Implementation problem

Question

#include <stdio.h>
#include <ctype.h>
#include <cs50.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main(int count, string input[])
{
    // array of small alphabets
    char smaAlph[26] = {
        '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'
    };

// capital aphabets
char capAlph[26] = {
    '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'
};

int key;
if (count == 2)
{
    int digit, n;

    // assigning the string length
    n = strlen(input[1]);

    for (int i = 0; i < n; i++)
    {
        if (input[1][i] < '0' || input[1][i] > '9')
        {
            printf("Usage: ./caesar key\n");
            return 1;
        }
    }

    // getting the key
    key = atoi(input[1]);

    // trying encryption
    //getting normal text
    string text = get_string("plaintext: ");
    int n2 = strlen(text);


    for (int i = 0; i < n2; i++)
    {
        // testing if the char alphabet
        if (isalpha(text[i]))
        {
            // testing if the char upper text
            if (isupper(text[i]))
            {
                // looping through the cap alphabet array
                for (int p = 0; p < 26; p++)
                {
                    // testing if the char matches the char in the array
                    // switcing from ASCII index to alphabet index
                    if (capAlph[p] == text[i])
                    {
                        // applying the formula & returning to ascii
                        text[i] = ((capAlph[p] + key) % 26) + 'A';
                    }
                }
            }
            // testing if the char lower text
            if (islower(text[i]))
            {
                for (int p = 0; p < 26; p++)
                {
                    // looping through the cap alphabet array
                    // doing same as the CAP alphabet
                    if (smaAlph[p] == text[i])
                    {
                        // switching to smaAlph index and ascii
                        text[i] = ((smaAlph[p] + key) % 26) + 'a';
                    }
                }
            }
        }
    }
    printf("ciphertext: %s\n", text);

    return 0;

}
else
{
    printf("Usage: ./caesar key\n");
    return 2;
}
}

my code is compiling correctly but it doesn't function correctly (doesn't print the expected output) That is required in the assessment. It should cypher the input text by shifting the input alphabets (and only alphabets) by the value specified in the key input.

Edit

#include <stdio.h>
#include <ctype.h>
#include <cs50.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main(int count, string input[])
{
    // array of small alphabets
    char smaAlph[26] = {
        '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'
    };

 



// capital aphabets
    char capAlph[26] = {
        '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'
    };

int key;
if (count == 2)
{
    int digit, n;

    // assigning the string length
    n = strlen(input[1]);

    for (int i = 0; i < n; i++)
    {
        if (input[1][i] < '0' || input[1][i] > '9')
        {
            printf("Usage: ./caesar key\n");
            return 1;
        }
    }

    // getting the key
    key = atoi(input[1]);

    // trying encryption
    //getting normal text
    string text = get_string("plaintext: ");
    int n2 = strlen(text);


    for (int i = 0; i < n2; i++)
    {
        // testing if the char alphabet
        if (isalpha(text[i]))
        {
            // testing if the char upper text
            if (isupper(text[i]))
            {
                // looping through the cap alphabet array
                for (int p = 0; p < 26; p++)
                {
                    // testing if the char matches the char in the array
                    // switcing from ASCII index to alphabet index
                    if (capAlph[p] == text[i])
                    {
                        // applying the formula & returning to ascii
                        text[i] = ((capAlph[p] + key) % 26);
                        
                        // returning to ascii
                        text[i] += 'A';
                    }
                }
            }
            // testing if the char lower text
            if (islower(text[i]))
            {
                for (int p = 0; p < 26; p++)
                {
                    // looping through the cap alphabet array
                    // doing same as the CAP alphabet
                    if (smaAlph[p] == text[i])
                    {
                        // switching to smaAlph index and ascii
                        text[i] = ((smaAlph[p] + key) % 26);
                        
                        // returning to ascii
                        text[i] += 'a';
                    }
                }
            }
        }
    }
    printf("ciphertext: %s\n", text);

    return 0;

}
else
{
    printf("Usage: ./caesar key\n");
    return 2;
}
}

Answer 1

Simply put, the logic of encoding the letters is wrong. The logic of the current code is to take the letter from the input string, find the same matching letter in an array (ie, replace q with q, which does nothing), add the key to that ASCII value and apply %26. Finally, applying %26 directly to an ASCII value doesn't produce a valid result.

The code needs to first convert an ASCII value to a number from 0 to 25 inclusive. Then add the key and apply %26 to that result. Finally, convert the number (0 to 25 inclusive) back to an ASCII value.

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