class="code">scanf</code> is simply <code class="code">scanf fmt f</code>, meaning that <code class="code">f</code> should be applied to the arguments read according to the format string <code class="code">fmt</code>. For instance, if <code class="code">f</code> is defined as <code class="code"><span class="keyword">let</span> f x = x + 1</code>, then <code class="code">scanf <span class="string">"%d"</span> f</code> will read a decimal integer <code class="code">i</code> from <code class="code">stdin</code> and return <code class="code">f i</code>; thus, if we enter <code class="code">41</code> at the keyboard, <code class="code">scanf <span class="string">"%d"</span> f</code> evaluates to <code class="code">42</code>. <p> This module provides general formatted input functions that read from any kind of input, including strings, files, or anything that can return characters. Hence, a typical call to a formatted input function <code class="code">bscan</code> is <code class="code">bscan ib fmt f</code>, meaning that <code class="code">f</code> should be applied to the arguments read from input <code class="code">ib</code>, according to the format string <code class="code">fmt</code>. <p> The Caml 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 simply the regular function application not the variable assigment based mechanism which is typical of formatted input in imperative languages; the format strings also feature useful additions to easily define complex tokens; as expected of a functional programming language feature, the formatted input functions support polymorphism, in particular arbitrary interaction with polymorphic user-defined scanners. Furthermore, the Caml formatted input facility is fully type-checked at compile time.<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><div class="info"> Scanning buffers </div> <pre><span class="keyword">exception</span> <a name="EXCEPTIONScan_failure"></a>Scan_failure <span class="keyword">of</span> <code class="type">string</code></pre> <div class="info"> The exception raised by formatted input functions when the input cannot be read according to the given format.<br> </div> <pre><span class="keyword">type</span> <a name="TYPEscanner"></a><code class="type">('a, 'b, 'c, 'd)</code> scanner = <code class="type">('a, <a href="Scanf.Scanning.html#TYPEscanbuf">Scanning.scanbuf</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 scanning buffer according to some format string; more precisely, if <code class="code">scan</code> is some formatted input function, then <code class="code">scan ib 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 some scanning buffer <code class="code">ib</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">stdib</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">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 <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 ib <span class="string">"%r;"</span> read_elem f</code> reads a value of type <code class="code">t</code> followed by a <code class="code"><span class="string">';'</span></code> character.<br> </div> <br> <a name="6_Formattedinputfunctions"></a> <h6>Formatted input functions</h6><br> <pre><span class="keyword">val</span> <a name="VALbscanf"></a>bscanf : <code class="type"><a href="Scanf.Scanning.html#TYPEscanbuf">Scanning.scanbuf</a> -> ('a, 'b, 'c, 'd) <a href="Scanf.html#TYPEscanner">scanner</a></code></pre><div class="info"> <code class="code">bscanf ib fmt r1 ... rN f</code> reads arguments for the function <code class="code">f</code> from the scanning buffer <code class="code">ib</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 <code class="code">bscanf</code>. 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 a <code class="code">%r</code> conversion. <p> The format is a character string which contains three types of objects:<ul> <li>plain characters, which are simply matched with the characters of the input,</li> <li>conversion specifications, each of which causes reading and conversion of one argument for <code class="code">f</code>,</li> <li>scanning indications to specify boundaries of tokens.</li> </ul> Among plain characters the space character (ASCII code 32) has a special meaning: it matches ``whitespace'', that is any number of tab, space, line feed and carriage return characters. Hence, a space in the format matches any amount of whitespace in the input. <p> 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 formats for hexadecimal (<code class="code">0x[d]+</code> and <code class="code">0<span class="constructor">X</span>[d]+</code>), octal (<code class="code">0o[d]+</code>), and binary <code class="code">0b[d]+</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.</li> <li><code class="code">o</code>: reads an unsigned octal integer.</li> <li><code class="code">s</code>: reads a string argument that spreads as much as possible, until the next white space, the next scanning indication, or the end-of-input is reached. 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 Caml).</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 Caml).</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 Caml (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 (decimal, hexadecimal, etc).</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.</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.</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>.</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>.scanbuf <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 to the format specified by the internal format <code class="code">fmt</code>. The format string to be read must have the same type as the internal format <code class="code">fmt</code>. For instance, "%{%i%}" reads any format string that can read a value of type <code class="code">int</code>; hence <code class="code"><span class="constructor">Scanf</span>.sscanf <span class="string">"fmt:\\\"number is %u\\\""</span> <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 format substitution. Reads a format string to replace <code class="code">fmt</code>. The format string read must have the same type as <code class="code">fmt</code>.</li> <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> </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>, 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> 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> 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 constant 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 scanning indication <code class="code">@c</code> does not follow a string conversion, it is treated as a plain <code class="code">c</code> character. <p> Raise <code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scan_failure</span></code> if the input does not match the format. <p> Raise <code class="code"><span class="constructor">Failure</span></code> if a conversion to a number is not possible. <p> 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. 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 read. <p> Raise <code class="code"><span class="constructor">Invalid_argument</span></code> if the format string is invalid. <p> Notes: <p> <ul> <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 by the <code class="code"><span class="constructor">Printf</span></code> module. However, scanning indications are similar to those of 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 <code class="code"><span class="constructor">Format</span></code> (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> <ul> <li>in addition to relevant digits, <code class="code"><span class="string">'_'</span></code> characters may appear inside numbers (this is reminiscent to the usual Caml 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> </div> <pre><span class="keyword">val</span> <a name="VALfscanf"></a>fscanf : <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 channel. <p> Warning: since all formatted input functions operate from a scanning buffer, be aware that each <code class="code">fscanf</code> invocation must allocate a new fresh scanning buffer (unless you make careful use of partial application). Hence, there are chances that some characters seem to be skipped (in fact they are pending in the previously used scanning buffer). This happens in particular when calling <code class="code">fscanf</code> again after a scan involving a format that necessitated some look ahead (such as a format that ends by skipping whitespace in the input). <p> To avoid confusion, consider using <code class="code">bscanf</code> with an explicitly created scanning buffer. Use for instance <code class="code"><span class="constructor">Scanning</span>.from_file f</code> to allocate the scanning buffer reading from file <code class="code">f</code>. <p> This method is not only clearer it is also faster, since scanning buffers to files are optimized for fast buffered reading.<br> </div> <pre><span class="keyword">val</span> <a name="VALsscanf"></a>sscanf : <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 class="keyword">val</span> <a name="VALscanf"></a>scanf : <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 scanning buffer <a href="Scanf.Scanning.html#VALstdib"><code class="code"><span class="constructor">Scanf</span>.<span class="constructor">Scanning</span>.stdib</code></a> that is connected to <code class="code">stdin</code>.<br> </div> <pre><span class="keyword">val</span> <a name="VALkscanf"></a>kscanf : <code class="type"><a href="Scanf.Scanning.html#TYPEscanbuf">Scanning.scanbuf</a> -><br> (<a href="Scanf.Scanning.html#TYPEscanbuf">Scanning.scanbuf</a> -> exn -> 'a) -> ('b, 'c, 'd, 'a) <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 scanning buffer and the exception that aborted the scanning process.<br> </div> <pre><span class="keyword">val</span> <a name="VALbscanf_format"></a>bscanf_format : <code class="type"><a href="Scanf.Scanning.html#TYPEscanbuf">Scanning.scanbuf</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 ib fmt f</code> reads a format string token from the scannning buffer <code class="code">ib</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 doesn't have the same type as <code class="code">fmt</code>.<br> </div> <pre><span class="keyword">val</span> <a name="VALsscanf_format"></a>sscanf_format : <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> </div> <pre><span class="keyword">val</span> <a name="VALformat_from_string"></a>format_from_string : <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, doesn't have the same type as <code class="code">fmt</code>.<br> </div> </body></html>