<html> <head> <!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc --> <title>Statements</title> <link rel="stylesheet" href="theme/style.css" type="text/css"> <link rel="prev" href="operators.html"> <link rel="next" href="binders.html"> </head> <body> <table width="100%" height="48" border="0" background="theme/bkd2.gif" cellspacing="2"> <tr> <td width="10"> </td> <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Statements</b></font> </td> <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" align="right" border="0"></a></td> </tr> </table> <br> <table border="0"> <tr> <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> <td width="30"><a href="operators.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="20"><a href="binders.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <a name="lazy_statements"></a><h2>Lazy statements</h2><p> The primitives and composite building blocks presented before are sufficiently powerful to construct quite elaborate structures and facilities. We have presented lazy-functions and lazy-operators. How about lazy-statements? First, an appetizer:</p> <p> Print all odd-numbered contents of an STL container using std::for_each (sample4.cpp):</p> <code><pre> <span class=identifier>for_each</span><span class=special>(</span><span class=identifier>c</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(), </span><span class=identifier>c</span><span class=special>.</span><span class=identifier>end</span><span class=special>(), </span><span class=identifier>if_</span><span class=special>(</span><span class=identifier>arg1 </span><span class=special>% </span><span class=number>2 </span><span class=special>== </span><span class=number>1</span><span class=special>) [ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=literal>' ' </span><span class=special>] ); </span></pre></code> <p> Huh? Is that valid C++? Read on...</p> <p> Yes, it is valid C++. The sample code above is as close as you can get to the syntax of C++. This stylized C++ syntax differs from actual C++ code. First, the if has a trailing underscore. Second, the block uses square brackets instead of the familiar curly braces {}.</p> <p> Here are more examples with annotations. The code almost speaks for itself.</p> <p> <b>1) block statement:</b></p> <code><pre> <span class=identifier>statement</span><span class=special>, </span><span class=identifier>statement</span><span class=special>, .... </span><span class=identifier>statement </span></pre></code> <p> Basically, these are comma separated statements. Take note that unlike the C/C++ semicolon, the comma is a separator put *in-between* statements. This is like Pascal's semicolon separator, rather than C/C++'s semicolon terminator. For example:</p> <code><pre> <span class=identifier>statement</span><span class=special>, </span><span class=identifier>statement</span><span class=special>, </span><span class=identifier>statement</span><span class=special>, // </span><span class=identifier>ERROR</span><span class=special>! </span></pre></code> <p> Is an error. The last statement should not have a comma. Block statements can be grouped using the parentheses. Again, the last statement in a group should not have a trailing comma.</p> <code><pre> <span class=identifier>statement</span><span class=special>, </span><span class=identifier>statement</span><span class=special>, ( </span><span class=identifier>statement</span><span class=special>, </span><span class=identifier>statement </span><span class=special>), </span><span class=identifier>statement </span></pre></code> <p> Outside the square brackets, block statements should be grouped. For example:</p> <code><pre> <span class=identifier>for_each</span><span class=special>(</span><span class=identifier>c</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(), </span><span class=identifier>c</span><span class=special>.</span><span class=identifier>end</span><span class=special>(), ( </span><span class=identifier>do_this</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>), </span><span class=identifier>do_that</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>) ) ); </span></pre></code> <p> <b>2) if_ statement:</b></p> <p> We have seen the if_ statement. The syntax is:</p> <code><pre> <span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) [ </span><span class=identifier>sequenced_statements </span><span class=special>] </span></pre></code> <p> <b>3) if_ else_ statement:</b></p> <p> The syntax is</p> <code><pre> <span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) [ </span><span class=identifier>sequenced_statements </span><span class=special>] .</span><span class=identifier>else_ </span><span class=special>[ </span><span class=identifier>sequenced_statements </span><span class=special>] </span></pre></code> <p> Take note that else has a prefix dot and a trailing underscore: .else_</p> <p> Example: This code prints out all the elements and appends " > 5", " == 5" or " < 5" depending on the element's actual value:</p> <code><pre> <span class=identifier>for_each</span><span class=special>(</span><span class=identifier>c</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(), </span><span class=identifier>c</span><span class=special>.</span><span class=identifier>end</span><span class=special>(), </span><span class=identifier>if_</span><span class=special>(</span><span class=identifier>arg1 </span><span class=special>> </span><span class=number>5</span><span class=special>) [ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" > 5\n" </span><span class=special>] .</span><span class=identifier>else_ </span><span class=special>[ </span><span class=identifier>if_</span><span class=special>(</span><span class=identifier>arg1 </span><span class=special>== </span><span class=number>5</span><span class=special>) [ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" == 5\n" </span><span class=special>] .</span><span class=identifier>else_ </span><span class=special>[ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" < 5\n" </span><span class=special>] ] ); </span></pre></code> <p> Notice how the if_ else_ statement is nested.</p> <p> <b>4) while_ statement:</b></p> <p> The syntax is:</p> <code><pre> <span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) [ </span><span class=identifier>sequenced_statements </span><span class=special>] </span></pre></code> <p> Example: This code decrements each element until it reaches zero and prints out the number at each step. A newline terminates the printout of each value.</p> <code><pre> <span class=identifier>for_each</span><span class=special>(</span><span class=identifier>c</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(), </span><span class=identifier>c</span><span class=special>.</span><span class=identifier>end</span><span class=special>(), ( </span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--) [ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", " </span><span class=special>], </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>val</span><span class=special>(</span><span class=string>"\n"</span><span class=special>) ) ); </span></pre></code> <p> <b>5) do_ while_ statement:</b></p> <p> The syntax is:</p> <code><pre> <span class=identifier>do_ </span><span class=special>[ </span><span class=identifier>sequenced_statements </span><span class=special>] .</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) </span></pre></code> <p> Again, take note that while has a prefix dot and a trailing underscore: .while_</p> <p> Example: This code is almost the same as the previous example above with a slight twist in logic.</p> <code><pre> <span class=identifier>for_each</span><span class=special>(</span><span class=identifier>c</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(), </span><span class=identifier>c</span><span class=special>.</span><span class=identifier>end</span><span class=special>(), ( </span><span class=identifier>do_ </span><span class=special>[ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", " </span><span class=special>] .</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--), </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>val</span><span class=special>(</span><span class=string>"\n"</span><span class=special>) ) ); </span></pre></code> <p> <b>6) for_ statement:</b></p> <p> The syntax is:</p> <code><pre> <span class=identifier>for_</span><span class=special>(</span><span class=identifier>init_statement</span><span class=special>, </span><span class=identifier>conditional_expression</span><span class=special>, </span><span class=identifier>step_statement</span><span class=special>) [ </span><span class=identifier>sequenced_statements </span><span class=special>] </span></pre></code> <p> It is again almost similar to C++ for statement. Take note that the init_statement, conditional_expression and step_statement are separated by the comma instead of the semi- colon and each must be present (i.e. for_(,,) is invalid).</p> <p> Example: This code prints each element N times where N is the element's value. A newline terminates the printout of each value.</p> <code><pre> <span class=keyword>int </span><span class=identifier>iii</span><span class=special>; </span><span class=identifier>for_each</span><span class=special>(</span><span class=identifier>c</span><span class=special>.</span><span class=identifier>begin</span><span class=special>(), </span><span class=identifier>c</span><span class=special>.</span><span class=identifier>end</span><span class=special>(), ( </span><span class=identifier>for_</span><span class=special>(</span><span class=identifier>var</span><span class=special>(</span><span class=identifier>iii</span><span class=special>) = </span><span class=number>0</span><span class=special>, </span><span class=identifier>var</span><span class=special>(</span><span class=identifier>iii</span><span class=special>) < </span><span class=identifier>arg1</span><span class=special>, ++</span><span class=identifier>var</span><span class=special>(</span><span class=identifier>iii</span><span class=special>)) [ </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", " </span><span class=special>], </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>val</span><span class=special>(</span><span class=string>"\n"</span><span class=special>) ) ); </span></pre></code> <p> As before, all these are lazily evaluated. The result of such statements are in fact composites that are passed on to STL's for_each function. In the viewpoint of for_each, what was passed is just a functor, no more, no less.</p> <table width="80%" border="0" align="center"> <tr> <td class="note_box"> <img src="theme/note.gif"></img> Unlike lazy functions and lazy operators, lazy statements always return void. </td> </tr> </table> <table border="0"> <tr> <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> <td width="30"><a href="operators.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="20"><a href="binders.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <br> <hr size="1"> <p class="copyright">Copyright © 2001-2002 Joel de Guzman<br> <br> <font size="2">Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </font> </p> </body> </html>