Книга: Standard Template Library Programmer

transform

transform

Category: algorithms

Component type: function

Transform is an overloaded name; there are actually two transform functions.

template <class InputIterator, class OutputIterator, class UnaryFunction>
OutputIterator transform(InputIterator first, InputIterator last, OutputIterator result, UnaryFunction op);
template <class InputIterator1, class InputIterator2, class OutputIterator, class BinaryFunction>
OutputIterator transform(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, OutputIterator result, BinaryFunction binary_op);

Transform performs an operation on objects; there are two versions of transform, one of which uses a single range of Input Iterators and one of which uses two ranges of Input Iterators.

The first version of transform performs the operation op(*i) for each iterator i in the range [first, last) , and assigns the result of that operation to *o, where o is the corresponding output iterator. That is, for each n such that 0 <= n < last – first, it performs the assignment *(result + n) = op(*(first + n)). The return value is result + (last – first).

The second version of transform is very similar, except that it uses a Binary Function instead of a Unary Function: it performs the operation op(*i1, *i2) for each iterator i1 in the range [first1, last1) and assigns the result to *o, where i2 is the corresponding iterator in the second input range and where o is the corresponding output iterator. That is, for each n such that 0 <= n < last1 – first1, it performs the assignment *(result + n) = op(*(first1 + n), *(first2 + n). The return value is result + (last1 – first1).

Note that transform may be used to modify a sequence "in place": it is permissible for the iterators first and result to be the same. [1]

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

For the first (unary) version:

• InputIterator must be a model of Input Iterator.

• OutputIterator must be a model of Output Iterator.

• UnaryFunction must be a model of Unary Function.

• InputIterator's value type must be convertible to UnaryFunction's argument type.

• UnaryFunction's result type must be convertible to a type in OutputIterator's set of value types.

For the second (binary) version:

• InputIterator1 and InputIterator2 must be models of Input Iterator.

• OutputIterator must be a model of Output Iterator.

• BinaryFunction must be a model of Binary Function.

• InputIterator1's and InputIterator2's value types must be convertible, respectively, to BinaryFunction's first and second argument types.

• UnaryFunction's result type must be convertible to a type in OutputIterator's set of value types.

For the first (unary) version:

• [first, last) is a valid range.

• result is not an iterator within the range [first+1, last). [1]

• There is enough space to hold all of the elements being copied. More formally, the requirement is that [result, result + (last – first)) is a valid range.

For the second (binary) version:

• [first1, last1) is a valid range.

• [first2, first2 + (last1 – first1)) is a valid range.

• result is not an iterator within the range [first1+1, last1) or [first2 + 1, first2 + (last1 – first1)).

• There is enough space to hold all of the elements being copied. More formally, the requirement is that [result, result + (last1 – first1)) is a valid range.

Linear. The operation is applied exactly last – first times in the case of the unary version, or last1 – first1 in the case of the binary version.

Replace every number in an array with its negative.

const int N = 1000;
double A[N];
iota (A, A+N, 1);
transform(A, A+N, A, negate<double>());

Calculate the sum of two vectors, storing the result in a third vector.

const int N = 1000;
vector<int> V1(N);
vector<int> V2(N);
vector <int> V3(N);
iota(V1.begin(), V1.end(), 1);
fill(V2.begin(), V2.end(), 75);
assert(V2.size() >= V1.size() && V3.size() >= V1.size());
transform(V1.begin(), V1.end(), V2.begin(), V3.begin(), plus <int>());

[1] The Output Iterator result is not permitted to be the same as any of the Input Iterators in the range [first, last), with the exception of first itself. That is: transform(V.begin(), V.end(), V.begin(), fabs) is valid, but transform(V.begin(), V.end(), V.begin() + 1, fabs) is not.

The function object overview, copy, generate, fill

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