Doubt in Compare Function

#1

In bubble sort we use condition a [ j ] > a [ j + 1 ] for sorting in increasing order.
I am confused when we use compare function and return a > b how does that make return array in decreasing order

#2

I mean when we pass compare function in sort(STL) function

#3

please see this code for how to sort function in decreasing order using sort function

#4

Yes but my doubt is that if I use the same comparator function in bubble sort to compare in second loop of the bubble sort. I will get the array is increasing order. And the code you shared outputs array in decreasing order. So why does this happen in STL sort function?

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#5

In bubble sort what are you planning to use for the condition,
let say a[j] > a[j+1] is true
then, (what you want? is the question)
#Case1 , if above condition is true then, keep the numbers as it is otherwse swap. This will result in decreasing order sort.

#Case2 , if above condition is true then, swap the numbers otherwse keep as it is. This will result in increasing order sort.

So, inbuilt sort uses Case1 type comparison always.

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#6

Yes that’s true. But what about STL sort function when compare function return a > b the array returned is in decreasing order. Should i just remember when,
CASE 1:
bool compare(int a, int b) {
return a > b;
}
sort(a, a + n, compare)
returns array in decreasing order.
CASE 2:
bool compare(int a, int b) {
return a < b;
}
sort(a, a + n, compare)
returns array in increasing order.

Or do you have anything to add about details of STL sort function working

#7

when you simply write
sort(a,a+n)
this will return increasing order by default.
and when you give some comparator, it will go according to that

the idea is intuitive:
for example given a[] = 1 5 6 2 3
and I say you to sort with comparator a>b
then what u think? you will sort in such way that a>b should hold for number a appearing before b

that is 6 > 2 and 6 is before 2, this is in good position but,
2 !> 3 so we have to rearrange them

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#8

I just needed this clarification and didnt want to just remember it. This idea clears my doubt. Thanks!

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#9

If you are happy with this clarification!
Please mark this as resolved doubt and rate me!

#10

How do I rate you Rahul?

#11

you might have rated while marking this doubt as resolved !
Nothing serious! if you didn’t find that, then its OK,

Regards from rkrishna :blush::blush:

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#12

Ohh ha! 5 start dont worry :grin: