Home » C++ » <algorithm> function for finding last item less-than-or-equal to, like lower_bound

# <algorithm> function for finding last item less-than-or-equal to, like lower_bound

Questions:

Is there a function in that uses binary search, like `lower_bound` but that returns the last item less-than-or-equal-to according to a given predicate?

`lower_bound` is defined to:

Finds the position of the first element in an ordered range that has a value greater than or equivalent to a specified value, where the ordering criterion may be specified by a binary predicate.

and `upper_bound`:

Finds the position of the first element in an ordered range that has a value that is greater than a specified value, where the ordering criterion may be specified by a binary predicate.

Specifically, I have a container of time ordered events and for a given time I want to find the last item that came before or at that point. Can I achieve this with some combination of upper/lower bound, reverse iterators and using `std::greater` or `std::greater_equal` ?

EDIT:
A tweak was needed to user763305’s suggestion to cope with if you ask for a point before the start of the array:

``````iterator it=upper_bound(begin(), end(), val, LessThanFunction());
if (it!=begin()) {
it--; // not at end of array so rewind to previous item
} else {
it=end(); // no items before this point, so return end()
}
return it;
``````

In a sorted container, the last element that is less than or equivalent to `x`, is the element before the first element that is greater than `x`.

Thus you can call `std::upper_bound`, and decrement the returned iterator once.
(Before decrementing, you must of course check that it is not the begin iterator; if it is, then there are no elements that are less than or equivalent to `x`.)

Questions:

Here is a wrapper function around upper_bound which returns the largest number in a container or array which is less than or equal to a given value:

``````template <class ForwardIterator, class T>
ForwardIterator largest_less_than_or_equal_to ( ForwardIterator first,
ForwardIterator last,
const T& value)
{
ForwardIterator upperb = upper_bound(first, last, value);

// First element is >, so none are <=
if(upperb == first)
return NULL;

// All elements are <=, so return the largest.
if(upperb == last)
return --upperb;

return upperb - 1;
}
``````

For a better explanation of what this is doing and how to use this function, check out:

C++ STL — Find last number less than or equal to a given element in an array or container