Thread: Funny array behaviour. Please Help!

Dear all,

I have been having trouble with this code . This program is supposed to brute-force verbal arithmetic, but when it is testing large numbers, the values in array x1 screw up badly. They become negative and I have given up. Could anyone please tell me what's wrong? This same error came up in another program I tried to write.

Code:

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

int pows(int a,int b)
{
    int p=1;
    for(;b>0;b--)
    {
        p*=a;
    }
    return p;
}

int check(int data[],int dataCheck[],int k)
{
    int i;
    int flag=1;
    for(i=0;i<k;i++)
    {
        if(data[i]!=dataCheck[i])
        {
            flag=0;
            break;
        }
    }
    return flag;
}
int sortNum(int x1[],int x)
{
    int i,j;
    int y=x;
    for(i=0;x!=0;i++)
    {
        x/=10;
    }
    x1[i]=10;
    x1[i+1]=10;
    j=i;
    for(i--;i>=0;i--)
    {
        x1[i]=y%10;

        y/=10;
    }



    return j;
}
int main()
{
  char a[30],b[30],c[30];
  char x[30];
  int x1[30];
  int aNum,bNum,cNum;
  int data[60]={0},dataCheck[59]={0};
  int aLen,bLen,cLen;
  int cLent,cNumt;
  int xLen;
  int i,j,m,k=0,k1=0;


  fgets(a,10,stdin);
  fgets(b,10,stdin);
  fgets(c,11,stdin);


  aLen=strlen(a);
  bLen=strlen(b);
  cLen=strlen(c);

  a[aLen-1]='\0';
  b[bLen-1]='\0';

  strcpy(x,a);
  strcat(x,b);
  strcat(x,c);

  for(i=0;x[i]!='\n';i++)
  {
      for(j=i+1;x[j]!='\n';j++)
      {
          if(x[i]==x[j])
          {
              data[k]=i;
              data[k+1]=j;
              k+=2;
              break;
          }
      }

  }

  for(m=0;m<pows(10,aLen+bLen-2);m++)
  {
      aNum=m%(pows(10,aLen-1));
      bNum=(m-aNum)/pows(10,aLen-1);
      cNum=aNum+bNum;
      cNumt=cNum;
      for(cLent=0;cNumt!=0;cLent++)
      {
          cNumt/=10;
      }
      xLen=sortNum(x1,(aNum*pows(10,bLen+cLent-1)+bNum*pows(10,cLent)+cNum));

      if(xLen!=aLen+bLen+cLen-3)
      {
          continue;
      }
      for(i=0,k1=0;i<xLen;i++)
      {
          for(j=i+1;j<xLen;j++)
          {
              if(x1[j]==x1[i])
              {
                  dataCheck[k1]=i;
                  dataCheck[k1+1]=j;
                  k1+=2;
                  break;
              }
          }

      }

      if(k!=k1)
      {
          continue;
      }
       

      if(check(data,dataCheck,k)==1)
      {
          
          printf("%d+%d=%d\n",aNum,bNum,cNum);
      }
     

  }
  return 0;
}

Originally Posted by quzah

Sounds like you are simply overflowing your variable. See variables have a specific range they can hold. If you try to go beyond those ranges, you run into situations like what you are encountering. Use smaller numbers, or use a bigger data type.


Quzah.

what do you mean by overflowing? what are the ranges? To use a bigger data type do i use long unsigned int or do I increase the array size? Thanks in advance.

You can test the ranges using the MAX values....
For example...

Code:

printf("%d", INT_MAX);

The reason they don't say "an int can hold values between -6000 and + 6000 is that it won't be the same on on compilers or on all machines... On a 16 bit system INT_MAX is 32767 ... on a 32 bit system it's 2147483647 ... and as soon as compilers catch up on 64 bit systems it wil be... 9223372036854775807 ( the current value for long long int)...

Frankly I think this is a terrible mistake made back in the beginnings of the language... they should have standardized by bit sizes int16 int32 int 64 etc. Many libraries have these definitions but they are simply aliases for the native types...

So what happens when you add 1 to each of those values? Yep, you end up with -1.

Standard grantee minimum value. Question 1.1
As CT suggested, you can print MAX value. Take a look at limits.h
C99 introduced stdint.h - Wikipedia, the free encyclopedia

Thanks a lot! I'll try to correct it...

I fixed that issue, but some of the array values that I set to 0 initially and subsequently did not modify, start to have values? What's going on???

That probably means you've walked off the end of one array and wandered into another.

But that happened within the range of the array. How can it be modified by some phantom?

Originally Posted by polarbear

But that happened within the range of the array. How can it be modified by some phantom?

Ok think about 2 arrays declared in sequence...

Code:

char a[16], b[16];

These will be in memory like this...

Code:

AAAAAAAAAAAAAAAABBBBBBBBBBBBBBBB

C does not do range checking... so you can easily walk off the end of one array into the beginning of the next...

Code:

AAAAAAAAAAAAAAAABBBBBBBBBBBBBBBB
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So now you are writing to a[i] but actually changing the contents of array b.