[SOLVED] Book example of own type variable initialization wont work

paliga · 11-08-2010, 12:37 PM

Hi...

Its an example froom a book... and it does not work...

Code:

#include <stdio.h>

int main ( int argc, const char * argv[] ) {

        struct a_struct {
                unsigned char first;
                unsigned char second;
        };
        typedef struct a_struct my_atyp;

        my_atyp my_var;
        my_var = {1, 2};

        printf("%c, %c", my_var.first, my_var.second);
}

compiler gives me:

Code:

playing_with_struct.c: In function ‘main’:
playing_with_struct.c:12: error: expected expression before ‘{’ token

Anybody?
Thanks in advance!

Sergei Steshenko · 11-08-2010, 12:46 PM

Quote:

Originally Posted by paliga

Hi...

Its an example froom a book... and it does not work...

Code:

#include <stdio.h>

int main ( int argc, const char * argv[] ) {

        struct a_struct {
                unsigned char first;
                unsigned char second;
        };
        typedef struct a_struct my_atyp;

        my_atyp my_var;
        my_var = {1, 2};

        printf("%c, %c", my_var.first, my_var.second);
}

compiler gives me:

Code:

playing_with_struct.c: In function ‘main’:
playing_with_struct.c:12: error: expected expression before ‘{’ token

Anybody?
Thanks in advance!

It is not initialization, it is assignment. Look up initialization in http://www.open-std.org/JTC1/SC22/wg...docs/n1124.pdf .

paliga · 11-08-2010, 01:23 PM

well... now I know it has to be in the same line... kind of weird...

in the case of struct initialization and assignment have to be in the same line...

wje_lq · 11-08-2010, 01:27 PM

Sergei is correct. This is assignment:

Code:

my_atyp my_var;
my_var = {1, 2};

This is initialization:

Code:

my_atyp my_var = {1, 2};

Furthermore, once you correct that and run it, the output won't make much sense to you unless you change that line to this:

Code:

my_atyp my_var = {'1', '2'};

Making that correction fills those fields not with the numbers 1 and 2, which you originally coded, but the ASCII characters "1" and "2", which numerically are hex 31 and hex 32 (decimal 49 and 50), respectively. The %c character spits out single ASCII characters exactly as they are in your data. Hex 01 and hex 02 are, of course, also ASCII characters, but they won't ordinarily show up on your teletype (or, if you're a young person, your screen).

Hope this helps.

wje_lq · 11-08-2010, 01:31 PM

Quote:

Originally Posted by paliga

in the case of struct initialization and assignment have to be in the same line...

No, that's not quite Sergei's point.

You're not doing both "initialization and assignment" in the same line. In that one line, you're doing initialization. Initialization is what happens when you put data in the variable at the same time you declare it, which is what you're doing in the corrected code. Assignment is what you can do later, on some other line, any time you want:

Code:

my_var.first = '3';

/* and / or: */

my_var.second = '4';