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231 lines
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<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
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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>
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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>
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<p>
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Print all odd-numbered contents of an STL container using std::for_each (sample4.cpp):</p>
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<code><pre>
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<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>(),
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</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>)
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[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=literal>' '
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</span><span class=special>]
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);
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</span></pre></code>
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<p>
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Huh? Is that valid C++? Read on...</p>
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<p>
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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>
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<p>
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Here are more examples with annotations. The code almost speaks for itself.</p>
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<p>
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<b>1) block statement:</b></p>
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<code><pre>
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<span class=identifier>statement</span><span class=special>,
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</span><span class=identifier>statement</span><span class=special>,
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....
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</span><span class=identifier>statement
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</span></pre></code>
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<p>
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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>
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<code><pre>
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<span class=identifier>statement</span><span class=special>,
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</span><span class=identifier>statement</span><span class=special>,
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</span><span class=identifier>statement</span><span class=special>, // </span><span class=identifier>ERROR</span><span class=special>!
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</span></pre></code>
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<p>
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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>
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<code><pre>
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<span class=identifier>statement</span><span class=special>,
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</span><span class=identifier>statement</span><span class=special>,
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(
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</span><span class=identifier>statement</span><span class=special>,
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</span><span class=identifier>statement
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</span><span class=special>),
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</span><span class=identifier>statement
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</span></pre></code>
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<p>
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Outside the square brackets, block statements should be grouped. For example:</p>
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<code><pre>
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<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>(),
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(
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</span><span class=identifier>do_this</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>),
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</span><span class=identifier>do_that</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>)
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)
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);
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</span></pre></code>
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<p>
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<b>2) if_ statement:</b></p>
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<p>
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We have seen the if_ statement. The syntax is:</p>
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<code><pre>
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<span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
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[
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</span><span class=identifier>sequenced_statements
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</span><span class=special>]
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</span></pre></code>
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<p>
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<b>3) if_ else_ statement:</b></p>
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<p>
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The syntax is</p>
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<code><pre>
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<span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
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[
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</span><span class=identifier>sequenced_statements
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</span><span class=special>]
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.</span><span class=identifier>else_
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</span><span class=special>[
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</span><span class=identifier>sequenced_statements
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</span><span class=special>]
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</span></pre></code>
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<p>
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Take note that else has a prefix dot and a trailing underscore: .else_</p>
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<p>
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Example: This code prints out all the elements and appends " > 5", " == 5" or " < 5" depending on the element's actual value:</p>
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<code><pre>
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<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>(),
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</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>)
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[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" > 5\n"
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</span><span class=special>]
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.</span><span class=identifier>else_
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</span><span class=special>[
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</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>)
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[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" == 5\n"
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</span><span class=special>]
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.</span><span class=identifier>else_
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</span><span class=special>[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>" < 5\n"
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</span><span class=special>]
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]
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);
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</span></pre></code>
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<p>
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Notice how the if_ else_ statement is nested.</p>
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<p>
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<b>4) while_ statement:</b></p>
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<p>
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The syntax is:</p>
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<code><pre>
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<span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
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[
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</span><span class=identifier>sequenced_statements
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</span><span class=special>]
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</span></pre></code>
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<p>
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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>
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<code><pre>
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<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>(),
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(
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</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--)
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[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", "
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</span><span class=special>],
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</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>)
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)
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);
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</span></pre></code>
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<p>
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<b>5) do_ while_ statement:</b></p>
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<p>
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The syntax is:</p>
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<code><pre>
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<span class=identifier>do_
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</span><span class=special>[
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</span><span class=identifier>sequenced_statements
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</span><span class=special>]
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.</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
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</span></pre></code>
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<p>
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Again, take note that while has a prefix dot and a trailing underscore: .while_</p>
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<p>
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Example: This code is almost the same as the previous example above with a slight twist in logic.</p>
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<code><pre>
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<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>(),
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(
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</span><span class=identifier>do_
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</span><span class=special>[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", "
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</span><span class=special>]
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.</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--),
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</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>)
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)
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);
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</span></pre></code>
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<p>
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<b>6) for_ statement:</b></p>
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<p>
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The syntax is:</p>
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<code><pre>
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<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>)
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[
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</span><span class=identifier>sequenced_statements
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</span><span class=special>]
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</span></pre></code>
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<p>
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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>
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<p>
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Example: This code prints each element N times where N is the element's value. A newline terminates the printout of each value.</p>
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<code><pre>
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<span class=keyword>int </span><span class=identifier>iii</span><span class=special>;
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</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>(),
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(
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</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>))
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[
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</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>arg1 </span><span class=special><< </span><span class=string>", "
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</span><span class=special>],
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</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>)
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)
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);
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</span></pre></code>
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<p>
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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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<table width="80%" border="0" align="center">
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<td class="note_box">
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<img src="theme/note.gif"></img> Unlike lazy functions and lazy operators, lazy statements always return void. </td>
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<br>
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<hr size="1">
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<p class="copyright">Copyright © 2001-2002 Joel de Guzman<br>
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<br>
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<font size="2">Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt) </font> </p>
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