Need help in Caesar Cipher in C

Question

I am new to programming and I am trying to write a program in C for a Caesar Cipher.

Input consists of an integer ilength equal to the length of the string, followed by the string str and an integer encrypt.

My input is:

11
middle-Outz
2

Output:

okffng-Qwv@

Required output is:

okffng-Qwvb

Below is the code I have written. Could someone help me why I am getting the last character wrong in the output!

I'm totally clueless.

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
int main()
{
 int ilength = 0, encrypt = 0, i = 0, j = 0;
 char alph_base[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
 scanf("%d", &ilength);
 char str[ilength + 1];
 scanf("%s", str);
 scanf("%d", &encrypt);
 //printf("%c\n", str[5]);
 char outputString[ilength + 1];
 char temp[ilength + 1];
 for (j = 0; j <= ilength; j++)
 {
 temp[j] = str[j];
 i = 0;
 if (str[j] == '0円')
 {
 outputString[j] = '0円';
 }
 while ((i >= 0) && (i < 26))
 {
 if (temp[j] == alph_base[i])
 {
 if (i == 25 && encrypt == 0)
 {
 outputString[j] = alph_base[25];
 }
 if ((i + encrypt) == 26)
 {
 outputString[j] = alph_base[(i + encrypt) % 26];
 }
 else
 outputString[j] = alph_base[(i + encrypt) % 26];
 }
 if ((temp[j] < 65 || temp[j] > 90) && temp[j] < 97)
 outputString[j] = temp[j];
 if ((temp[j] < 97 || temp[j] > 122) && temp[j] > 90)
 outputString[j] = temp[j];
 i++;
 }
 while ((i > 25) && (i < 52))
 {
 if (temp[j] == alph_base[i])
 {
 if (i == 51 && encrypt == 0)
 {
 outputString[j] = alph_base[51];
 }
 if ((i + encrypt) == 51)
 {
 outputString[j] = alph_base[51];
 }
 if ((i + encrypt) > 51)
 {
 outputString[j] = alph_base[((i + encrypt) % 51) + 25];
 }
 else
 outputString[j] = alph_base[(i + encrypt) % 51];
 }
 if ((temp[j] < 65 || temp[j] > 90) && temp[j] < 97)
 outputString[j] = temp[j];
 if ((temp[j] < 97 || temp[j] > 122) && temp[j] > 90)
 outputString[j] = temp[j];
 i++;
 }
 }
 printf("%s\n", outputString);
 return 0;
}

• r3mainer: After the conditional block if (str[j] == '0円') { ... }, you probably need to wrap the remainder of the loop body with else { ... }.
• Aravind_krishnan: @Giorgi : I am very new to coding. Will do my best to improve. Thanks
• Aravind_krishnan: @squeamishossifrage : Ok will check with that. Thank you
• John Demetriou: The suggested answer is pretty good. It covers all cases and helps with reducing your code. You should use that
• Aravind_krishnan: @JohnDemetriou : Sure. Thanks

Answer 1

Your code is too complex for what you want to do.

Your issue seems related to looping back from 'z' to 'a'.

A simple function like this can do the work for a character:

#include <ctype.h>
char caesar_encrypt(char input, int key)
{
 char output = input;
 char base, offset;
 // If not a letter, return the char unmodified
 if (! isalpha(input))
 {
 return output;
 }
 base = isupper(input) ? 'A' : 'a'; // Check if upper/lower case
 offset = input - base; // Take offset from 'a'
 offset += key; // Add key to offset
 offset %= 26; // Wrap offset to the 26 letters
 output = base + offset;
 return output;
}

Many ideas here:

1. Use functions from <ctype.h> (isalpha, isupper), which avoids many comparaisons in your code.

2. Consider your characters as an 'offset' from the letter A (uppercase or lowercase A). Thus you're working with numbers in range [0;25], and you can wrap using a simple modulus

3. Charaters are 'integers', so you can add or subtract them. To get the third letter of the uppercase alphabet, you can do char c = 'A' + 2;, which is simpler than your huge array.

Disclaimer: Code written here, not tested, may contain typos ;)

• Aravind_krishnan: Thanks a ton. I was not aware of those useful functions in <ctype.h>
• Aravind_krishnan: worked like a charm! thanks. The key learning is that I can use the <ctype.h> functions and that characters can be added/ subtracted.

Answer 2

Perfect answer by NiBZ. To add to it, if you still want to write your own checks rather than using library functions you can do something like:

int my_isalpha(char c){
 return ((c >= 'a' && c <= 'z')|| (c >= 'A' && c <= 'Z'));
}
int my_isupper(char c){
 return (c >= 'A' && c <= 'Z');
}

Warning: Above implementation is fine for ASCII but not so good for ISO 8859-1 or its relatives. Ref: https://stackoverflow.com/a/2169293/5183246

• Aravind_krishnan: Thanks @d_geeks. I shall try this one too