Dont get the output out of char palindrom[]

Question

i trying to make a palindrom out of the user input.

Im saving the user input as array in char eingabe[] with int i im counting the index of the used arrays.

Then i want to copy the reversed value of eingabe in palindrom and then printf both of them to get the palindrom.

My i and j are counting correctly, but i dont get the output for palindrom.

I hope you can help me.

#include <stdio.h>

int main(){
char eingabe[20];
char palindrom[20];
int i = 0;
int j = 0;  
char c;

do{
    c = getchar();
    if (c != EOF){
        eingabe[i] = c;
        i++;
    }
} while (c != EOF);

eingabe[i] = '\0';


do{ 
    palindrom[j] = eingabe[i];

    j++;
    --i;


} while (i != 0);

    palindrom[j] = '\0';

    printf("\n\n%s",eingabe);
    printf("%s\n\n",palindrom);

    return 0;
}

Answer 1

It is sometimes very helpful to add printf's to see what is going on. For example, if I add:

do { 
    printf("palindrom[%d] = eingabe[%d]\n", j, i);
    palindrom[j] = eingabe[i];

then the output is:

abcde
palindrom[0] = eingabe[5]
palindrom[1] = eingabe[4]
palindrom[2] = eingabe[3]
palindrom[3] = eingabe[2]
palindrom[4] = eingabe[1]
eingabe:   "abcde"
palindrom: ""

The problem is immediately obvious: the array index is off by 1. Since the first character of palindrom ([0]) gets set to the last character of eingabe ([5]) which is "\0", then C sees it as an empty string.

This is easily corrected by moving the --i to the top of the loop:

do { 
    --i;
    palindrom[j] = eingabe[i];
    j++;
} while (i != 0);