Sorting two different parts of the same array C

Question

I've been playing around a bit and I've done a program that plays the lottery (EuroMillions), and I was able to do it.

I got four arrays, two for the numbers and two for the stars, and then I'd sort both so I could more easily compare them.

So here's my question, can I sort only the first five numbers and then the last two, separately? Or do I really need to have four arrays like I did?

What I used to sort:

int cmpfunc (const void * a, const void * b)
{
   return ( *(int*)a - *(int*)b );
}

and

qsort(b, 7, sizeof(int), cmpfunc);

If the code is needed I can post it, but it's rather big.

Add 100 to the stars so they get sorted above the non-stars. — Ian Abbott
– Ian Abbott, Commented Nov 26, 2015 at 16:42

Manos Nikolaidis · Accepted Answer · 2015-11-26 16:48:22Z

3

If b is an int array with 7 elements you can do this

qsort(b, 5, sizeof(int), cmpfunc); // sort 5 elements starting from b 
qsort(b + 5, 2, sizeof(int), cmpfunc); // sort 2 elements starting from b+5

In each case the first 2 arguments are the pointer to the start of a memory block and the number of elements.

So if b is defined as

int b[] = {8, 2, 12, 4, 5, 7, 6};

you will get

{2, 4, 5, 8, 12, 6,7}

edited Nov 26, 2015 at 16:48

answered Nov 26, 2015 at 16:40

Manos Nikolaidis

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5 Comments

Miguel Garcia Over a year ago

This answer looks correct assuming qsort is the one defined in stdlib

EnderEgg Over a year ago

I tried using b[5] instead of b+5. Thank you for the help!

this Over a year ago

This is so trivial, compared to actually figuring out qsort itself, I'm surprised OP was actually asking for this answer.

user4842163 Over a year ago

@this It's probably fairly easy to skip understanding how pointers actually work and start looking for code samples of qsort that can be minimally-adapted.

this Over a year ago

@Ike Unfortunately. But samples found online must be correct because they are online and their author claims they are correct. /s

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