caesar cipher algorithm c++

Question

i'm trying to implement Ceaser cipher in c++ language

#include <iostream>
#include <string>
#include <locale>

using namespace std;

int main()
{
    string word;


    getline(cin,word);

    for(int i=0; i<word.length();i++)
    {
        if(isalnum(word[i]))
        {
//shift by 3
            word[i]+= 3;
        }
    }
    cout << word ;
    return 0;
}

what i want is to limit the output also for only letter and number . for example if i want to shift z by 3 the output would be 'c' and not '}' as in my code .

Answer 1

Compilers are much better at handling the tedious details than humans, so in this case, I would write the code to show clearly what you intend, and then let the compiler figure out the numbers.

For example, if you want to shift a letter, don't you really just want to add 3 to the index of the letter in the range A to Z, and then mod by 26 -- the number of letters from A to Z? This is really what you want -- rotate around the circle of LETTERS from A to Z, of which there are 26, and not worry about ASCII values.

In that case, you can let the compiler figure it out for you:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main()
{
    int shift = 3;

    char* input = "Is it 90 with a ZERO?";
    printf("%s\n", input);

    int length = strlen(input);

    char* output = malloc(length + 1);
    output[length] = '\0';

    for (int i = 0; i < length; i++)
    {
        char c = input[i];
        if (c >= 'A' && c <= 'Z')
        {
            c = (((c - 'A') + shift) % 26) + 'A';
        }
        else if (c >= 'a' && c <= 'z')
        {
            c = (((c - 'a') + shift) % 26) + 'a';
        }
        else if (c >= '0' && c <= '9')
        {
            c = (((c - '0') + shift) % 10) + '0';
        }
        output[i] = c;
    }
    printf("%s\n", output);
}

Why would you want to take on that responsibility, if you are not worried about speed or memory footprint?

Answer 2

This should work.This assumes that there will be only lower case letters.

        word[i]+= 3;
        //at this point word[i] might have cross the ascii limit for lower case letters

ie may be word[i]>122 .Ascii range for lower case letters is 97-122 So we use mod to wrap it around. But now may be word[i]<97 which is again out of range so we add 97 to it.

        word[i]%=123;
        if(word[i]<97)word[i]+=97;

Example z

     word[i]+=3 makes word[i] as 125  //out of range
     word[i]%=123      //word[i]=3
     word[i]+=97          //word[i]=99=c

Answer 3

You have to make sure it does not go out of the valid range for ASCII letters. A way of doing this is to convert the input to lowercase, then make sure that when the shift is added, it does not exceed 122 ( z 's value in ASCII).

if (word[i] + SHIFT > 'z') {
    SHIFT -= 123 - word[i];
    word[i] = 'a';    // go back to the beginning
    word[i] += SHIFT; // add on the remaining amount
}