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')
    {
      outputString[j] = '\0';
    }

    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;
}

Answer 1

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

Answer 2

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 ;)