Lecture 11 – Conditional Statements and Iteration

DSC 10, Fall 2022





Recap: Booleans

Boolean operators; not

There are three operators that allow us to perform arithmetic with Booleans – not, and, and or.

not flips True ↔️ False.

The and operator

The and operator is placed between two bools. It is True if both are True; otherwise, it's False.

The or operator

The or operator is placed between two bools. It is True if at least one is True; otherwise, it's False.

Order of operations

Booleans can be tricky!

For instance, not (a and b) is different than not a and not b! If you're curious, read more about De Morgan's Laws.

Note: & and | vs. and and or

Concept Check ✅ – Answer at cc.dsc10.com

Suppose we define a = True and b = True. What does the following expression evaluate to?

not (((not a) and b) or ((not b) or a))

A. True

B. False

C. Could be either one

Aside: the in operator

Sometimes, we'll want to check if a particular element is in a list/array, or a particular substring is in a string. The in operator can do this for us:



if <condition>:


else: Do something else if the specified condition is False.


What if we use if instead of elif?

Example: Percentage to letter grade

Below, complete the implementation of the function, grade_converter, which takes in a percentage grade (grade) and returns the corresponding letter grade, according to this table:

Letter Range
A [90, 100]
B [80, 90)
C [70, 80)
D [60, 70)
F [0, 60)

Your function should work on these examples:

>>> grade_converter(84)

>>> grade_converter(60)


def mystery(a, b):
    if (a + b > 4) and (b > 0):
        return 'bear'
    elif (a * b >= 4) or (b < 0):
        return 'triton'
        return 'bruin'

Without running code:

  1. What does mystery(2, 2) return?
  2. Find inputs so that calling mystery will produce 'bruin'.




Example: Squares

The line print(num, 'squared is', num ** 2) is run four times:

This happens, even though there is no num = anywhere.


Using the array colleges, write a for-loop that prints:

Revelle College
John Muir College
Thurgood Marshall College
Earl Warren College
Eleanor Roosevelt College
Sixth College
Seventh College

Click here to see the solution after you've tried it yourself.
for college in colleges:
    print(college + ' College')


Example: Goldilocks and the Three Bears

We don't have to use the loop variable!

Randomization and iteration

Storing the results


Example: Coin flipping

The function flip(n) flips n fair coins and returns the number of heads it saw. (Don't worry about how it works for now.)

Let's repeat the act of flipping 10 coins, 10000 times.

Now, heads_array contains 10000 numbers, each corresponding to the number of heads in 10 simulated coin flips.

The accumulator pattern

for-loops in DSC 10

Working with strings

String are sequences, so we can iterate over them, too!

Example: Vowel count

Below, complete the implementation of the function vowel_count, which returns the number of vowels in the input string s (including repeats). Example behavior is shown below.

>>> vowel_count('king triton')

>>> vowel_count('i go to uc san diego')

Summary, next time


Next time