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programming·Programmingbymilon

Why does isalpha() fail to identify an alphabetic character?

This is in C language. When I call rotate() in main, the function returns false for isalpha() even though the string entered for plaintext uses alphabetic characters. Perhaps it's identifying an alphabetic character by its ASCII value ('A' = 65)? I tried to test that out and used (char) with the letter variable in rotate() but it didn't change anything.

PORTION OF MAIN

string plaintext = get_string("plaintext:  ");

    int length = strlen(plaintext);
    char ciphertext[length];

    for (int i = 0; i < length; i++)
    {
        ciphertext[i] = rotate(plaintext[i], key);
    }

ROTATE FUNCTION

char rotate(char letter, int key)
{
    if (isalpha(letter) == true)
    { ...
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8 replies

jjagaimo
lemmy.ca

isalpha documentation:

Return value

Non-zero value if the character is an alphabetic character, zero otherwise.

You should be either checking for not equal to 0 instead of true, as its not necessarily guaranteed to be 1 ~= true, or removing the comparison entirely

Also make sure that your loop condition is < and not "& lt" without the space unless that's a weird formatting issue

For more information, make sure to check the documentation for the standard library functions

15
pHr34kYreply
lemmy.world

I would drop the "== true" entirely. C will evaluate any nonzero int as true in an "if" statement.

15
milonreply
lemm.ee

Ah ha! Yes, I did check the docs but I think I just glanced over that portion. Be more careful next time. Now that I took another look at the other ctype.h functions, they all return 1 or 0. I think I confused equivalent python built-in functions as those evaluated to true/false. The < is a less than sign but it seems it doesn't render correctly on Lemmy.

4
lolcatnipreply
reddthat.com

Never, ever write "== true" or "== false" unless it's absolutely necessary. In C that's pretty close to never.

2
breadsmasher
lemmy.world

what language? needs more context

portion of main and function are just titles for me, if that is supposed to contain code

4
milonreply
lemm.ee

Sorry. It's in C. Updated post. Yes those are titles. I just included the relevant portions rather than the entire code.

3

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