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| <H2>13.8 The for_each() Algorithm</H2> |
| <A NAME="idx343"><!></A> |
| <P>The algorithm <SAMP>std::for_each()</SAMP> applies a function to all elements in a collection. This algorithm takes three arguments: the first two provide the <B><I><A HREF="../stdlibref/iterator.html">iterator</A></I></B>s that describe the sequence to be evaluated, and the third is a one-argument function. The algorithm <SAMP>std::for_each()</SAMP> applies the function to each value of the sequence, passing the value as an argument:</P> |
| |
| <UL><PRE> |
| namespace std { |
| Function for_each(InputIterator first, |
| InputIterator last, Function); |
| } |
| </PRE></UL> |
| <P>For example, the following code fragment, which uses the <SAMP>print_if_leap()</SAMP> function, prints a list of the leap years that occur between 1900 and 1997: </P> |
| |
| <UL><PRE> |
| std::cout << "leap years between 1990 and 1997 are: "; |
| std::for_each(1990, 1997, print_if_leap); |
| std::cout << std::endl; |
| </PRE></UL> |
| <P>The argument function is guaranteed to be invoked only once for each element in the sequence. The <SAMP>std::for_each()</SAMP> algorithm itself returns the value of the third argument, although this is usually ignored.</P> |
| <BLOCKQUOTE><HR><B> |
| NOTE -- The function passed as the third argument is not permitted to make any modifications to the sequence, so it can only achieve a result by means of a side effect, such as printing, assigning a value to a global or static variable, or invoking another function that produces a side effect. If the argument function returns any result, it is ignored. |
| </B><HR></BLOCKQUOTE> |
| <P>The following example searches an array of <SAMP>int</SAMP> values representing dates, to determine which vintage wine years were also leap years:</P> |
| |
| <UL><PRE> |
| int vintageYears[] = {1947, 1955, 1960, 1967, 1994}; |
| ... |
| |
| std::cout << "vintage years which were also leap years are: "; |
| std::for_each(vintageYears, vintageYears + 5, print_if_leap); |
| std::cout << std::endl; |
| </PRE></UL> |
| <P>Side effects need not be restricted to printing. Assume we have a function <SAMP>countCaps()</SAMP> that counts the occurrence of capital letters:</P> |
| |
| <UL><PRE> |
| int capCount = 0; |
| |
| void countCaps(char c) { if (std::isupper(c)) capCount++; } |
| </PRE></UL> |
| <P>The following example counts the number of capital letters in a string value:</P> |
| |
| <UL><PRE> |
| std::string advice = "Never Trust Anybody Over 30!"; |
| std::for_each(advice.begin(), advice.end(),countCaps); |
| std::cout << "upper-case letter count is " |
| << capCount << std::endl; |
| </PRE></UL> |
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