What Is Recursion?

What Is Recursion?
👉 Recursion is a type of program where you get a subroutine to call itself. It gets kinda hard to explain. I'd very much recommend this book, which does an excellent job of explaining this concept.
Recursion lets us solve problems in a more human way. Some mathematical problems can just be solved better using recursion.
For example:

We want to print out a sequence of the same emoji, reducing by 2 emojis per line. (eg print a row of 9, then a row of 7, then 5, etc. until we get none).

We will end up with a reverse pyramid pattern.
We could use range(), but it's a bit odd with counting backwards. Or a loop, but that would be pretty long.

👉 Here's an example of a recursive solution.

def reverse(value):
if value <= 0:
print("Done!")
return
# This `if` provides the 'stop' condition for the program. Otherwise it would run forever.

else: # if we're not at the stop condition.
for i in range(value):
print("💯", end="") # Outputs 'value' emojis
print() # New line
reverse(value - 2) # takes 2 off the value and calls the subroutine again with this new number. Eg if value was 7 it would call 'reverse(value)' again with value as 5.
reverse(5)

Comments

IF AND ELSE STATEMENT

If Statements These statements are a bit like asking a question. You are telling the computer: if something is true, then do this specific block of code. Double equals ( == ) is asking the computer to compare if these two things are exactly the same. EXAMPLE : myName = input("What's your name?: ") if myName == "David": What is else? IF the condition is not met with the if statement, then we want the computer to do the else part instead. Likewise, if the condition is met in the if statement, then the else bit is ignored by the computer. The else statement must be the first thing unindented after the if statement and in line with it EXAMPLE : myName = input("What's your name?: ") if myName == "David": print("Welcome Dude!") print("You're just the baldest dude I've ever seen...

FOR LOOP , RANGE

FOR LOOP A while loop is perfect to use when we don't know how many times we want the loop to repeat. If we have an idea of how many times we want the loop to repeat, we can use a for loop to loop code in exactly the same way the while loop did. EXAMPLE : for counter in range(10): print(counter) RANGE The range function creates a list of numbers in the range you create. If you only give it one number, it will start at 0 and move to a state where the final number is one less than the number in the brackets. In this case, the final number would be 9 . EXAMPLE : total = 0 for number in range(100) : total += number print(total)

Parameters , Call the Subroutine , Adding More Arguments

Parameters In a subroutine, the () are for the argument (FYI argument is another word for parameter). These are the pieces of information we pass to the code. These can be variable names that are made up for the first time within the argument () . EXAMPLE : def whichCake(ingredient): if ingredient == "chocolate": print("Mmm, chocolate cake is amazing") elif ingredient == "broccoli": print("You what mate?!") else: print("Yeah, that's great I suppose...") How do we call the subroutine? We call it in the same way as before. However, instead of leaving the () blank, we send the code a message. EXAMPLE : def whichCake("chocolate"): if ingredient == "chocolate": print("Mmm, chocolate cake is amazing") elif ingredient == "broccoli": print("You what mate?!") else: print("Yeah, that's great I suppose...") Addin...

PYTHON SYNTAX

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