CS 100 Introduction to Computing Seminar October 7, 2015

Topics Introduction to Repetition Structures The while Loop: a Condition-Controlled Loop The for Loop: a Count-Controlled Loop Calculating a Running Total Sentinels Input Validation Loops Nested Loops

Introduction to Repetition Structures Often have to write code that performs the same task multiple times  Disadvantages to duplicating code Makes program large Time consuming May need to be corrected in many places Repetition structure: makes computer repeat included code as necessary  Includes condition-controlled loops and count- controlled loops

The while Loop: a Condition- Controlled Loop while loop: while condition is true, do something  Two parts: Condition tested for true or false value Statements repeated as long as condition is true  In flow chart, line goes back to previous part  General format: while condition: statements

The while Loop: a Condition- Controlled Loop (cont’d.)

In order for a loop to stop executing, something has to happen inside the loop to make the condition false Iteration: one execution of the body of a loop while loop is known as a pretest loop  Tests condition before performing an iteration  Will never execute if condition is false to start with  Requires performing some steps prior to the loop

Infinite Loops Loops must contain within themselves a way to terminate  Something inside a while loop must eventually make the condition false Infinite loop: loop that does not have a way of stopping  Repeats until program is interrupted  Occurs when programmer forgets to include stopping code in the loop

Examples (1) count = 0 if count < 5: print ("Hello, I am an if statement and count is", count) while count < 5: print ("Hello, I am a while and count is", count) count = count + 1

Examples (2) password = " " while password != "cs_100": password = input("Please enter the password: ") if password == "cs_100": print("You have entered the correct password") else: print("Sorry the value entered in incorrect - try again")

Examples (3) flag = True while flag: number = int(input("Please enter a number from 0 to 100: ")) if number >= 0 and number <= 100: flag = False else: print("Sorry number must be between 0 and 100") print("Please try again") print("You entered: ", number) print("This is a valid number. ")

Infinite Loop count = 0 while count < 5: print ("Hello, I am a while and count is", count) count = count - 1

Let’s Work A Problem (1) A student takes 5 exams. Write a program that using looping to collect the test scores. The determine the student’s average for the 5 exams. What do we do first?

Let’s Work A Problem (2) Running on a treadmill burns 6.6 calories per minute. Write a program that uses a loop to display the number of calories burned after 5, 10, 15, 20, 25, and 30 minutes. What do we do first?