Is there any better way to sort three integers in C++?

Question

In the programming textbook I'm using (Programming: Principles and Practice using C++ by Bjarne Stroustrup), I'm trying to do one of the exercises found in the very early chapters (before the introduction of arrays or anything), but I can only solve it using an algorithm that looks quite weird and "backwards" to me. The exercise is to read from the console 3 integers and to sort them according to size, separated by commas. Here is what I wrote:

#include <iostream>

using namespace std;

int main()
{
    int a, b, c;
    cout << "Enter three integers: ";
    cin >> a >> b >> c;
    if (a <= b and a <= c) {
        cout << a << ", ";
        if (b < c)
            cout << b << ", " << c;
        else
            cout << c << ", " << b;
        return 0;
    }
    if (b <= a and b <= c) {
        cout << b << ", ";
        if (a < c)
            cout << a << ", " << c;
        else
            cout << c << ", " << a;
        return 0;
    }
    if (c <= a and c <= b) {
        cout << c << ", ";
        if (a < b)
            cout << a << ", " << b;
        else
            cout << b << ", " << a;
        return 0;
    }
    return 0;
}

I know it's very long, but I can't think of any other way to do it with the tools at my disposal (if statements). Can you please help me and show me if there is any other way to do it? Thanks!

Sort of related: https://stackoverflow.com/questions/2786899/fastest-sort-of-fixed-length-6-int-array — hyde, Jun 13 '20 at 09:19
For a more general answer look up https://en.wikipedia.org/wiki/Sorting_network. The lowest number of comparisons to sort 3 numbers is 3. — A.Hristov, Jun 13 '20 at 09:22

Aykhan Hagverdili · Accepted Answer · 2020-06-13T10:05:30.947

Depends on what you mean by "better". There is a shorter way to do that, just like most other things in C++:

#include <iostream>
#include <iterator>
#include <algorithm>
#include <cstdio>

int main() {
    std::istream_iterator<int> it{std::cin};
    int a[]{ *it++, *it++, *it++ };
    std::sort(std::begin(a), std::end(a));
    std::printf("%d, %d, %d\n", a[0], a[1], a[2]);
}

Whether or not shorter is better is another discussion.

score 2 · Answer 2 · answered Jun 13 '20 at 09:59

You can also do this without invoking std::sort, sorting by hand:

// Put the smallest number in a.
if (b < a)
    std::swap(a, b);
if (c < a)
    std::swap(a, c);
// Arrange the other two numbers.
if (c < b)
    std::swap(b, c);
std::cout << a << ", " << b << ", " << c << '\n';

score 1 · Answer 3 · answered Jun 13 '20 at 10:08

1

Hope this helps:

#include <iostream>

using namespace std;

int main()
{
    int a, b, c, x, mid, max;
    cout << "Enter three integers: ";
    cin >> a >> b >> c;
    if (a<b){
        x = a;
        max = b;

    }
    else {
        x = b;
        max = a;
    }

    if (c < x){
       mid = x;
       x = c;
    }
    if (c > max){
        mid = max;
        max = c;
    }
    else
      mid = c;

    cout << x << ", " << mid <<", "<<max;

  }

answered Jun 13 '20 at 10:08

nullElement

11
1

I was assuming we were limited to if statements like OP mentioned, you could probably do better finding mid but this is what I came up with and it seems to work. – nullElement Jun 13 '20 at 10:12

Is there any better way to sort three integers in C++?

3 Answers3