I thought I understood recur, but the following usage doesn't make sense:
(fn gcd [a b]
(if (= b 0)
a
(recur b (rem a b))))
The function retrieves the greatest common divisor for two numbers. For 4 and 2, the function would give 2.
I know that recur can be bound to functions, but I would think that 'b' is just cycled through the recur without any change. You generally need to put in something like a (inc b) to allow the value in the loop to change.
What am I missing?
The gcd function here uses the Euclidean algorithm to find the Greatest Common Divisor of two numbers.
The function works and does terminate because the argument list contains [a b] but the recur is called for b, (rem a b). Note that the place of b is changed here (from seond place to first place).
The value of a is changed because the value of b is assigned to it. Also, the value of b is changed because (rem a b) is assigned to it (thus decreasing). Therefore both values decrease when the calls are repeated and eventually one of them reaches 0 (that stops the recursion).