Trouble with vigenere cipher

Question

I feel i almost got this locked in but my keyword doesn't seem to to work. It isn't rotating through the keyword. please tell me what i did wrong my brain hurts lol.

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
int main(int argc,string argv[])
{

string sKeyWord = argv[1];                   
int iKeyLength = strlen(sKeyWord);           
int iKey[iKeyLength];                        
string sPlainText = "";
int counter = 0;
int iAccept = 0;
do
{
    if(argc != 2)
        {
            printf("Invalid argument! Please enter program name and keyword.\n");
            return 1;
        }
    else if(argv[1])
        {
        for(int i = 0; i < iKeyLength; i++)
            {
                if (!isalpha(argv[1][i]))
                {
                    printf("Invalid entry, please use letters only.\n");
                    return 1;
                }
                else
                {
                    iAccept = 1;
                }
            }
        }    
}while(iAccept == 0);


for(int i = 0; i < iKeyLength; i++)           
    {
        iKey[i] = toupper(sKeyWord[i]) - 65;     
    }

sPlainText = GetString();                    
int iPlainText = strlen(sPlainText);        


for(int j = 0; j < iPlainText; j++)
    {
        if(!isalpha(sPlainText[j]))
            {
                printf("%c",sPlainText[j]);
                counter++;
            }
        if(islower(sPlainText[j]))
            {
                printf("%c",((((sPlainText[j] - 97) + iKey[(j - counter)%iKeyLength])%26)+ 97));
            }
        if(isupper(sPlainText[j]))
            {
                printf("%c",((((sPlainText[j] - 65) + iKey[(j - counter)%iKeyLength])%26)+ 65));
            }

    }
printf("\n");

return 0;
}

When I run what I have, it works fine as far as I can tell except for it only uses the first letter of the keyword instead of rotating through it.

Answer 1

The code doesn't increment k, so it never changes from 0. A for loop has three clauses, initialization, test and increment. The code initializes k in the first or initialization clause. After the initialization, the initialization clause doesn't have any effect on anything. It doesn't cause k to change as i changes. K needs to be updated somewhere inside the loop or in the incrementation clause. Just remember that k will only be updated when a letter is actually encoded. If it's linked to i, it will get out of synch.

Some side comments: if((argc == 1) || (argc < 2) || (argc >2)) Why these three tests? If the first test is true, then the second is true. All three could be replaced by argc != 2, simplifying the code.

if (isalpha(keyWord[i]) == false) can be simplified to if (!isalpha(keyWord[i]) ).

Looks like you could benefit from understanding the NOT operator, !.

And what happens if the key is uppercase?

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