C语言：输出的最后一个字符后面有一个结尾字符

Question

I am making a Caesar's Cipher for my lab sheet, and have made it able to encrypt 3 subtitution(Caesar's Cipher), which is the point of the exercise. But there has been one thing bugging me. First, there is a trailing character if i put it other than 3. For example, by typing "malware", and 2 for key. This is my code :

#include<stdio.h>
#include<stdlib.h>

int main()
{
   char text[100];
   int key,i;

   printf("Please enter a word/sentence (lowercaps) for encrypting :\n ");
   fgets(text,100,stdin);
   printf("Please enter the key that you desire : eg:14\n");
   scanf("%d", &key);
   for(i=0;i<strlen(text);i++)
   {
      if (key>=26)
      {
         key=key%26;
      }
      if (text[i]==' ')
      {
         continue;
      }
      if(text[i]+key>'z')
      {
         text[i]-=97;
         text[i]+=26;
         text[i]+=key;
         text[i]%=26;
         text[i]+=97;
      }
      else
      {
         text[i]=text[i]+key;
      }
   }

   printf("this is your encrypted text : %s", text );
}

I hope I followed the correct indentation methods for coding. Got a lot of dislikes because of that

Answer 1

Code is 1) not properly detecting when a char is a lower case letter 2) encrypting non-letters including '\\n' from fgets() which is causing OP's "trailing character after the last character of my output".

Instead:

if (text[i] >= 'a' && text[i]<= 'z') {
   text[i] = (text[i] - 'a' + key)%26 + `a`;
}
else {
  ; // nothing 
}

Alternatively

if (islower((unsigned char) text[i]) {
   text[i] = (text[i] - 'a' + key)%26 + `a`;
}

Note: the above depends on char are encoded as ASCII .

A solution that does not depend on ASCII.

static const char lowercase[] = "abcdefghijklmnopqrstuvwxyz";
char *p = strchr(lowercase, text[i]);
if (p) {
  int offset = (p - lowercase + key)%26;
  text[i] = lowercase[offset];
}

Answer 2

As Blake_Lead said, this '\\0' character was changed in your cypher

Indeed I was wrong about the length of the buffer as fgets() puts a '\\0'
From the manual page:

A terminating null byte ('\\0') is stored after the last character in the buffer.

So, you just need to change your test

if (text[i]==' ')

by something like:

 if (text[i] < 'A' || text[i] > 'z' || (text[i] > 'Z' && text[i] < 'a') )

Answer 3

I will simplify and correct this code to

#include <stdio.h>

int main() {
    char text[100];
    int key, i;
    printf("Enter a word / sentence (lowercaps) for encrypting : ");
    fgets(text, 100, stdin);
    printf("Enter the key that you desire (eg. 14) : ");
    scanf("%d", &key);
    key %= 26;    // Pull this out of the loop and remove the unnecessary if
    for (i = 0; text[i]; ++i) {    // Correct the loop condition
        if (text[i] == ' ') continue;
        if (text[i] + key > 'z')
            text[i] = (text[i] - 97 + 26) % 26 + 97;    // Simplify
        else
            text[i] += key;
    }
    printf("Encrypted text : %s\n", text);
    return 0;
}

Input

Enter a word / sentence (lowercaps) for encrypting : malware
Enter the key that you desire (eg. 14) : 2

Output

Encrypted text : ocnyctg