How to deal with overflow in C

Question

I'm pretty new to programming and I was just playing around coding today trying to use functions and I made a simple piece of code that worked a math problem I had in one of my classes. it basically takes the formula (bacteria amount) * 2^hours and calculates that.

My issue is that when I get a really large number it doesn't return correctly, I always get -2147483648 back after a certain size of number. I'm guessing this has something to do with overflow but i'm not 100% sure how that works. What i'm trying to figure out is how to get the actual numbers i'm looking for after I hit this overflow. So what can I do to deal with the overflow?

I initially was only setting everything to int, but after some reading thought maybe changing everything over to long might help me out, but it didn't, if this is a terrible thing for me to do go ahead and let me know! Also the test numbers i'm using are 1500 and 24, that always returns the number above.

Here's the code Thanks!

#include<stdio.h>
#include<math.h>
long bacteria(long b, long h);

int main(void)
{
        long f,g;

        scanf("%ld%ld",&f,&g);
        f = bacteria(f,g);
        printf("%ld\n",f);

return 0;
}

long bacteria(long b,long h)
{
        long d;
        printf("%ld %ld\n",b,h);
        d = b * (pow(2,h));

return d;
}

I've got some code [here](https://groups.google.com/d/topic/comp.lang.apl/Lu2tgqanK5Q/discussion) mostly derived from example code in the CFAQ. — luser droog, Oct 09 '14 at 10:13

score 2 · Accepted Answer · edited May 23 '17 at 11:59

2

Yes your suspect about overflow is right. C datatype have some range. You need to use some bignum library to handle cases where you need broader range. Also note that pow returns double and not long as you might expect.

If you don't care about precision, you can use double instead of long which serves much broader range.

Live example here

edited May 23 '17 at 11:59

Community

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1

answered Oct 09 '14 at 05:17

Mohit Jain

30,259
8
73
100

Vaibhav · Answer 2 · 2014-10-10T06:24:17.920

Good Question. I faced same issue some time back used the below example. Check it out if this helps you

http://discuss.codechef.com/questions/7349/computing-factorials-of-a-huge-number-in-cc-a-tutorial

Code below from the link above.

#include<stdio.h>
int main()
{
    int t;
    int a[200]; //array will have the capacity to store 200 digits.
    int n,i,j,temp,m,x;

    scanf("%d",&t);
    while(t--)
    {
       scanf("%d",&n);
       a[0]=1;  //initializes array with only 1 digit, the digit 1.
       m=1;    // initializes digit counter

       temp = 0; //Initializes carry variable to 0.
       for(i=1;i<=n;i++)
       {
            for(j=0;j<m;j++)
            {
               x = a[j]*i+temp; //x contains the digit by digit product
               a[j]=x%10; //Contains the digit to store in position j
               temp = x/10; //Contains the carry value that will be stored on later indexes
            }
             while(temp>0) //while loop that will store the carry value on array.
             { 
               a[m]=temp%10;
               temp = temp/10;
               m++; // increments digit counter
             }
      }
              for(i=m-1;i>=0;i--) //printing answer
              printf("%d",a[i]);
              printf("\n");
    }
    return 0;
}

score 1 · Answer 3 · answered Oct 09 '14 at 05:21

1

use longlong datatype instead of simple long for containing larger values.

answered Oct 09 '14 at 05:21

Wajahat

27
1
3

2

I think this isn't an unreasonable answer - it doesn't ultimately deal with the question of what to do with overflow when it happens, but it is at least likely to work for longer! – Synchro Oct 09 '14 at 07:12
I accept that the solution provided will not cater the overflow issue. But there are very high chances that using longlong data type will solve the issue of Yosvan10. – Wajahat Oct 10 '14 at 05:11

How to deal with overflow in C

3 Answers3