How to sort C++ array in ASC and DESC mode?

Question

I have this array:

array[0] = 18;
array[1] = -10;
array[2] = 2;
array[3] = 4;
array[4] = 6;
array[5] = -12;
array[6] = -8;
array[7] = -6;
array[8] = 4;
array[9] = 13;

how do I sort the array in asc/desc mode in C++?

Solution 1

To sort an array in ascending, use:

#include <algorithm>

int main()
{
   // ...
   std::sort(array, array+n); // where n is the number of elements you want to sort
}

To sort it in descending, use

#include <algorithm>
#include <functional>  

int main()
{
   // ...
   std::sort(array, array+n, std::greater<int>());
}

Solution 2

You can pass custom comparison functor to the std::sort function.

Solution 3

#include <iostream>
#include <stdlib.h>
using namespace std;

int main (int argc, char *argv[])
    {
        int num[10]={18,-10,2,4,6,-12,-8,-6,13,-1};
        int temp;

        cout << "Ascending Sort : \n\n";

        for(int i=0; i<=10; i++)
            {
                for(int j=i+1; j<=10; j++)
                    {
                        if(num[i]>num[j])
                            {
                                temp=num[i];
                                num[i]=num[j];
                                num[j]=temp;
                            }
                    }
                cout << num[i] << "\n";
            }

        cout << "\nDescending Sort : \n\n";

        for(int i=0; i<=10; i++)
            {
                for(int j=i+1; j<=10; j++)
                    {
                        if(num[i]<num[j])
                            {
                                temp=num[j];
                                num[j]=num[i];
                                num[i]=temp;
                            }
                    }
                cout << num[i] << "\n";
            }

    return 0;
}

Solution 4

Well first I'm hoping your array assignment was just an error when posting but all your numbers are being assigned to the same memory location. There's nothing to sort.

After that, you can use the sort() function. The example linked shows an easy method for using it. Note that there is a third parameter that's not being used that will specify how to compare the elements. By default if you don't specify the parameter it uses 'less-than' so you get an ascending order sort. Change this to specify 'greater-than' comparator to get a descending order sort.

Solution 5

Generally, you can just swap the two variables in

http://www.cplusplus.com/reference/algorithm/sort/

Change

bool myfunction (int i,int j) { return (i<j); }

to

bool myfunction (int i,int j) { return (j<i); }

you can rename it to something else so that you have two comparison functions to use when the result needs to be ascending or descending.

If the function body has complicated expressions and involves i and j multiple times, then it is easier to swap the i and j in the parameter list instead of every i and j in the body:

bool myfunction (int j,int i) { return (i<j); }

The same goes for

http://www.cplusplus.com/reference/clibrary/cstdlib/qsort/

View more solutions

Comments

[sth]

@steven_desu: What non-standard headers?

[fredoverflow]

@steven: algorithm and functional are standard headers as defined by the international C++ standard (see 17.6.1.2 in the current draft). Hand-rolling your own sorting functions is a waste of time. It's error-prone and will most likely yield less efficient code. And what makes you think this is homework? Sometimes people just want to sort arrays and get on with their lives...

[SwiftMango]

Or a simple lambda expression instead of include a whole functional header, which is also high performance because it is expanded out inline.

[Galik]

This code is broken as it accesses the array out of bounds.

[diralik]

std::greater<>() works too (without specyifing template argument)

[Dale Woods]

Change all the i/j <= 10 to i/j < 10 to fix the out of bounds error.