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nvteighen
March 23rd, 2008, 03:54 PM
Hi there!
I've been experimenting these days with some C just because of fun and need some help.

This code tries to substitute one single letter into a number (A=0, B=1, C=2, etc.), but for some reason it doesn't work and gets stuck into an infinite loop. This is the first step for a little Caesar Cipher encoder, where letters are "summed" by a key. So, I need to convert letters into numbers first. (I don't want any help on the rest)

#include <stdio.h>
#include <string.h> //for strlen();
#include <ctype.h> //for toupper();

char *abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //Alphabet defined as global variable

int subst(char x); //Function defined

int main(void)
{
char key;
int key2;

puts("Enter a 1 character long text");
scanf("%c", &key);

key = toupper(key); //converts all letters to uppercase
key2 = subst(key); //calling substitution function

printf("%d\n", key2);

return 0;
}

int subst(char x)
{
int out, i, len;
len = strlen(abc); //do len be abc string's length

for((i = 0); (i = len); i++)
{
if(abc[i] == x)
{
out = i;
}
/*This should search each abc's characters and compare it with x (which is the key variable) and, if they're the same, the function will return the corresponding character's place in the array as result (so, A = 0, B = 1, etc.)*/
}

return out;
}

Thanks!

mike_g
March 23rd, 2008, 04:03 PM
for((i = 0); (i = len); i++)
Here you set i to len each loop. You want:

for((i = 0); (i < len); i++)
;)

ruy_lopez
March 23rd, 2008, 04:04 PM
The "for" loop looks weird, try this:

for (i = 0; i < len; i++)

or using pointer arithmetic:

/* include <string.h> */
int subst(char x)
{
char *p;

p = index(abc, x);
return (p - abc);
}

nvteighen
March 23rd, 2008, 04:08 PM
Thank you very much! Compiled and substitution works perfectly. Now I'll try to make it ignore numbers (if I enter "2", I get "0", which is an "a").

But, just curious, why shouldn't I use (i <= len) instead?

ruy_lopez
March 23rd, 2008, 04:21 PM
But, just curious, why shouldn't I use (i <= len) instead?

array indexes start at 0. So when you test using "< len", it stops at len - 1, which is correct for an array.

In other words, the length of str = "string" is 6 chars, but the last char is at str[5].

Can+~
March 23rd, 2008, 04:23 PM
Tip: You know that you can get the ascii code of any character by using 'something'. So use that, and for each character on a string, substracting the first letter:

char string[20] = "abcdef"

//blah blah loop
numb = string[i] - 'a'

it would produce something like:
'a' - 'a' = 0
'b' - 'a' = 1
'c' - 'a' = 2
...

And you can filter that the input is not < 'a' and > 'z'

nvteighen
March 23rd, 2008, 04:39 PM
Tip: You know that you can get the ascii code of any character by using 'something'. So use that, and for each character on a string, substracting the first letter:

char string[20] = "abcdef"

//blah blah loop
numb = string[i] - 'a'

it would produce something like:
'a' - 'a' = 0
'b' - 'a' = 1
'c' - 'a' = 2
...

And you can filter that the input is not < 'a' and > 'z'
Well, that's easier and avoids using that global array. And also avoids the using ctype's isalpha()! Thanks!

nvteighen
March 23rd, 2008, 07:16 PM