1. EE 194/Bio 196: Modeling biological systems
Spring 2018
Tufts University
Instructor: Joel Grodstein
Loops
EE 194/Bio 196 Joel Grodstein

2. 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

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

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

5. 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')
Printing 'Hello' 1000 times without a loop would be a bit unwieldy… but again, perhaps not that common.
EE 194/Bio 196 Joel Grodstein

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

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

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

9. 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

10. Multiply 1*2*3*4 fact=1 for i in range(2,5): fact = fact*i U 3 4 2 i24 2 6 U 1
fact
This is called “factorial,” and written as 4!
EE 194/Bio 196 Joel Grodstein

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

12. Triangle printing Here's a size-3 triangle: Output:
for i in range (1,4):
print ('*'*i)
Output: U 3 2 1 i * **
***
EE 194/Bio 196 Joel Grodstein

13. Slight variations on for loop
for i in range(4,10,2): # Counts 4,6,8
for i in range(4,9,2): # Counts 4,6,8
for i in range(10,5,-1): # Counts 10,9,8,7,6
for i in range(10,9): # Doesn’t count!
for i in range(10,5,-2): # Counts 10,8,6
EE 194/Bio 196 Joel Grodstein

14. 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

15. How about this code? Output: i=7 while (i is not a good password):
print (i, 'does not work')
i=i+1
print (i, 'is the password!’)
Output:
Try 7: no good
Try 8: no good
9 gets me in! U 8 7 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
7 does not work
8 does not work
9 is the password!
EE 194/Bio 196 Joel Grodstein

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

17. Nested loops U 3 2 U 1 u v for u in range(2,4): clap u times
for v in range (1,2,3):
spin left v times
nod your head U 3 2 U 1 u v
EE 194/Bio 196 Joel Grodstein

18. 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 . Despite what the web site says, you do not need to write your own functions.
EE 194/Bio 196 Joel Grodstein