same two codes, one giving error other not?

[OZGUR OZZEYBEK]

hi!

I m a beginner at this trying to learn C, I have this .pdf c-in-7-days where
told me to work on dev-C++. I have the version 4.9.9.2 and win7 as OS. I can't
understand why this code:

include <stdio.h>

main()
{
printf("hello world!\n");
system("pause"); //this line is only needed under windows
}

is working for the first time I used it and not in the other times I want to
use?

it keeps giving me the same error about the system line as being undeclared.

when I compile, it puts an x mark on the left of the line and at the bottom
gives these messages:
in function 'int main()':
'system' undeclared (first use this function)
(Each undeclared identifier is reported only once for each function it appears
in.)

I can't figure out a way to solve this since I've only began today.

I would be grateful if you help me! Thanks...

[cpns]

If that is truly what the document you referred you suggested as initial code,
then discard it. It is astounding how many errors, bad-practice, and
misinformation you can cram into six lines of code. Get a better reference.
For that matter, a better development tool would help. The recommendation to
use Dev-C++ may be indicative of the age of the material you are using; there
are now better choices than perhaps when that was written.

#include <stdio.h>
#include <stdlib.h>  // where system() function is declared

int main()  // The only mandated return type for main() in C is int
{ 
    printf( "hello world!\n" ) ; 
    system( "pause" )  ; // Required to prevent the application from terminating 
                        // immediately causing the OS to close its window.

    return 0 ;
}