123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230 |
- <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>
|