1. While Loops CMSC 201

2. Overview Today we will learn about: Looping Structures While loops

3. Looping Loops let us do something until a boolean condition is met. The simplest loop is called a while loop.

4. The while loop The syntax for a while loop is as follows: line-1 while someCondition: line-2 line-3 line-4

5. The while loop line-1 while someCondition: line-2 line-3 line-4 line-1 Condition is true line-2 line-3 Condition is false line-4 Condition is false Condition is true

6. Example a = 5 while a > 0: print(a) a = a – 1 Prints 5 4 3 2 1

7. Notes If the condition is false to begin with, the loop never executes! a = 4 b = 5 while a == b: print("Hello") print("Goodbye") This will only print goodbye.

8. Notes It is very important that the variable in the condition be altered at some point during the loop. a = 5 while a > 0: print(a) This is called an infinite loop, since a will always be greater than zero.

9. Factorial Example Computing factorial with a while loop. n = int(input("Enter a number ")) counter = n answer = 1 while counter > 0: answer = answer * counter counter = counter - 1 print("n factorial is ", answer)

10. Factorial Example

12. Exercise Use a while loop to print out every number between 0 and 100 that is divisible by three.

13. Loops + If Statements Now that you know how to use loops, if statements, and variables, you can create fairly complicated algorithms. However, figuring out how to combine them can be difficult! In the next few homeworks and exercises, you will start needing to figure out when to use each tool you’ve been given so far. These are the basic elements of programming!

14. Nested Loops Nested loops are loops inside loops. The first loop is the outer loop The second one is the inner loop. a = 0 while a < 5: b = 0 while b < 4: print(b) b = b + 1 a = a + 1

15. Nested Loops