Hello World
Recursion
p88 It is legal for a function to call itself (one of the most magical things a program can do).
# Recursion:
#
# A function can call himself
def countdown(n):
if n <= 0:
print ("Time!")
else:
print(n)
countdown(n-1)
countdown(3)
# 3
# 2
# 1
# Time!
Flow
What happens if we call this function like this?
countdown executions begins with n=3 ...
since n is greater than 0 it outputs 3 ..
then it calls itself ...
countdown executions begins with n=2 ...
since n is greater than 0 it outputs 2 ..
then it calls itself ...
countdown executions begins with n=1 ...
since n is greater than 0 it outputs 1 ..
then it calls itself ...
countdown executions begins with n=0 ...
since n is not greater than 0 it outputs Time! ..
then returns ...
the countdown that got n=1 returns
the countdown that got n=2 returns
the countdown that got n=3 returns
After that we are back in the __main__
Repeat
p89 As another example, we can print a string n times.
# It is easier to write a for loop ...
# but there are times when we need recursion.
def repeat(s, n):
if n <=0 :
return
print(s)
repeat(s, n-1)
repeat('abc', 3)
# abc
# abc
# abc
➥ Questions
Last update: 60 days ago
