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I am working on a code in C that, based on a 26-character key input by the user in command line (ex: BCDEFGHIJKLMNOPQRSTUVWXYZA), encrypts a source message (plaintext) to an encrypted message (ciphertext) by replacing each letter of the source with the letter contained in the key at the i position, where i is the position of the source message letter in the classic alphabet. Uppercase and lowercase should be kept.

The expected output is the following: plaintext: "Hello" ciphertext: "Ifmmp"

To do this, I need an "upkey", a string made of the uppercase characters the user input as key, and a "lowkey", a string made of the lowercase characters the user input as key.

Example: upkey = argv[1] = [BCDEFGHIJKLMNOPQRSTUVWXYZA] lowkey = [bcdefghijklmnopqrstuvwxyza]

I append here below the piece of code that is giving me problems. In the cyle below, what I aim to do is to simply iterate the upkey and to replace each i character of it with the correspondant lowercase character, under the hypothesis the key the user input is made of uppercase only, as in the example above (argv[1] = [BCDEFGHIJKLMNOPQRSTUVWXYZA]).

string upkey = argv[1]; //key uppercase
string lowkey = upkey; //key lowercase temporary initialized as upkey

//generate lowkey
for (int i = 0, n = strlen(upkey); i < n; i++) 
{ 
     lowkey[i] = upkey[i] + 32; //convert to lowercase
}

What happens when I run it, is that both lowkey and upkey are manipulated.

i=0:
lowkey = [bCDEFGHIJKLMNOPQRSTUVWXYZA] //correct
upkey = [bCDEFGHIJKLMNOPQRSTUVWXYZA] //why?!

i=1:
lowkey = [bcDEFGHIJKLMNOPQRSTUVWXYZA] //correct
upkey = [bcDEFGHIJKLMNOPQRSTUVWXYZA] //why?!

and so on.

Can anyone please explain me what I am not seeing?

Vlad from Moscow
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Andrea De Arcangelis
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  • lowkey and upkey are pointing to the same memory area. you need to make a copy of upkey. – OldProgrammer Sep 19 '21 at 14:41
  • What is your typedef `string`? It looks as though this is an exercise in understanding pointers-to-arrays. When you set `lowkey = upkey` you are pointing to the same storage. Figure out how to declare or allocate separate storage for the two arrays. – mpez0 Sep 19 '21 at 14:41

1 Answers1

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The reason of the problem is that the both pointers upkey and lowkey points to the same string argv[1]

string upkey = argv[1]; //key uppercase
string lowkey = upkey; //key lowercase temporary initialized as upkey

The last line is equivalent to

string lowkey = argv[1]; //key lowercase temporary initialized as upkey

You need to allocate an array for lower case letters. If your compiler supports variable length arrays then you could write

#include <string.h>
#include <ctype.h>

//...

string upkey = argv[1]; //key uppercase
size_t n = strlen( upkey );

char lowkey[ n + 1 ]; //key lowercase temporary initialized as upkey
lowkey[ n ] = '\0';

and then

//generate lowkey
for ( size_t i = 0; i < n; i++ ) 
{ 
     lowkey[i] = tolower( ( unsigned char )upkey[i] ); //convert to lowercase
}

If the compiler does not support VLAs then you need to allocated memory dynamically like

string lowkey = malloc( n + 1 );
lowkey[ n ] = '\0';
Vlad from Moscow
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