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?

Answer 1

for(j=i; j>0; j--) {
    printf("%c", str[j-1]);
}

Would be another option.
For a beginner maybe easier to understand.
But the other answers would be better.

edit: i'd say the best would be for(j=i; j-- > 0;) by l3x.
decrementing j after checking if it is greater then 0.

Using a do{}while() loop would also work.

j = i;
do {
   j--;
   printf("%c", str[j]);
} while (j > 0);

Answer 2

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.

Answer 3

You could change j from size_t to long this makes sure all the data still fits and you can reach the -1 value.

Another option is to end the for loop with the following statement:

for (j = i - 1;;--j)
{
    // code
    if (j == 0) break;
}

as a side-note: your first while loop does the same as the strlen() in string.h.