CODEWITHSUNDEEP
cprintf
user2636600
user2636600

Reputation: 1

How can a single printf statement changes value in an array?

The following program prints prime numbers between 1 and 10.

#include <stdio.h>
int* prime(int x,int y,int* range);

void main()
{
    int *x,s=10;
    int i=0; 
    x=prime(1,10,&s);

    for(i=0;i<s;i++)
    {
        printf("%d\n",x[i]);
    }


}
int* prime(int x,int y,int *range){

    int num[100],i,j,flag,inc=0;

    for(i=x;i<=y;i++)
    {
        flag=1;
        for(j=2;j<=(i/2);j++)
        {
            if(i%j == 0)
                flag=-1;
        } 
        if(flag==1)
        {

            num[inc]=i;
            inc++;
        }

    }
    *range=inc;
    //printf("$$%d$$",*range);
    return num;
}

the output is 1 2 3 5 0 in the above case, but if we remove the the comment in printf statement in prime function and give as normal printf statement the output is 1 2 3 5 7 How is this even possible?? What's the bug here??

The compiler i used is GCC compiler in linux platform.

Upvotes: 0

Views: 2328

Answers (3)

Amarnath Krishnan
Amarnath Krishnan

Reputation: 1263

You are passing the local variable address to the main. It is not a good practice. Scope of local variable is within that module. So you can directly pass your array to the prime and work on the result directly. Note: Your code works perfectly too without printf in the prime.. Undefined Behaviour.

#include <stdio.h>
void prime(int x,int y,int* range, int * arr);
void main()
{
    int n[10],s=10;
    int i=0; 
    prime(1,10,&s,n);
    for(i=0;i<s;i++)
    {
        printf("%d\n",n[i]);
    }
}
void prime(int x,int y,int *range,int *arr)
{
    int i,j,flag,inc=0;
    for(i=x;i<=y;i++)
    {
        flag=1;
        for(j=2;j<=(i/2);j++)
            if(i%j == 0) flag=-1;
        if(flag==1) *(arr+inc++)=i;
   }
   *range=inc;
}

Upvotes: 0

Devolus
Devolus

Reputation: 22094

Your are allocating an int array on the stack. So when you return num; it goes out of scope and the behaviour is undefined, because the pointer becomes invalid and you corrupt the stack.

You must allocate the array inside main and pass it to prime() or malloc() inside of prime() and return that, freeing it in main.

Upvotes: 1

John3136
John3136

Reputation: 29266

Your prime() method returns local variable num which is sitting on the stack, so it's not in scope (aka invalid) after theprime() call returns.

A couple of fixes:

  • you might be able to make num static in prime
  • you could dynamically allocate num (remember to free it)
  • you could pass num into the prime method.

Upvotes: 3

Related Questions