C++ STL Algorithms – Sort – Part-2

In the last post, the basic use of STL sort() method is explained. In this post, I will discuss some more options available with this method.
We have seen in the previous post that sort takes the beginning and ending of the array as arguments.
sort( array.begin(), array.end() );
In addition to those parameters, it can also take a third parameter (predicate) which decides the ordering of the elements. If we do not pass the third parameter, the default ordering is assumed. For integer data types such as int,  float, char etc, ascending order is followed. For strings alphabetical order is followed. To sort an array of integers in descending order, simply call the sort routine like this.

sort( array.begin(), array.end(), greater<int>() );

std::greater is a built in comparator object provided by STL. we have to pass the type of objects to be sorted. similarly we have std::less, std::less_equal, std::greater_equal etc…

So far we are sorting only built in data types such as int, double and library class string. We can also sort objects of user defined classes also.

For example in the following program I have defined a class called Pair, which contains two fields. To sort user defined/custom data types we also have to write the comparison functions to decide the ordering. We can sort Pairs either according to the first value or the second value. I have written two comparator functions and passed these while sorting. 

Here is the code which explains these concepts.