Python Spring 2020 Tufts University Instructor Joel Grodstein

Python Spring 2020 Tufts University Instructor: Joel Grodstein joel. grodstein@tufts. edu Loops Python Joel Grodstein 1

What is a loop? • A piece of code that you repeat several times • Why are loops useful? – Do you ever do things more than once in real life? Python Joel Grodstein 2

Simple loop • Simple code: for i in range(3): print ('Hello') • This code prints out Python Joel Grodstein 3

How it executes • Simple code: for i in range(3): print ('Hello') • Output: 201 U i Hello Python Joel Grodstein 4

Is this really useful? • How useful is printing 'Hello' 3 times? • Anyway, this would work too: print ('Hello') print ('Hello’) • How about 1000 times instead of 3? Python Joel Grodstein 5

What does this code do? • Simple code: for i in range(3): print (i) 01 U 2 i • Output: 0 1 2 Python Joel Grodstein 6

How about this code? • Simple code: for i in range (2, 5): print (i) 32 U 4 i • Output: 2 3 4 Python Joel Grodstein 7

Running sums • Sum the numbers from 2 to 4: sum=0 for i in range(2, 5): sum = sum+i 23 U 4 i 0295 U sum Python Joel Grodstein 8

Group activity • Can you write code to compute 1*2*3*4? • Can you write code to compute 1*2*3*…*n? • Can you write code to print * ** ***** Python Joel Grodstein 9

Multiply 1*2*3*4 fact=1 for i in range(2, 5): fact = fact*i 23 U 4 i 24 2 U 61 fact This is called “factorial, ” and written as 4! Python Joel Grodstein 10

Multiply 1*2*3*…*n fact=1 for i in range (1, n+1): fact=fact*i Python Joel Grodstein 11

Triangle printing • Here’s a size-3 triangle: for i in range (1, 4): print ('*'*i) 12 U 3 i • Output: * ** *** Python Joel Grodstein 12

Slight variations on for loop • • • for i in range(4, 10, 2): for i in range(4, 9, 2): for i in range(10, 5, -1): for i in range(10, 9): for i in range(10, 5, -2): Python Joel Grodstein 13

while loop • While loop looks like this: while (condition): statements… • It keeps executing the statements, in order, until the condition becomes false. Python Joel Grodstein 14

How about this code? i=7 while (i is not a good password): print (i, 'does not work') i=i+1 print (i, 'is the password!’) • Output: 7 does not work 8 does not work 9 is the password! 9 gets Try 8: no good in! 7: me 87 U 9 i • How do we know that “i=i+1” is part of the same loop as the “print” above it? – The indentation! – Ditto for a ‘for’ loop Python Joel Grodstein 15

Group activity: dancing • Try the following dances: for i in range(3): spin left for i in range(2, 7, 2): spin right hop i%3 times leader=pick somebody while (leader has hand raised): for i in range(3): copy leader's move for u in range(2, 4): clap u times for v in range (1, 2, 3): spin left v times nod your head i=10 j=1 while (j<i): spin left j times spin right j=j*2 Python Joel Grodstein Code your own dance! 16

Nested loops for u in range(2, 4): clap u times for v in range (1, 2, 3): spin left v times U 32 u nod your head Python Joel Grodstein U 1 v 17

Follow-up activities • Try the examples from this lecture yourself – Vary them, or even mis-type some to see what happens • More exercises. Write a program that… – Computes the future value of a specified principal amount, rate of interest, and a number of years – Sums the first n positive integers. – Takes a positive integer and returns the sum of the cube of all the positive integers smaller than the specified number – Draws an N-by-N game board as shown in http: //www. practicepython. org/exercise/2014/12/27/24 -draw-agame-board. html. Despite what the web site says, you do not need to write your own functions. Python Joel Grodstein 18