When to use return in recursion

#1

void bubblesort(int *arr, int n){
if(n==1){
return;
}
for(int i=0; i<n-1;i++){
if(arr[i]>arr[i+1]){
swap(arr[i], arr[i+1]);
}
bubblesort(arr,n-1); // if I put return bubblesort(), the function doesnt work
}
}

I do not understand when to use return for the recursive statement, because when we write another function for an appraoch without any loop we have to use the return statement:
void bubblesort2(int *arr, int n, int j){

if(n==1){
    return;
}
if(j==n-1){
    return bubblesort2(arr, n-1, 0);
}
if(arr[j]>arr[j+1]){
    swap(arr[j], arr[j+1]);
}
return bubblesort2(arr, n, j+1); // here the return works and without it i get an unsorted array.

}

#2

hello @amanb25

a) there is mistake , this function call should be outside the for loop.

b) use of return .

  • if we encounter base case then we use return to return back to calling function.

  • we use return to return answer of subproblem to the calling function.

    • example. let say we want to compute sum of first n natural number.
      then we can write it as
      sum(n)=n+sum(n-1) ; // sum of first n number = n + sum of first n-1 natural number
      so what we do . we call sum(n-1) to get sum of first n-1 natural number and then add n .
      its recursive function should look like
      int sum(int n){
      if(n==1){
      return 1; // base case return is mandatory
      }
      int temp=sum(n-1); // this will return sum of first n-1 natural number which we store in temp
      return n + temp; // we add n to temp and return to calling function.
      }
      }

  • it also depends on algorithm u r implementing

#3

thank you, I get it some what now maybe with some more practice I’ll be sure about it.

#4

yeah ,recursion is tricky and demands practice.

#5

one last thing, so in the code above its optional to use the return function in the recursive step because we basically are not returning anything.

#6

yeah . . . . . . . . . . .

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#7

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