Standard Template Library Programmer's Guide

find , mismatch , equal , search

Category: algorithms

Component type: function

find_first_of is an overloaded name; there are actually two find_first_of functions.

template

InputIterator find_first_of(InputIterator first1, InputIterator last1, ForwardIterator first2, ForwardIterator last2);

template

InputIterator find_first_of(InputIterator first1, InputIterator last1, ForwardIterator first2, ForwardIterator last2, BinaryPredicate comp);

Find_first_of is similar to find , in that it performs linear seach through a range of Input Iterators. The difference is that while find searches for one particular value, find_first_of searches for any of several values. Specifically, find_first_of searches for the first occurrance in the range [first1, last1) of any of the elements in [first2, last2) . (Note that this behavior is reminiscent of the function strpbrk from the standard C library.)

The two versions of find_first_of differ in how they compare elements for equality. The first uses operator== , and the second uses and arbitrary user-supplied function object comp . The first version returns the first iterator i in [first1, last1) such that, for some iterator j in [first2, last2) , *i == *j . The second returns the first iterator i in [first1, last1) such that, for some iterator j in [first2, last2) , comp(*i, *j) is true . As usual, both versions return last1 if no such iterator i exists.

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

For the first version:

• InputIterator is a model of Input Iterator.

• ForwardIterator is a model of Forward Iterator.

• InputIterator 's value type is EqualityComparable , and can be compared for equality with ForwardIterator's value type.

For the second version:

• InputIterator is a model of Input Iterator.

• ForwardIterator is a model of Forward Iterator.

• BinaryPredicate is a model of Binary Predicate.

• InputIterator 's value type is convertible to BinaryPredicate 's first argument type.

• ForwardIterator 's value type is convertible to BinaryPredicate 's second argument type.

• [first1, last1) is a valid range.

• [first2, last2) is a valid range.

At most (last1 – first1) * (last2 – first2) comparisons.

Like strpbrk , one use for find_first_of is finding whitespace in a string; space, tab, and newline are all whitespace characters.

int main() {

const char* WS = "\t\n ";

const int n_WS = strlen(WS);

char* s1 = "This sentence contains five words.";

char* s2 = "OneWord";

char* end1 = find_first_of(s1, s1 + strlen(s1), WS, WS + n_WS);

char* end2 = find_first_of(s2, s2 + strlen(s2), WS, WS + n_WS);

printf("First word of s1: %.*s\n", end1 – s1, s1);

printf("First word of s2: %.*s\n", end2 – s2, s2);

}

find , find_if , search

Category: algorithms

Component type: function

Count is an overloaded name: there are two count functions.

template

iterator_traits::difference_type count(InputIterator first, InputIterator last, const EqualityComparable& value);

template

void count(InputIterator first, InputIterator last, const EqualityComparable& value, Size& n);

Count finds the number of elements in [first, last) that are equal to value . More precisely, the first version of count returns the number of iterators i in [first, last) such that *i == value . The second version of count adds to n the number of iterators i in [first, last) such that *i == value .

The second version of count was the one defined in the original STL, and the first version is the one defined in the draft C++ standard; the definition was changed because the older interface was clumsy and error-prone. The older interface required the use of a temporary variable, which had to be initialized to 0 before the call to count .

Both interfaces are currently supported [1], for reasons of backward compatibility, but eventually the older version will be removed.

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

For the first version, which takes three arguments:

• InputIterator is a model of Input Iterator.

• EqualityComparable is a model of Equality Comparable.

• InputIterator 's value type is a model of Equality Comparable.

• An object of InputIterator 's value type can be compared for equality with an object of type EqualityComparable .

For the second version, which takes four arguments:

• InputIterator is a model of Input Iterator.

• EqualityComparable is a model of Equality Comparable.

• Size is an integral type that can hold values of InputIterator 's distance type.

• InputIterator 's value type is a model of Equality Comparable.

• An object of InputIterator 's value type can be compared for equality with an object of type EqualityComparable .

• [first, last) is a valid range.

For the second version:

• [first, last) is a valid range.

• n plus the number of elements equal to value does not exceed the maximum value of type Size .

Linear. Exactly last – first comparisons.

int main() {

int A[] = { 2, 0, 4, 6, 0, 3, 1, –7 };

const int N = sizeof(A) / sizeof(int);

cout << "Number of zeros: " << count(A, A + N, 0) << endl;

}

[1] The new count interface uses the iterator_traits class, which relies on a C++ feature known as partial specialization . Many of today's compilers don't implement the complete standard; in particular, many compilers do not support partial specialization. If your compiler does not support partial specialization, then you will not be able to use the newer version of count , or any other STL components that involve iterator_traits .