Published

2025-03-31

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Control Flow

  • Control flow just refers to the order in which a language executes code

  • In python

    • Using conditional logic (if, elif, and else)
    • Looping (for or while for instance)

Note: These types of webpages are built from Jupyter notebooks (.ipynb files). You can access your own versions of them by clicking here. It is highly recommended that you go through and run the notebooks yourself, modifying and rerunning things where you’d like!

Conditional Statements

  • Choose which portions of your code to execute by using conditional statements!

  • An if statement changes how a program behaves based on a condition

    • Condition comes in the form of a boolean

  • Recall: Booleans are True or False

    • Can be treated as 1 and 0
    • Many functions to create bools (.is_*() methods, bool() function)

if Syntax

  • Example of if with no further conditional logic:
if boolean:
    #If boolean is true, execute the chunk of code that is indented
    #Four spaces is recommended but any indentation can technically be used
    statement1
    statement2
    
#code not indented would then execute as normal

if Example

Printing different strings using if statements

temp = 30
if temp < 50:
    print(temp, "degrees is cold.")
30 degrees is cold.
temp = 100
if temp < 50:
    print(temp, "degrees is cold.")

We can have multiple statements that are executed within a block if the condition is True.

temp = 30
if temp < 50:
    print(temp, "degrees is cold.")
    print("Wear a jacket outside!")
30 degrees is cold.
Wear a jacket outside!
temp = 100
if temp < 50:
    print(temp, "degrees is cold.")
    print("Wear a jacket outside!")
if temp >= 50:
    print(temp, "degrees is not cold.")
100 degrees is not cold.

if with else Syntax

With the last example above, we see something we often want to do: - check a condition (temp < 50), if True execute some code - if that same condition is False - or the opposite is True (temp >= 50) - then execute something else

This can be taken care of with the else statement. The else statement immediately following an if block allows for execution of code only when the above condition(s) were (all) False

if boolean:
    execute this code
else:
    execute this code

this is logically equivalent to

if boolean:
    execute this code
if not boolean:
    execute this code
temp = 100
if temp < 50:
    print(temp, "degrees is cold.")
    print("Wear a jacket outside!")
else:
    print(temp, "degrees is not cold.")
100 degrees is not cold.

if, elif, and else

You can check additional conditions using elif which stands for ‘else if’.

This condition is only checked if all the above conditions were False.

if boolean1:
    #if boolean1 is True
    execute this code  
elif boolean2:
    #if boolean1 is False, check if boolean2 is True
    #if True
    execute this code
elif boolean3:
    #if boolean1 and boolean2 are False, check if boolean3 is True
    #if True
    execute this code
else:
    # if no conditions met
    execute this code
temp = 40
if temp < 50:
    print(temp, "degrees is cold.")
    print("Wear a jacket outside!")
elif temp < 70:
    print(temp, "degrees is kind of cold...")
    print("You may want to bring an umbrella in case it rains!")
else:
    print(temp, "degrees is not cold.")
40 degrees is cold.
Wear a jacket outside!
temp = 60
if temp < 50:
    print(temp, "degrees is cold.")
    print("Wear a jacket outside!")
elif temp < 70:
    print(temp, "degrees is kind of cold...")
    print("You may want to bring an umbrella in case it rains!")
else:
    print(temp, "degrees is not cold.")
60 degrees is kind of cold...
You may want to bring an umbrella in case it rains!
temp = 100
if temp < 50:
    print(temp, "degrees is cold.")
    print("Wear a jacket outside!")
elif temp < 70:
    print(temp, "degrees is kind of cold...")
    print("You may want to bring an umbrella in case it rains!")
else:
    print(temp, "degrees is not cold.")
100 degrees is not cold.

Loops

Conditionally executing code is really useful! Another useful thing is to repeatedly execute code. Each execution you may want to change something in the computation.

Loop Example

  • Suppose we’ve observed the eye colors of 15 people
  • Eye color coded as either 1 (blue), 2 (brown), 3 (green), 4 (other)
  • Want to create a new variable that has the descriptive values
#data stored in a 'list'
eye_color = [3, 2, 2, 1, 2, 1, 2, 4, 3, 2, 2, 1, 2, 2]

Goal: Create a new variable that has the descriptive values - Recall list elements are indexed using [index] (like other sequence type objects such as strings)

print(eye_color[0])
print(eye_color[1])
3
2
  • We could consider using conditional logic to print out the descriptive string
eye_color = [3, 2, 2, 1, 2, 1, 2, 4, 3, 2, 2, 1, 2, 2]

if eye_color[0] == 1:
    print("blue")
elif eye_color[0] == 2:
    print("brown")
elif eye_color[0] == 3:
    print("green")
else:
    print("other")
green

Loop Syntax

  • Instead of repeating and modifying code, use a loop!
for index in values:
     code to be run
  • index argument defines a counter, or variable, that varies each time the code within the loop is executed
  • values argument defines which values the index takes on in these iterations
    • These values do not have to be numeric!

Loop Toy Examples

  • Run the code below and feel free to modify things to see what happens!
for index in ["cat", "hat", "worm"]:
    print(index)
cat
hat
worm
values = list(range(10)) #recall range is an iterator-type object, list gets the values out
print(values)
for i in values:
    print(i)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
0
1
2
3
4
5
6
7
8
9
#we can get the indices of the eye_color object
print(list(range(len(eye_color))))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
#alternatively, we can just iterate over the range() object itself
values = iter(range(len(eye_color)))
print(next(values))
print(next(values))
print(next(values))
0
1
2

Back to our example. We want to print out the more descriptive name depending on what the numeric value is.

Let’s loop through all the eye_color values using a for loop. To do so, we’ll use the range() function with the len(eye_color) as its argument.

Recall: range() is an iterator-type object. A for loop will automatically go over the values the range indicates. When we give range() just one argument it defaults to a range of 0 to that value (but that value isn’t included).

for i in range(len(eye_color)):
    if eye_color[i] == 1:
        print("blue")
    elif eye_color[i] == 2:
        print("brown")
    elif eye_color[i] == 3:
        print("green")
    else:
        print("other")
green
brown
brown
blue
brown
blue
brown
other
green
brown
brown
blue
brown
brown

We don’t really need to use a set of numeric values to iterate over (via range()). We can iterate over anything that is iterable. All sequence type objects are iterable (like lists and strings).

Here we’ll iterate over the eye_color list itself.

for i in eye_color:
    if i == 1:
        print("blue")
    elif i == 2:
        print("brown")
    elif i == 3:
        print("green")
    else:
        print("other")
green
brown
brown
blue
brown
blue
brown
other
green
brown
brown
blue
brown
brown

Other Looping Commands

  • Occassionally we want to jump out of a for loop. This can be done with break
for i in range(5):
    if i == 3:
        break
    print(i)
0
1
2
  • The continue command jumps to the next iteration of the loop without finishing the current iteration
for i in range(5):
    if i == 3:
        continue
    print(i)
0
1
2
4

While Loops

  • While loops are similar to for loops but they loop until a condition is reached

    • Useful when we don’t know in advance how many loop iterations we should execute

  • General syntax of a while loop:

while expression:
    #code block to execute
    block
  • the expression is sometimes called the loop condition
  • At each iteration of the loop, python evaluates the expression
    • If the expression evaluates to False, the loop exits
    • If the expression evaluates to True, the loop body is executed again

While Loop Example

When using while loops, we usually modify the condition within the body of the while loop (or use a break to jump out when needed).

rabbits = 3
while rabbits > 0:
    print(rabbits)
    rabbits = rabbits - 1
3
2
1

Video Demo

This quick video demo shows an example of implementing a loop in python. We’ll look at the Fizzbuzz example. Then we’ll write a quick number guessing game! Remember to pop the video out into the full player.

The notebook written in the video is available here.

from IPython.display import IFrame
IFrame(src="https://ncsu.hosted.panopto.com/Panopto/Pages/Embed.aspx?id=ed3118c1-feb1-4d7f-a98c-b0f800ebfd46&autoplay=false&offerviewer=true&showtitle=true&showbrand=true&captions=false&interactivity=all", height="405", width="720")

Recap

  • Conditional logic with if, elif, and else
if boolean1:
    execute this code
elif boolean2:
    execute this code
elif boolean3:
    execute this code
else:
    execute this code
  • Looping via for or while
for index in values:
     code to be run
while expression:
    #code block to execute
    block

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