Luhn Algorithm sometimes work sometimes fail

Question

If it fits the algorithm, it should have an output of true and if it doesn't, the output should be false. Any idea where it goes wrong?

I tried

1586455534096 ; output : false(fail)

49927398716 ; output : true (ok)

984697300577 ; output : false (fail)

Code

#include<stdio.h>
#include<string.h>

int main()
{
  char str[100];
  int n,num[100],prod[100],remainder,sum,total;
  scanf("%s", str);
  for(n=0;n<100;n++)
    {
      if(str[n]==0) break;
      num[n] = str[n] - '0';
      if(n%2!=0)
        {
          prod[n]=num[n]*2;
          sum=0;
          while(prod[n]>=10)
            {
              remainder=prod[n]%10;
              sum=sum+remainder;
              prod[n]=prod[n]/10;
            }
          num[n]=sum;
          }
      total = total + num[n];
      }
 if(total%10==0)
    printf("true\n");
  else
    printf("false\n");
  return 0;
}

Have you tried to debug the code for a failing example? – MrSmith42 May 06 '16 at 08:52 — MrSmith42, May 06 '16 at 08:52

LPs · Answer 1 · 2016-05-06T07:35:29.030

Your code working.

#include<stdio.h>
#include<string.h>

int main()
{
  char str[100];
  int n,num[100],prod[100],remainder,sum,total=0;
  scanf("%s", str);
  for(n=0;n<100;n++)
    {
      if(str[n]==0) break;
      num[n] = str[n] - '0';
      if(n%2!=0)
        {
          prod[n]=num[n]*2;
          sum=0;
          while(prod[n]>0)
            {
              remainder=prod[n]%10;
              sum=sum+remainder;
              prod[n]=prod[n]/10;
            }
          num[n]=sum;
          }
      total = total + num[n];
      }
 if(total%10==0)
    printf("true\n");
  else
    printf("false\n");
  return 0;
}

Into your code there are 2 main problems:

no init value for total var at least
the inner while have to use >0 as condition.

My correction on your code

#include<stdio.h>
#include<stdint.h>
#include<string.h>

int main()
{
    char format[32];
    char str[100]={0};
    uint32_t n=0;
    uint32_t num = 0;
    uint32_t total=0;

    snprintf(format, sizeof(format), "%%%zus", (sizeof(str)/sizeof(str[0]))-1 );

    scanf(format, str);

    while (str[n] != '\0')
    {
        num = str[n] - '0';

        if(n%2!=0)
        {
            num*=2;
            while(num>0)
            {
                total += num%10;
                num/=10;
            }
        }
        else
        {
            total += num;
        }

        n++;
    }

    if(total%10==0)
        printf("true\n");
    else
        printf("false\n");

    return 0;
}

score 0 · Answer 2 · answered May 06 '16 at 06:53

0

At first glance total is uninitialized and having uninitialized values in the code leads to indeterminate values

answered May 06 '16 at 06:53

Gopi

19,784
4
24
36

Should I initialize total=0 ? – M.H.F May 06 '16 at 06:58

score 0 · Answer 3 · answered May 06 '16 at 06:53

0

You never initialize total before you use it, which means its value is indeterminate and giving you undefined behavior. Might be the same with other variables.

Initialize the variables on declaration, like e.g.

int n, num[100] = { 0 }, prod[100] = { 0 }, remainder = 0, sum = 0,total = 0;

answered May 06 '16 at 06:53

Some programmer dude

400,186
35
402
621

Sorry, but I don't quite understand. Do I put total=0 before any calculations? – M.H.F May 06 '16 at 06:55
Tried, but still failed. – M.H.F May 06 '16 at 07:01
1

@M.H.F Then you need to use a debugger to step through the code, line by line, while monitoring variables and values to see when and where it goes wrong. Start with some input that you *know* the values in each step, so you can easily verify. – Some programmer dude May 06 '16 at 07:15

Luhn Algorithm sometimes work sometimes fail

3 Answers3