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

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)

OS LIBRARY , TIME LIBRARY

What is the os library? It allows us to "talk" to the console. One of the most powerful things we can do with this library is allow it to clear the console EXAMPLE: import os print("Welcome") print("to") print("Replit") os.system("clear") username = input("Username: ") Time Library We can import a second library by placing a , after the name of the first library. EXAMPLE: import os, time print("Welcome") print("to") print("Replit") time.sleep(10) os.system("clear") username = input("Username: ") NOTE: from replit import audio import os, time def play(): source = audio.play_file('audio.wav') source.paused = False # unpause the playback while True: stop_playback = int(input("Press 2 anytime to stop playback and go back to the menu : ")) # giving the user the option to stop playback if stop_playback == 2: source.paused = True # let'...

ALL ABOUT WHILE LOOP

WHILE LOOP A while loop allows your code to repeat itself based on a condition you set. EXAMPLE : counter = 0 while counter < 10: print(counter) counter +=1 Infinite Loop You have to be really careful that you don't accidentally invoke an infinite loop! This is where the computer will loop code until the end of time. Without a break. Forever. This is just saying "count to 10 by 1 each time." to make the loop end. Don't forget, if your condition is a > then you might need to -= . This will subtract from the variable instead of adding to it. EXAMPLE : counter = 0 while counter < 10: print(counter) counter += 1

PYTHON SYNTAX

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