Vigenere 的 Cipher CS50 if 循环

Question

As part of pset2 CS50, for vigerne's cipher. My question is how should I arrange my loops of 'if's? I don't think i did them exactly right.

And pls point out any errors/bad things in my code as well. (my indentation seems bad)

So far, the encryption I got matches the one they provided on cs50. But im looking to improve this. Thanks in advance.

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

int main(int argc ,string argv[])
{ 
    // Make sure 1 argument is given
    if(argc !=2)
    {
    printf("Please enter only 1 alphabetical word as key\n");
    return 1; //returns an error if no 2nd arg is provided.
    }

    // Converting 2nd argument into an integer

    string k=argv[1];

    for(int i=0,n=strlen(k);i<n;i++)
    {   
        if(!isalpha(k[i]))
        {
        char c= k[i];
        printf("%c is not an alphabet. Please enter only alphabets.\n",c);
        return 2; //returns an error if key has non-alphabets
        }
    }

    // Prompt for plaintext to be encrypted    
    printf("Please enter the plain text you wish to encrypt\n");
    string p=GetString();
    printf("This is your text: %s\n",p);



    // Converting upper/lower case alphabets to 0-25 indexed
   for(int i=0,j=0,n=strlen(p),m=strlen(k);i < n; i++,j++)
   {
        if(p[i]==' ')
        {
        j--;
        }
                    if(isupper(k[j]))
                    {  
                    k[j] = (int)(k[j]-'A'); 
                    }

                    if(islower(k[j]))
                    {  
                    k[j] = (int)(k[j]-'a');   
                    }


                  //  printf("%d\n",k[i]); // value of k[i] is still stored as int here for ea char
                         if(isalpha(p[i]))
                         {  
                            if(isupper(p[i]))
                            {
                            int newpindex =(p[i]-'A');
                            int x = (newpindex + k[(j+m)%(m)])%26;
                            int cipher = x + 'A';
                            printf("%c",cipher);
                            }

                            else if(islower(p[i]))
                            {
                            int newpindex =(p[i]-'a');
                            int x = (newpindex + k[(j+m)%(m)])%26;
                            int cipher = x + 'a';
                            printf("%c",cipher);
                            }
                         } 

                         else 
                         printf("%c",p[i]);
  }

              printf(" \n");           
} 

Answer 1

One possible error in the code is the identification of characters that are neither letter nor spaces.

Looking at your code, j increments on every iteration of the loop, requiring it to decrement when a space is encountered. However, what if the non-alphabetic character is an exclamation point or other symbol?

Consider if there would be a better way to handle j within the loop to make sure you don't lose your place in the key in such situations. This can also eliminate your need to check to check for spaces (or symbols in general), thereby making the code a little more concise.

Edit: Just saw something else, isupper and islower encompass isalpha (the manual page for isalpha details that it is the equivalent of isupper || is lower). So you have the option of removing the isalpha condition and only checking for isupper/islower.

Answer 2

    if(p[i]==' ')
    {
    j--;
    } 

This if statement wouldn't work considering the first character in the string k is a space since that would make j=-1 so using that in the next if statement as p[-1] wouldn't work. A better option would be removing this if statement so in a situation when p[i] is a space, it is displayed just as it is with no modifications.

To make your code look cleaner you could remove the curly brackets from the if statements that only have one instruction.

For example:

      if(isupper(k[j]))  
            k[j] = (int)(k[j]-'A');