pset2//caesar- relational operator result unused error

Question

my code-->

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

int main(int argc, string argv[])           
{
    if(argc!=2)
    {
        printf("Enter the key as well, in the command line !\n");
        return 1;
    }

    int key=atoi(argv[1]);

    printf("plaintext: ");

    string ptext=get_string();
    int len=strlen(ptext);              //needed to pass arguments in for-loop later
    int temp=key;                       //can't change value of key, so assign a temporary variable for performing calculations
    int z=0;
    while(temp>0)
    {
        temp=temp/10;
        z++;                                   
    }
    int array[z];
                                                        // z is now the length of the key
    for(int k=0;k<z;k++)
    {
        array[k]=temp%10;
        temp=temp/10;
    }                                                   // all the individual numbers in key are now stored in an array--123 is now 1,2,3

    for(int i=0,k=z;i<len,k>=0;i++,k--)             //whilst the loop runs over the length of the entire input string, the key is used again and again
                                                    //since the array elements are stored in reverse order of that in which they're to be used, i can't initialise from k=0 rather, from k=z and decreasing
    {
        if(k==0)
        {
            k=z;                                    //so that the loop runs again and again
        }

        if(!isalpha(ptext[i]))
        {
            k++;                                    //do not increase the value of key element if the element of input is NOT alphanumeric
        }

        if(isalpha(ptext[i]))                       //perform the operation for alphanumeric chars only
        {
            if(islower(ptext[i]))
            {
                ptext[i]=ptext[i]-97;
                ptext[i]=((ptext[i]+array[k])%26)+97;       //ciphering lower case letters
            }
            if(isupper(ptext[i]))
            {
                ptext[i]=ptext[i]-65;
                ptext[i]=((ptext[i]+array[k])%26)+65;           //ciphering uppercase letters
            }
        }

    }

    for(int i=0;i<len;i++)
    {
        printf("%c",ptext[i]);
    }

    printf("\n");
    return 0;
}

the error-->

caesar.c:38:22: error: relational comparison result unused [-Werror,-Wunused-comparison]
    for(int i=0,k=z;i<len,k>=0;i++,k--)             //the marks show that error is related to i<len condition//

i have absolutely no idea why that condition would not be used in the loop- i goes to one and k goes to one, the ascii value changes, i increases and k increases and the cycle repeats !

is my understanding of the error correct at all? i believe it means that the condition has zero significance in the output of the program, i might as well delete that condition..

thanks!

Answer 1

First to the error: i<len,k>=0 uses the comma operator, which evaluates its left side, discards the result, and evaluates to its right side. So i<len,k>=0 is the same as k>=0, the value of i<len is ignored.

You're doing Caesar's cipher, right? This one looks like a strange crossover of caesar and vigenère.

In Caesar's cipher, you shift all characters by the same number, so there's no need for any array, just shift a letter to range 0-25, add the key, apply %26, and shift it back into the letter range.

In Vigenère, you instead have a keyword, composed of various letters, and shift the letters of the plaintext by the alphabet position of the current keyword letter, advancing the keyword index on encrypting a letter.

Oh, and be aware of get_string returning NULL on [edit]a problem when reading from stdin, for example when it finds an EOF (End-of-file, also Ctrl+D)[/edit]. Your call to strlen on the next line would instantly segfault. Not part of the tests, but doesn't hurt to test for.