Geting some garbage value on the second index

Question

I'm trying to calculate HCF in C with pointers.

int-type-Pointer ptr points to an array of integers. The inputs that i have given are 30,60,18,a. And here "a" is to terminate the list of integers and breaks off the "while".

I tried the debug mode, and found the values:

*ptr = 30
*(ptr+1)= -1163005939  //the garbage that i'm talking of. 
*(ptr+2)= 60
*(ptr+3)= 30

while what i should get are 30, 60,18.

#include<stdio.h>

void main(){

int* ptr=(int*) malloc( sizeof(int)* 50);
int input=0;
int smallest;

printf ("Enter the numbers (press any alphabet when you're done )\n");

    while (1)
    {   input++;     

        if (input==1 && scanf("%d", ptr))  // the first number is stored in smallest
        {smallest = *ptr; continue;}

        if (!scanf("%d",ptr+input ))   // if the input is a character , scanf says 0, 
        {input--;                      //! makes it 1, and we jump out of  the loop
            break;}                     

        if (smallest > *(ptr+input))  // swapping
        { smallest += *(ptr+input);
            *(ptr+input) = smallest- *(ptr+input);
            smallest= smallest- *(ptr+input);
        }
    }

// code for determining the HCF
char c;

if (smallest <=0)
{
printf("", scanf("%c",&c ), printf("Answer is 0")); // it will print that the answer 
exit(0);                                            //is 0 then waits for you to 
}                                                   //press any key and then it exits

//if smallest greater than 0

int i=2;
int HCF=1;
int j;

for (; i<smallest/2; i++)
    { for (j=0; j<=input;j++)

         // this is where the problem is suspected 
         //as i ve seen in the debug mode 
         //it gives results divides the garbage value to i

       {if (! (*(ptr+j)%i == 0))   // if the number stored in the location is not 
          break;                   //divisible by i, then leave that number i 
        }                          //and check for the next i 

        if (j>input)
        HCF *= i;
    }

    printf( "", scanf("%c", c), printf("The HCF is %d ", HCF)); 
    free(ptr);

}

So what is the problem? And i didnt want to allocate the 50 ints memory. I wanted to just use the pointer wildly without any allocation. I know its bad practice but i just want to apply it . Is that any harm to other programs? How?

Answer 1

It's garbage because you never write anything to it. Look at this code

while (1)
{   input++;     

    if (input==1 && scanf("%d", ptr))  // the first number is stored in smallest
    {smallest = *ptr; continue;}

    if (!scanf("%d",ptr+input ))   // if the input is a character , scanf says 0, 
    {input--;                      //! makes it 1, and we jump out of  the loop
        break;} 

    //code that doesn't assign values to ptr
}

by the time you get to scanf("%d",ptr+input ), input will be 2 if scanf("%d", ptr) returned a truthy value. That's because of this if statement:

    if (input==1 && scanf("%d", ptr))  // the first number is stored in smallest
    {smallest = *ptr; continue;}

notice how you continue here when input is equal to 1? that means that the while loop will skip everything else and begin again from the beginning, and the first thing that it's going to do is increment input from 1 to 2.