Modeling Biological Systems with Loops in Spring 2018 at Tufts

EE 194/Bio 196: Modeling biological systems Spring 2018 Tufts University Instructor: Joel Grodstein joel.grodstein@tufts.edu Loops EE 194/Bio 196 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? EE 194/Bio 196 Joel Grodstein 2

Simple loop Simple code: for i in range(3): print ('Hello') This code prints out Hello Hello Hello what do you think? EE 194/Bio 196 Joel Grodstein 3

How it executes Simple code: for i in range(3): print ('Hello') Output: 0 1 U 2 i Hello Hello Hello EE 194/Bio 196 Joel Grodstein 4

Is this really useful? Printing 'Hello' 3 times isn't really something we need to do too often. Even if we did, this would work too: print ('Hello') print ('Hello') print ('Hello') Printing 'Hello' 1000 times without a loop would be a bit unwieldy but again, perhaps not that common. EE 194/Bio 196 Joel Grodstein 5

What does this code do? Simple code: for i in range(3): print (i) 0 1 U 2 i Output: 0 1 2 EE 194/Bio 196 Joel Grodstein 6

How about this code? Simple code: for i in range (2,5): print (i) 2 3 U 4 i Output: 2 3 4 EE 194/Bio 196 Joel Grodstein 7

Running sums Sum the numbers from 2 to 4: sum=0 for i in range(2,5): sum = sum+i 2 3 U 4 i U 0 2 5 9 sum EE 194/Bio 196 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 * ** *** **** ***** EE 194/Bio 196 Joel Grodstein 9

Multiply 1*2*3*4 fact=1 for i in range(2,5): fact = fact*i 2 3 U 4 i U 1 2 6 24 fact This is called factorial, and written as 4! EE 194/Bio 196 Joel Grodstein 10

Multiply 1*2*3**n fact=1 for i in range (1,n+1): fact=fact*i EE 194/Bio 196 Joel Grodstein 11

Triangle printing Here's a size-3 triangle: for i in range (1,4): print ('*'*i) 1 2 U 3 i Output: * ** *** EE 194/Bio 196 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): # Counts 4,6,8 # Counts 4,6,8 # Counts 10,9,8,7,6 # Doesn t count! # Counts 10,8,6 EE 194/Bio 196 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. EE 194/Bio 196 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! ) Try 7: no good Try 8: no good 9 gets me in! 7 8 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 Output: 7 does not work 8 does not work 9 is the password! EE 194/Bio 196 Joel Grodstein 15

Group activity: dancing for u in range(2,4): clap u times for v in range (1,2,3): spin left v times nod your head Try the following dances: for i in range(2,7,2): for i in range(3): spin left spin right hop i%3 times i=10 j=1 while (j<i): spin left j times spin right j=j*2 leader=pick somebody while (leader has hand raised): for i in range(3): copy leader's move Code your own dance! EE 194/Bio 196 Joel Grodstein 16

Nested loops for u in range(2,4): clap u times for v in range (1,2,3): spin left v times nod your head 2 U 3 1 U u v EE 194/Bio 196 Joel Grodstein 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-a- game-board.html . Despite what the web site says, you do not need to write your own functions. EE 194/Bio 196 Joel Grodstein 18