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Showing content from https://cplusplus.com/reference/iterator/prev/ below:

function template

<iterator>

std::prev 
template <class BidirectionalIterator>  BidirectionalIterator prev (BidirectionalIterator it,       typename iterator_traits<BidirectionalIterator>::difference_type n = 1);

Get iterator to previous element

Returns an iterator pointing to the element that it would be pointing to if advanced -n positions.

If it is a random-access iterator, the function uses just once operator+ or operator-. Otherwise, the function uses repeatedly the increase or decrease operator (operator++ or operator--) on the copied iterator until n elements have been advanced.



Parameters
it
Iterator to base position.
BidirectionalIterator shall be at least a bidirectional iterator.
n
Number of element positions offset (1 by default).
difference_type is the numerical type that represents distances between iterators of the BidirectionalIterator type.

Return value An iterator to the element n positions before it.

Example 
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// prev example
#include <iostream>     // std::cout
#include <iterator>     // std::next
#include <list>         // std::list
#include <algorithm>    // std::for_each

int main () {
  std::list<int> mylist;
  for (int i=0; i<10; i++) mylist.push_back (i*10);

  std::cout << "The last element is " << *std::prev(mylist.end()) << '\n';

  return 0;
}

Output:


Complexity Constant for random-access iterators.
Linear in n for bidirectional iterators.

Iterator validity No effect.

Data races The function accesses the iterator, but it is never dereferenced (no pointed object is accessed by the call).

Exception safety Throws if any of the arithmetical operations performed on the copied iterator throws, providing the same level of guarantee as such operations.

See also
next
Get iterator to next element (function template)
advance
Advance iterator (function template)

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