Flip coin procedure

Question

I tried to solve this problem. The flip coin takes no arguments and evenly returns the heads or tails of characters with equal probability. This is what I got, but I don’t understand why it gives me the “impossible”, can it be stopped?

(define flip-coin (lambda ( ) (cond [ (= (random 2 ) 1 ) "heads" ] [ (= (random 2 ) 0 ) "tails" ] [else "impossible" ] ) ) ) 

Answer 1

The flip-coin procedure returns only one of two possible values, it can be simplified a bit, also noting that random should be called only once - and there is no need to save its value in a variable, since the result is used immediately:

 (define (flip-coin) (if (zero? (random 2)) "tails" "heads")) 

Answer 2

You have two different random calls in your cond statement. Both are independent and may give you different results. Thus, it is possible that the first (random 2) evaluates to 0 , and the second evaluates to 1 , which causes both of these cases to fail and gives you "impossible" .

The solution is to put the result (random 2) in a local variable with let-statement, making sure that random is called only once.