1<html> 2<head> 3<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc --> 4<title>Statements</title> 5<link rel="stylesheet" href="theme/style.css" type="text/css"> 6<link rel="prev" href="operators.html"> 7<link rel="next" href="binders.html"> 8</head> 9<body> 10<table width="100%" height="48" border="0" background="theme/bkd2.gif" cellspacing="2"> 11 <tr> 12 <td width="10"> 13 </td> 14 <td width="85%"> 15 <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Statements</b></font> 16 </td> 17 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" align="right" border="0"></a></td> 18 </tr> 19</table> 20<br> 21<table border="0"> 22 <tr> 23 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 24 <td width="30"><a href="operators.html"><img src="theme/l_arr.gif" border="0"></a></td> 25 <td width="20"><a href="binders.html"><img src="theme/r_arr.gif" border="0"></a></td> 26 </tr> 27</table> 28<a name="lazy_statements"></a><h2>Lazy statements</h2><p> 29The 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> 30<p> 31Print all odd-numbered contents of an STL container using std::for_each (sample4.cpp):</p> 32<code><pre> 33 <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>(), 34 </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>) 35 [ 36 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=literal>' ' 37 </span><span class=special>] 38 ); 39</span></pre></code> 40<p> 41Huh? Is that valid C++? Read on...</p> 42<p> 43Yes, 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> 44<p> 45Here are more examples with annotations. The code almost speaks for itself.</p> 46<p> 47<b>1) block statement:</b></p> 48<code><pre> 49 <span class=identifier>statement</span><span class=special>, 50 </span><span class=identifier>statement</span><span class=special>, 51 .... 52 </span><span class=identifier>statement 53</span></pre></code> 54<p> 55Basically, 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> 56<code><pre> 57 <span class=identifier>statement</span><span class=special>, 58 </span><span class=identifier>statement</span><span class=special>, 59 </span><span class=identifier>statement</span><span class=special>, // </span><span class=identifier>ERROR</span><span class=special>! 60</span></pre></code> 61<p> 62Is 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> 63<code><pre> 64 <span class=identifier>statement</span><span class=special>, 65 </span><span class=identifier>statement</span><span class=special>, 66 ( 67 </span><span class=identifier>statement</span><span class=special>, 68 </span><span class=identifier>statement 69 </span><span class=special>), 70 </span><span class=identifier>statement 71</span></pre></code> 72<p> 73Outside the square brackets, block statements should be grouped. For example:</p> 74<code><pre> 75 <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>(), 76 ( 77 </span><span class=identifier>do_this</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>), 78 </span><span class=identifier>do_that</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>) 79 ) 80 ); 81</span></pre></code> 82<p> 83<b>2) if_ statement:</b></p> 84<p> 85We have seen the if_ statement. The syntax is:</p> 86<code><pre> 87 <span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) 88 [ 89 </span><span class=identifier>sequenced_statements 90 </span><span class=special>] 91</span></pre></code> 92<p> 93<b>3) if_ else_ statement:</b></p> 94<p> 95The syntax is</p> 96<code><pre> 97 <span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) 98 [ 99 </span><span class=identifier>sequenced_statements 100 </span><span class=special>] 101 .</span><span class=identifier>else_ 102 </span><span class=special>[ 103 </span><span class=identifier>sequenced_statements 104 </span><span class=special>] 105</span></pre></code> 106<p> 107Take note that else has a prefix dot and a trailing underscore: .else_</p> 108<p> 109Example: This code prints out all the elements and appends " > 5", " == 5" or " < 5" depending on the element's actual value:</p> 110<code><pre> 111 <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>(), 112 </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>) 113 [ 114 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" > 5\n" 115 </span><span class=special>] 116 .</span><span class=identifier>else_ 117 </span><span class=special>[ 118 </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>) 119 [ 120 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" == 5\n" 121 </span><span class=special>] 122 .</span><span class=identifier>else_ 123 </span><span class=special>[ 124 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" < 5\n" 125 </span><span class=special>] 126 ] 127 ); 128</span></pre></code> 129<p> 130Notice how the if_ else_ statement is nested.</p> 131<p> 132<b>4) while_ statement:</b></p> 133<p> 134The syntax is:</p> 135<code><pre> 136 <span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) 137 [ 138 </span><span class=identifier>sequenced_statements 139 </span><span class=special>] 140</span></pre></code> 141<p> 142Example: 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> 143<code><pre> 144 <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>(), 145 ( 146 </span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--) 147 [ 148 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", " 149 </span><span class=special>], 150 </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>) 151 ) 152 ); 153</span></pre></code> 154<p> 155<b>5) do_ while_ statement:</b></p> 156<p> 157The syntax is:</p> 158<code><pre> 159 <span class=identifier>do_ 160 </span><span class=special>[ 161 </span><span class=identifier>sequenced_statements 162 </span><span class=special>] 163 .</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>) 164</span></pre></code> 165<p> 166Again, take note that while has a prefix dot and a trailing underscore: .while_</p> 167<p> 168Example: This code is almost the same as the previous example above with a slight twist in logic.</p> 169<code><pre> 170 <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>(), 171 ( 172 </span><span class=identifier>do_ 173 </span><span class=special>[ 174 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", " 175 </span><span class=special>] 176 .</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--), 177 </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>) 178 ) 179 ); 180</span></pre></code> 181<p> 182<b>6) for_ statement:</b></p> 183<p> 184The syntax is:</p> 185<code><pre> 186 <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>) 187 [ 188 </span><span class=identifier>sequenced_statements 189 </span><span class=special>] 190</span></pre></code> 191<p> 192It 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> 193<p> 194Example: This code prints each element N times where N is the element's value. A newline terminates the printout of each value.</p> 195<code><pre> 196 <span class=keyword>int </span><span class=identifier>iii</span><span class=special>; 197 </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>(), 198 ( 199 </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>)) 200 [ 201 </span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", " 202 </span><span class=special>], 203 </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>) 204 ) 205 ); 206</span></pre></code> 207<p> 208As 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> 209<table width="80%" border="0" align="center"> 210 <tr> 211 <td class="note_box"> 212<img src="theme/note.gif"></img> Unlike lazy functions and lazy operators, lazy statements always return void. </td> 213 </tr> 214</table> 215<table border="0"> 216 <tr> 217 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 218 <td width="30"><a href="operators.html"><img src="theme/l_arr.gif" border="0"></a></td> 219 <td width="20"><a href="binders.html"><img src="theme/r_arr.gif" border="0"></a></td> 220 </tr> 221</table> 222<br> 223<hr size="1"> 224<p class="copyright">Copyright © 2001-2002 Joel de Guzman<br> 225 <br> 226<font size="2">Use, modification and distribution is subject to the Boost Software 227 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 228 http://www.boost.org/LICENSE_1_0.txt) </font> </p> 229</body> 230</html> 231
