ASCII value output of the last character in a user input string (using fgets) is 10 while expected value is 0

Question

I am writing a program to simulate strcmp(). This is my code.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 100
int strcmp(const char *str1, const char *str2);

char s1[MAX], s2[MAX];

int main()
{
    printf("Compare two user entered strings character by character.\n");
    printf("Enter string one: ");
    fgets(s1, MAX, stdin);
    printf("Enter string two: ");
    fgets(s2, MAX, stdin);
    printf("The user entered string one is: %s", s1);
    printf("The user entered string two is: %s", s2);
    printf("The value returned by strcmp() is: %d", strcmp(s1, s2));
    return 0;
}

int strcmp(const char *str1, const char *str2){
    int result;
    while(*str1 != '\0' && *str1 - *str2 == 0){
            str1++;
            str2++;
    }
    if(*str1 - *str2 != '\0'){
            printf("%d\n", *str1);
            printf("%d\n", *str2);
            result = *str1 - *str2;
        }else if(*str1 == '\0' && *str2 == '\0'){
            result = 0;
        }

    return result;
}

It works fine for the most part and the strcmp() function returns the correct results, except for when one string terminates and the other has characters left. I use the while loop to compare the characters and increment the pointers to the next character. When a string is incremented to '\0' the integer value displayed on doing a printf is 10. Why isn't it 0? Because the value is 10, deducting other strings' character from it gives a result which is larger by 10.

Why does this happen?

How is the title of this post useful to anyone? Please read [ask]. — Marco Bonelli, Feb 05 '20 at 13:52
@ra5 The function fgets can append the new line character '\n' (decimal 10) to the entered string. — Vlad from Moscow, Feb 05 '20 at 13:53
if you look at man ascii, the 10 is actually a newline as Vlad points out. You need to remove the newline character before processing the strings — Owl, Feb 05 '20 at 13:55
[**Removing trailing newline character from fgets() input**](https://stackoverflow.com/a/28462221/4756299) — Andrew Henle, Feb 05 '20 at 13:57
Thanks for the answers guys. I edited the title to better summarize the query. — ra5, Feb 05 '20 at 16:22

score 2 · Accepted Answer · answered Feb 05 '20 at 13:57

2

The function fgets can append the new line character '\n' - decimal 10 (that corresponds to the key Enter) to the entered string if there is enough space in the destination character array.

You should remove it. For example

#include <string.h>

//...

fgets(s1, MAX, stdin);
s1[ strcspn( s1, "\n" ) ] = '\0'; 
printf("Enter string two: ");
fgets(s2, MAX, stdin);
s2[ strcspn( s2, "\n" ) ] = '\0';

answered Feb 05 '20 at 13:57

Vlad from Moscow

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That's a neat way to do it. Thank you. `scanf(" %[^\n]",s1)` does the job as well. Many ASCII charts don't have the newline character listed. Found one now with it listed. [link](https://1.bp.blogspot.com/-1DARuE2wyUc/XIPF_1CisdI/AAAAAAAAAh8/frDwPTIhG94f9ciKx514duPPD4n2PRTHQCLcBGAs/s1600/Ascii.png) – ra5 Feb 05 '20 at 16:11
@ra5 Here is one drawback. If you need to read several strings you need in any case to remove the new line character from the input buffer before a next read. – Vlad from Moscow Feb 05 '20 at 16:19

ASCII value output of the last character in a user input string (using fgets) is 10 while expected value is 0

1 Answers1