PROGRAMMING

# REMEMBERS

Last update:   19-11-2021

### 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!
... 9 lines 

### 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__
... 11 lines 

### 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
... 6 lines 

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