bubble sort, ordering by a primary variable, then when the same, a secondary variable

**BpB** · 05-04-2017

Bit of a lazy question, can't even think about it right now, been a long day. How to make the well known bubble sort algorithm work/sort for two different types of variables? So say you've got a struct with a variable a. That's the primary variable to sort by. But then where all the a values, whatever they are, it doesn't matter, are the same, then these structs should be sorted by a secondary variable, b.

Here's a copy and paste of the bubble sort algorithm:

Code:

 for (c = 0 ; c < ( n - 1 ); c++)  {
    for (d = 0 ; d < n - c - 1; d++)
    {
      if (array[d] > array[d+1])
      {
        swap       = array[d];
        array[d]   = array[d+1];
        array[d+1] = swap;
      }
    }
  }

**GReaper** · 05-04-2017

Let's take your line that does the comparison:

Code:

array[d] > array[d+1]

and convert it to a function since you said these would actually be structs:

Code:

compare(&array[d], &array[d+1]) > 0

Inside that function, you should implement the correct way to compare those two elements so that member "a" takes precedence over member "b", like this:

Code:

int compare(const myStruct* e1, const myStruct* e2)
{
    // if e1->a > e2->a, return 1
    // if e1->a < e2->a, return -1

    // if e1->b > e2->b, return 1
    // if e1->b < e2->b, return -1

    // Return 0 to signal "equality"    
}

**BpB** · 05-04-2017

Yup, that makes a lot of sense. Last night, when I really wasn't thinking straight, and hadn't given it much thought either, I was thinking further for loops might be required to cover the extra comparisons, but that didn't seem right, and it wasn't. I'll give your suggestion a go, sure it'll work. Thanks.

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