Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Teams

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

*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.
atoi() expects a string. You only want the char code of the character... which is the character itself, since in C, the char is a normal integer type just like every other integer, and a string is an array of chars which hold the character codes themselves.
Consequently,
int letter = ptext[i];
would do the job.
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.
atoi() convert string to integer not character.
To store ascii value of character into integer variable Do direct assignment of character to integer variable.    
int letter = ptext[i];
        
Thanks for contributing an answer to Stack Overflow!
Please be sure to answer the question. Provide details and share your research!
But avoid …
Asking for help, clarification, or responding to other answers.
Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.