Comments

Comments are pieces of text in our code that the compiler ignores. We use them to explain code or to disable lines without deleting them

In C, a single-line comment starts with //

#include <stdio.h>

int main(void) {
    // this is a comment
    int a = 1 + 2 + 3;
    printf("%d\n", a);   // display variable a
    return 0;
}

Everything after // on a line is ignored. The program runs as if those parts weren't there

Comments are great for temporarily disabling code

#include <stdio.h>

int main(void) {
    int a = 1 + 2 + 3;
    // printf("%d\n", a);
    return 0;
}

Now nothing is printed, because the printf is commented out. Very useful when debugging: instead of deleting code and rewriting it later, you just comment it out

For longer comments that span multiple lines, C also has /* ... */

#include <stdio.h>

int main(void) {
    /*
    This is a
    multi-line
    comment
    */
    printf("Hello\n");
    return 0;
}

In old C code (before C99), only /* */ existed. // was added later, copied from C++. Today both work. Most modern code uses //

Mission: System Log

The system prints a boot log to the teletype, but one of the lines contains an internal access code that must not end up in the public log.

• Write four printf calls, in order, that print: PDP-11, Bell Labs Computing Center, ACCESS-7734-SECRET, and the number 1972
• Comment out the third printf call (the one with the access code), so that its line no longer appears in the output

Example

Your program should print

PDP-11
Bell Labs Computing Center
1972

Don't delete anything, just comment the line you don't want to run