iterating an array backwards in For loop condition to stop at 0 when using unsigned integers causing infinite loop

Question

I have a loop that has to go from j to 0 (inclusively). My

j

variable is of type

size_t

which is usually unsigned.

My code:

#include<stdio.h>

#include<conio.h>



#define SIZE 100



int main(){

    char str[SIZE];

    size_t i=0;

    size_t j;

    puts("Enter any string");

    scanf("%s",str);

    while(str[i]!='\0'){

        i++;

    }





    for(j=i-1;j>=0;j--){



        printf("%c",str[j]);

    }





    getch();

    return 0;

}

I get an infinite loop. If I remove the equality of zero it outputs the reverse of string without the first letter. so what's the problem here?

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Answer 1

size_t is an unsigned integer and it's never going to be less than 0. So the condition in for loop is always true:

for(j=i;j>=0;j--)

You can modify the condition to (a bit ugly though):

for(j=i; j-- > 0;){
   ...
}

Note that in your condition, you are printing the \0 null byte too, which is a non-printable character. (since the j starts with a value equal to the string length). The above condition takes care of that too.

Also:

you can use strlen()instead of looping over it yourself.
Check the return value of scanf() if input reading wad successful.

iterating an array backwards in For loop condition to stop at 0 when using unsigned integers causing infinite loop

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