Got stuck with Caesar.c

Question

I am trying to run the program assignment caesar.c from the edx Introduction to programming. It requires a program able to encrypt a string with the Caesar encryption: therefore, the user has to enter a key (command-line); for example with a key of 2 a 'A' character needs to be encrypted in a 'C' character; the problem starts when you have to enter a key greater than 26, which is the number of alphabetical letters. For a key of 27 and an 'A' character for example, the program must return 'B' like a key of 1.

I have tried to transform the ASCII values of the characters to alphabetical values from 0 to 26 in order to use the modulus operator when the key is equal or greater than 26. It returns me a segmentation fault. Can anyone help me with some suggestions of the causes of my error?

Here's the program:

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

int key;

// function for an alphabetic value with non capital letters

int alpha_low( char c )
{
    int alpha_value;
    alpha_value = (int) c - 97;
    return alpha_value + ( key % 26 );
}

// function to return to ascii valuee for non capital letters

char ascii_low( char c )
{
    return (char) alpha_low( c ) + 97;
}

// function for an alphabetic value with capital letters

int alpha_up( char c )
{
    int alpha_value;
    alpha_value = (int) c - 65;
    return alpha_value + ( key % 26 );
}

// function to return to ascii value for capital letters

char ascii_up( char c )
{
    return (char) alpha_up( c ) + 65;
}


int main(int argc, string argv[])
{
        int result;
        string p;
        key = atoi( argv[1] );

    if( argc != 2 || key < 0 )
    {
       printf("Usage: ./caesar key(positive integer)\n");
       return 1;
    }

    printf("Please, write a plaintext: ");
    p = GetString();

    for( int i = 0, n = strlen(p); i < n; i++)
    {
       if ( isalpha(p[i]) )
       {
          if ( islower(p[i]) )
          {
             result = alpha_low( p[i] );
             printf("%c", ascii_low( p[i] ));
          }
          else if( islower( p[i]) )
          {
              result = alpha_up( p[i] );
              printf("%c", ascii_up( p[i]) );
          }
        }  
    }      

    return 0;
}

Did you try to use a debugger? `if (islower) else if (islower)`? — Mathieu, Jul 04 '16 at 14:44
I can't reproduce any problem with `./caesar 27`, and there's nothing in there that could cause a segfault unless `GetString` is broken, or you forget the parameter (you're doing `atoi(argv[1]])` before checking whether `argv[1]` exists). — molbdnilo, Jul 04 '16 at 14:54
I've seen a similar question a couple of hours ago. Some homework assignment going on somewhere ? — Jabberwocky, Jul 04 '16 at 14:59

score 2 · Accepted Answer · answered Jul 04 '16 at 14:57

2

A function to caesar an alphabetic char should be like (decomposed in elementary steps):

int caesar_lower(int c,int key) {
    int v = c-'a'; // translate 'a'--'z' to 0--25
    v = v+key;     // translate 0--25 to key--key+25
    v = v%26;      // translate key--key+25 to key--25,0--key-1
    v = v+'a';     // translate back 0--25 to 'a'--'z'
    return v;
}

answered Jul 04 '16 at 14:57

Jean-Baptiste Yunès

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Thank you. Your function syntetise my two functions, but when I run the program it returns just the low characters encrypted, whereas the capital letters are not printed and there is no space between words. Do you have any suggestions to improve my code? I honestly don't know why it doesn't take into consideration the capital letters – Fabio Operti Jul 04 '16 at 16:32
I solved for the space problem (pretty stupid problem actually)... only need to understand why the caesar_upper function does not activate – Fabio Operti Jul 04 '16 at 16:38
because you used twice the same test `if (islower())`... replace with `if (islower()) ... else if (isupper())` ... – Jean-Baptiste Yunès Jul 04 '16 at 16:41
Yeah, I solved it without your suggestion but thank you very much again! – Fabio Operti Jul 04 '16 at 16:46

Got stuck with Caesar.c

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