C printf %c character printing

Question

The following code prints (compiled with clang)

Output

[A][?][?]

Code

#include <stdio.h>

int main(){
    char a = "A";
    printf("[A]");
    printf("[%c]", a);
    printf("[%c]", "A");
}

Gives a warning (clang does)

test.c:4:10: warning: incompatible pointer to integer conversion initializing
      'char' with an expression of type 'char [2]' [-Wint-conversion]
    char a = "A";
         ^   ~~~
test.c:7:20: warning: format specifies type 'int' but the argument has type
      'char *' [-Wformat]
    printf("[%c]", "A");
             ~~    ^~~
             %s

However

Output

[A][z][z]Characters: a A

Code

int main(){
    char a = "A";
    printf("[A]");
    printf("[%c]", a);
    printf("[%c]", "A");
    printf ("Characters: %c %c \n", 'a', 65);

}

I'm thinking it has something to do with the memory and integers (both because of the warning and because the "A" that is rendered as a "?" goes to a "z", that is "A"--).

turn on your compiler warnings, and read a beginners' tutorial about characters and strings in C. — The Paramagnetic Croissant, Feb 26 '15 at 16:32
The compiler's message perfectly explains what is the problem with your code. I don't know how to explain it any better than this. — 5gon12eder, Feb 26 '15 at 16:35
@5gon12eder I disagree is is completely in obvious (and stupid) to assume a differentiation between '' and "" when it comes to most languages that don't differentiate and I have never seen any mention of '' and "" being different from any tutorials or classes. — Zimm3r, Feb 26 '15 at 16:43

score 4 · Accepted Answer · answered Feb 26 '15 at 16:35

4

That's because "A" is a const char[2] string of 'A' and '\0'. Use:

char a = 'A';

to get what you appear to be after.

answered Feb 26 '15 at 16:35

Paul Evans

27,315
3
37
54

So using "" and '' produce different results? – Zimm3r Feb 26 '15 at 16:41
1

@Zimm3r `'A'` and `"A"` are different. Curious: Do you expect `'A'` and `"A"` to be the same thing? – chux - Reinstate Monica Feb 26 '15 at 16:45
@Zimm3r very much so, `""` is used to create C compile-time strings of type `const char []` where as the C language uses `''` to ceate compile-time `char` constants. – Paul Evans Feb 26 '15 at 16:45
@chux yes I have never seen it be different in Python, Javascript, or C++. – Zimm3r Feb 26 '15 at 16:46
@Zimm3r C++ uses these same rules -- it inherited then from from C – Paul Evans Feb 26 '15 at 16:46
Guess you learn new things. I have never ran into this, from C classes to tutorials etc. – Zimm3r Feb 26 '15 at 16:50
@Zimm3r But C is neither JavaScript nor Python. It's pointless to assume that one language's syntax/semantics follows that of another one. – The Paramagnetic Croissant Feb 26 '15 at 21:09

C printf %c character printing

1 Answers1