transform

Category : algorithms		Component type: function

Prototype

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);

Description

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

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 BinaryFunction instead of a UnaryFunction : 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]

Definition

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

Requirements on types

For the first (unary) version:

• InputIterator must be a model of InputIterator.
• OutputIterator must be a model of OutputIterator.
• UnaryFunction must be a model of UnaryFunction.
• 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 InputIterator.
• OutputIterator must be a model of OutputIterator.
• BinaryFunction must be a model of BinaryFunction.
• 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.

Preconditions

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.

Complexity

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.

Example

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>());

Notes

[1] The OutputIterator result is not permitted to be the same as any of the InputIterator 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.

See also

The functors, copy, generate, fill