Not the right result with Arithmetic Progression in C

Question

Im trying to sum the multiples of a number (x0) with a progression number (r) and a number of times (n). If I use the number x0 = 6, r = 3, n = 3, the result should be 6+9+12=27, but the program gives me always 18.

I try different times changing the formula but if I do on the paper the result is right, so Im afraid the problem can be the syntax...

So theres the program in C:

#include <stdio.h>

int sum_progression(int x0, int r, int n)
{
  return (n/2) * ((2 * x0) + ((n - 1) * (r)));
}

void test_sum_progression(void)
{
  int x0;
  int r;
  int n;
  scanf("%d", &x0);
  scanf("%d", &r);
  scanf("%d", &n);
  int z = sum_progression(x0,r,n);
  printf("%d\n", z);
}

int main(void)
{
  test_sum_progression();
  return 0;
}

Thanks for helping!

I don't understand :/ Im new on C and some things are not to good to understand. Why I should print 3/2? — Bruno Rodrigues, Sep 28 '14 at 20:48
@BrunoRodrigues: To get some insight into the root of your problem. — Oliver Charlesworth, Sep 28 '14 at 20:49
oh god i understand now :/ so to resolve I can use double right? — Bruno Rodrigues, Sep 28 '14 at 20:50
`return (2*x0 + ((n - 1)*r)) * n/2;` (do the division by 2 last.) — chux - Reinstate Monica, Sep 28 '14 at 22:37

atlanteh · Accepted Answer · 2014-09-28T20:52:10.873

1

When using ints with division the value is calculated and then truncated to int.
if you divide int by int you should do something like:
return (n/(double)2) * ((2 * x0) + ((n - 1) * (r)));

edited Sep 28 '14 at 20:52

answered Sep 28 '14 at 20:47

atlanteh

5,615
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2

Bad idea to go to floating-point. Though with limited range, and only a single division by 2, it might work. – Deduplicator Sep 28 '14 at 20:48
You are right. changed to double. – atlanteh Sep 28 '14 at 20:52
@atlanteh: `double` is also floating-point... – Oliver Charlesworth Sep 28 '14 at 20:55
thanks people... sorry about my no vision :/ sometimes when we focus in something we can't see the most simple thing :/ – Bruno Rodrigues Sep 28 '14 at 20:56
1

Absolutely don't use floating point here – David Heffernan Sep 28 '14 at 21:21
@atlanteh Using floating point numbers, like `float` or `double`, is really the wrong approach. Think of a simple integer only solution. – chux - Reinstate Monica Sep 28 '14 at 22:38
How about return n * ((2 * x0) + ((n - 1) * (r)))/2; ? – atlanteh Sep 29 '14 at 03:24

Not the right result with Arithmetic Progression in C

1 Answers1