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I take it you never bothered to read post #10 then.
NEVER use gets()

> the program won't crash!!!!! however my string has only 5 slots!!! and I assigned 'k' to the sixth slot, which doesn't even exist!!! how come it works?!
Pure dumb luck.

C doesn't check array...

> scanf("%s%s", first_name, last_name); //no space required between the two %s
Did you try this, to see what happened?
Preferably with two names shorter than 10 characters.

The presence (or...