Writing a recursive function in C

Question

I am studying how to program and have recently been working on a problem that calculates the total of 2 entered numbers from min to max. For example, if someone entered the numbers 4, 7. The calculation would be 4+5+6+7=22.

I've attempted what i think would be the definition of recSum but obviously it is wrong as I get a segmentation fault. What is wrong with my definition?

/* Define the recursive function */
int recSum (int x, int max)
{
int incrementalSum = 0;
if (x == max) 
{
   return x; /* Exit if summated lower to upper numbers */
}
else
{
   return (x + recSum(x++, max)); /* My recursive call */
}
} /* End of function call */

*new code is shown above, sorry, i used the wrong code.

If you are going to write code like if it were the 70s, at least make it readable. — Iharob Al Asimi, Mar 04 '17 at 11:41
Your `goto` construct is equivalent to a `while(1) { … }` loop. Loops are not recursion. — Emil Laine, Mar 04 '17 at 11:46
Do you want to write a recursive / or iterative solution for exercise purposes or aren't you aware of the formula to sum up all integers from 1-n? — MikeMB, Mar 04 '17 at 11:50

score 5 · Answer 1 · edited May 23 '17 at 11:46

5

Your code has 3 important problems

The expression
```
incrementalSum = x + x++;
```
is undefined, read this for more information
Your function is not recursive, a recursive function calls it self until a condition happens where it should end.
Also, noting that I am not an irrational "don't ever use goto person", this is precisely why some people advice against using goto.

edited May 23 '17 at 11:46

Community

1
1

answered Mar 04 '17 at 11:42

Iharob Al Asimi

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score 2 · Accepted Answer · edited May 23 '17 at 12:25

The reason your code doesn't work is this line:

return x + recSum(x++, max);

x++ increments x but returns the previous value, so in the recursive calls it never increments and you never reach the base case. Like an infinite loop. You have to replace x++ with ++x in order to give any result, even though it won't be correct. ++x is modifying x so it will alter the final sum of x + recSum. You'd better use:

return x + recSum(x + 1, max);

See What is the difference between ++i and i++?

Vlad from Moscow · Answer 3 · 2017-03-04T12:06:12.443

It seems you mean the following

int recSum(int x, int max)
{
    return max < x ? 0 : x + recSum( x + 1, max );
}

Or it would be even better to declare the return type of the function like long long int.

long long int recSum(int x, int max)
{
    return max < x ? 0 : x + recSum( x + 1, max );
}

The function can be called like

printf( "%lld\n", recSum( 4, 7 ) );

As for your function then it exits in the first call

int recSum(int x, int max)
{
    int incrementalSum = 0;
    recCall: if (x < max)
    return incrementalSum;
    ^^^^^^^^^^^^^^^^^^^^^

because usually it is called when x is less than max. So the function does not make sense. Moreover the function is not recursive because it does not call itself.

Excelent observation, this completes my answer. I think SO should have a method to allow answers to be merged and share the upvotes and downvotes among users. — Iharob Al Asimi, Mar 04 '17 at 11:50

Writing a recursive function in C

3 Answers3