1. EE 194/BIO 196: Modeling biological systems
Spring 2018
Tufts University
Instructor: Joel Grodstein
if then, logical types
EE 194/Bio 196 Joel Grodstein

2. Demo a bubble sort with cards
EE 194/Bio 196 Joel Grodstein

3. Card sort with if then
The code below is for a card sort. It already has an "if"
And hopefully it even makes sense .
Let's just fix the format a bit.
n_cards = 3
for pass in range(n_cards-1):
for i in range(n_cards-1):
if card #i and #(i+1) are backwards then
if (card[i] > card[i+1])
swap them
EE 194/Bio 196 Joel Grodstein

4. Follow the bouncing ball
n_cards = 3
for pass in range(n_cards-1):
for i in range(n_cards-1):
if (card[i] > card[i+1]):
swap them 10 6 8 10 6
[0] [1] [2]
10>6 is True U U U 3
n_cards pass i card
EE 194/Bio 196 Joel Grodstein

5. Follow the bouncing ball
n_cards = 3
for pass in range(n_cards-1):
for i in range(n_cards-1):
if (card[i] > card[i+1]):
swap them 10 8 10 8 6
[0] [1] [2]
10>8 is True 1 3
The “for i” loop is now done
n_cards pass i card
But the “for pass” loop is not done
EE 194/Bio 196 Joel Grodstein

6. Follow the bouncing ball
n_cards = 3 for pass in range(n_cards-1): for i in range(n_cards-1): if (card[i] > card[i+1]): swap them
Starting this loop all over again from the top 8 10 6
[0] [1] [2]
6>8 is False
skip over the “if” code 1 1 3
The “for i” loop is not done
n_cards pass i card
EE 194/Bio 196 Joel Grodstein

7. Follow the bouncing ball
n_cards = 3
for pass in range(n_cards-1):
for i in range(n_cards-1):
if (card[i] > card[i+1]):
swap them 8 10 6
[0] [1] [2]
8>10 is False
skip over the “if” code 1 1 3
The “for i=” loop is done
n_cards pass i card
The “pass=” loop is done
Finally finished!!!
EE 194/Bio 196 Joel Grodstein

8. if then – details The format is simple:
if (condition):
statement…
The usual Python note: if there are more than 1 “statement”s, they must all have the same indentation
As usual, there are several variations:
We can make the condition fairly complex
There is an “else”.
Let's look at both of these.
EE 194/Bio 196 Joel Grodstein

9. if then else Here’s some code with an obvious use: We can rewrite it:
if (it’s Sunday evening):
do your homework
if (it's not Sunday evening):
go to sleep early
We can rewrite it:
else:
“Else” replaces the inverted condition
Gain in clarity
EE 194/Bio 196 Joel Grodstein

10. More complex conditions
We don’t have homework due every week.
if (it’s Sunday evening and there's homework due)
do your homework
else:
go to sleep early
The condition can have 'and' and 'or', or really be any arbitrary expression.
But how arbitrary can an expression get, anyway?
In fact, very arbitrary.
Without the 'else', this would have been “if ((it's not Sunday evening) or (there's no homework due))”. That's a bit of a mess!
EE 194/Bio 196 Joel Grodstein

11. Logical expressions False and True are the simplest expressions.
if (False) → do not execute the statements
if (True) → do execute the statements
<, >, <=, >= work as expected
3<4 evaluates to True. 3>=4 evaluates to False.
And then see the case above.
So “if (3<4)” becomes “if (True)”, which executes the subsequent statements.
You can replace “3” with any variable that evaluates to 3.
a=3
if (a<4):
statements
These statements do happen, since 3<4.
EE 194/Bio 196 Joel Grodstein

12. Testing equality & inequality
Testing for equality is a bit weird
if (3=4) → error
if (3==4) → evaluates to false.
Single '=' is used only for assignment to a variable.
Testing for equality is '=='; test for inequality is ‘!='.
Example:
a=3
b=4
if (a==b):
statements
These statements do not happen, since 3 is not equal to 4.
EE 194/Bio 196 Joel Grodstein

13. AND, OR expression and expression is an expression Example:
Ditto for or.
And you can do this recursively…
Example:
a=3; b=4; c=5; d=6; e=7;
if ((a<4) and (b==c) and !((d==5) or (e==6))):
So in the end, this big expression just evaluates to 'False'
if ((3<4) and (4==5) and !((6==5)or(7==6))):
if (True and False and !(False or False)):
if (False):
if (True and False and True):
if (True and False and !False):
EE 194/Bio 196 Joel Grodstein

14. elif Sometimes you have a sequence of conditions:
If it's midnight-10am, keep the house at 55°
If 10-11am, 70°
11am-10pm, 55 °
10pm-midnight, 71°
if (time <= 10):
temp=55
if (time>10 && time<=11):
temp=70
if (time>11 && time<=22):
if (time>22 && time<=24):
The program on the right works, but
it’s inefficient, since it tests the last 3 "if"s even when the first is true.
it’s easy to mis-type the conditions
EE 194/Bio 196 Joel Grodstein

15. Nested if How about this? Pros and cons? More obvious that it works
More efficient
But the indentation is ugly
if (time <= 10):
temp=55
else:
if (time<=11):
temp=70
if (time<=22):
These must line up
These too
And these
EE 194/Bio 196 Joel Grodstein

16. Elif The solution is elif
Many (not all) languages have it.
Code is more obvious and elegant
We won’t need this until the Manduca homework.
if (time <= 10):
temp=55
elif (time<=11):
temp=70
elif (time<=22):
else:
EE 194/Bio 196 Joel Grodstein

17. Group activity What do each of these evaluate to? a=3 b=2
(a==3) and (b==2)
(a==4) or (b==2)
(a*b==6) and (a+b==6)
(a*b==6) and ((a+b==6) or (1==1))
True
True
False
True
EE 194/Bio 196 Joel Grodstein

18. Group activity What do each of these print? Assume a=3 and b=2
if (a==3) and (b==2):
print ('yes')
yes
if (a==4):
if (b==2):
print ('42')
else:
print ('4 not 2')
nothing
if (a==4) or (b==2):
print ('yes')
else:
print ('no')
yes
for i in range(2:6):
if (i>4):
print (i, '*', i, '=', i*i)
5*5=25
if (a==3):
if (b==2):
print ('32')
else:
print ('4 not 2') 32
EE 194/Bio 196 Joel Grodstein

19. 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…
Prints all of the even numbers in an array
Print all the elements of array1 that are not in array2
Makes a new array with “pos”, “neg” or “zero” based on a first array of numbers
Determines the indices of the largest and smallest numbers from an array
Checks whether a specified value is contained in an array
Tests if a number N is within 100 of 1000 or 2000.
Prints all divisors of a given number
Creates a 5-element array of random integers between 1 and 10, and then sums up only those elements that are 3 or higher
EE 194/Bio 196 Joel Grodstein

20. Another group exercise
Take an array p. Multiply every element by a random number r, and then bound them all to be ≥0 and ≤1.
You may want to Google “numpy clip”
This will be useful for HW3
EE 194/Bio 196 Joel Grodstein

21. Swapping
Let’s say we have two variables x and y, and we want to swap their values. Consider this code:
x=y
y=x
y is 7, so now x is too. 7 5
x is 7, so copy that to y. But y already was 7, so that does nothing. x 7 y
Once we set x to 7, there was no longer any variable anywhere with a value of 5. So there is no way to assign y=5 
EE 194/Bio 196 Joel Grodstein

22. Swapping – take 2
As always, there’s a trick to doing it. Try this code instead.
tmp=y
y=x
x=tmp
y is 7, so now tmp is too. 7 5
x is 5, so copy that to y. x
tmp=7, so copy 7 to x 5 7 y
It works  U 7
tmp
EE 194/Bio 196 Joel Grodstein