Recursive functions: can someone explain how this code printed 1 to 5?-CodePudding

def recursion_1(n):
    if n == 0:
        return
    recursion_1 (n-1)
    print(n)

I don't understand how this code printed 1 to 5 the same way a for loop would have printed a list of range from 1 to 5. Passing 5 as the argument here.

CodePudding user response：

The best way to understand it is probably to imagine how it would look if you replaced the function calls with its definition.

So recursion_1(5) becomes :

if 5 == 0:
    return
recursion_1 (4)
print(5)

Which in turn is equivalent to :

if 5 == 0:
    return
if 4 == 0:
    return
recursion_1 (4)
print(4)
print(5)

Et cetera until :

if 5 == 0:
    return
if 4 == 0:
    return
if 3 == 0:
    return
if 2 == 0:
    return
if 1 == 0:
    return
recursion_1 (0)
print(1)
print(2)
print(3)
print(4)
print(5)

Then recursion_1(0) will return because the condition is met (0 == 0 is true) and each precedent call will return after printing its number.