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| <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Statements</b></font> |
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| <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> |
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| <td class="note_box"> |
| <img src="theme/note.gif"></img> Unlike lazy functions and lazy operators, lazy statements always return void. </td> |
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| <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> |
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