Ting Ting - 4 months ago 12
C Question

*Beginner* C: incompatible integer to pointer conversion passing 'char' to parameter of type 'const char *'

I'm trying to convert each letter in a string to it's ASCII number. Using

int letter = (atoi(ptext[i]));


gives me this error:

error: incompatible integer to pointer conversion
passing 'char' to parameter of type 'const char *'; take the
address with & [-Werror]
int letter = (atoi(ptext[i]));
^~~~~~~~
&
/usr/include/stdlib.h:148:32: note: passing argument to parameter
'__nptr' here
extern int atoi (__const char *__nptr)


Here is some of the rest of my code that may be relevant:

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

int main(int argc, string argv[])
{

printf("Give a string to cipher:\n");
string ptext = GetString();

int i = 0;

if (isupper(ptext[i]))
{
int letter = (atoi(ptext[i]));
}
}


What am I doing wrong, and how do I fix this so I can convert a string into ASCII values?

Note:
cs50.h
lets me use "
string
" instead of "
char*
" in main.

Answer

You do not need to convert characters to a number. It is a matter of interpretation of your data.

Charater 'A' can be thought of 0x41 or 65 so this is perfectly fine:

int number = 'A';

and value on variable number is 0x41/65 or 1000001b depending how you want to present it/treat it.

As for interpretation : 0xFF may be treated as 255 if you present it as unsigned value or even as -1 when treated as signed and kept in 8 bits.

So your question:

can convert a string into ASCII values?

is kind of wrong - all characters of the string are already ascii values - it is just a matter of how you treat/print/interpret/present them.