Lecture 04

E21 Computer Engineering Fundamentals

Author

Emad Masroor

Published

September 10, 2026

Announcements

Please complete survey available on Moodle or here.
Homework 1 due at midnight tonight
Homework 2 to be released today
Labs start today

Running code on your Circuit Playground Express

The code that’s saved in the special file code.py on your CIRCUITPY drive is automatically run whenever the board is powered on with a USB cable or battery.
If the board is in interactive mode, automatic execution of code.py is paused unless you click
Enter interactive mode using
The code inside code.py runs once whenever the board is reset. You can reset by:
- pressing the physical reset button
- unplugging and replugging the USB cable

Importing the `time` package

Python comes with several packages that need to be imported into programs. These packages can be:

Part of the so-called standard library. Standard library packages must be imported. These include packages such as random, math, calendar and time.
Available for download from the Python package index. These include packages such as numpy, for numerical computing, or matplotlib, for plotting.
Local, unregistered packages

import time
while True:
  print("one iteration")
  time.sleep(2.5) # sleep for 2.5 seconds

It is good practice to insert a time.sleep() command into every while True: loop to prevent the program from getting out of hand.

Objects in Python

The line from adafruit_circuitplayground.express import cpx imports the class ‘Express’ which is known as cpx.
A class is an object associated with certain attributes and methods.
- Methods are functions. e.g. the cpx class posssess the function cpx.play_tone(440,1)
- Attributes are variables that are ‘part of’ the class. e.g., the cpx class possess the following attributes
  - touched — this attribute is set internally based on which pins are being touched.
    >>> cpx.touched [board.A2, board.A1, board.A3]
  - cpx.red_led — this attribute can be set by the user
    >>> cpx.red_led False

`if` statements

In Python, if statements are followed by indented blocks of code, similar to while.

from adafruit_circuiytplayground.express import cpx
if cpx.light < 100:
    cpx.play_tone(440,2)
elif cpx.light < 200:
    cpx.play_tone(660,1)
else:
    cpx.play_tone(880,0.5)

No end is needed.
elif stands for “else, if”
The code following an if or elif statement must be a conditional
- Recall: Conditionals are either True or False.
- e.g., if it’s dark in the room, cpx.light < 100 will be True.

`if` `else` and `elif`

The following code is equivalent:

Using else followed by if:

if cpx.light < 100:
    cpx.play_tone(440,2)
else:
    if cpx.light < 200:
        cpx.play_tone(660,1)
    else:
        cpx.play_tone(880,0.5)

Using elif

if cpx.light < 100:
    cpx.play_tone(440,2)
elif cpx.light < 200:
    cpx.play_tone(660,1)
else:
    cpx.play_tone(880,0.5)

Multiple `elif`s in one block

Inside the same ‘block’ of code, you can have:

One if:
As many elif:s as you want
One else:

if cpx.light < 100:
    cpx.play_tone(440,2)
elif cpx.light < 200:
    cpx.play_tone(660,1)
elif cpx.light < 400:
    cpx.play_tone(770,0.75)
else:
    cpx.play_tone(880,0.5)

Nesting `if` statements

If statements can be nested indefinitely:

if cpx.light < 200:
  cpx.play_tone(440,1)
  if cpx.light < 150:
      if cpx.light < 100:
          cpx.play_tone(330,1)
          if cpx.light < 50:
              cpx.play_tone(220,1)

It’s up to you to check if your nested if statements make logical sense. For example, run and compare the following programs

Option A:

# First, check if light < 200
if cpx.light < 200:
    # If yes, then ...
    cpx.play_tone(440,1)
    # check if light < 150
    if cpx.light < 150:
        # if light is less than 150:
        cpx.play_tone(330,1)
    else:
        # if light is not less than 150:
        cpx.play_tone(330,2)
else:
    # if no, light is not less than 200:
    cpx.play_tone(440,2)

Option B:

# First, check if light < 200
if cpx.light < 200:
    # If yes, then ...
    cpx.play_tone(440,1)
    # check if light > 350
    if cpx.light > 350:
        # if light is more than 150:
        cpx.play_tone(330,1)
    else:
        # if light is not less than 150:
        cpx.play_tone(330,2)
else:
    # if no, light is not less than 200:
    cpx.play_tone(440,2)

In this example, lines 5-8 will never execute.

Printing Text

Print statements are commonly used in programs, especially for debugging purposes.
The print() function takes as argument a string.
In Python, strings are enclosed in single quotes or double quotes.
- "This is a string"
- 'This is also a string'

Try out this code and shine a flashlight:

while True:
  if cpx.light < 200:
    print("Light is less than 200")
  else:
    print("Light is more than 200")

Strings embedded with variables

We often want to embed variables into a Python string, before printing it.

Variables can be directly printed

print("the light sensor reads ",cpx.light," in some unknown units")

Variables can be embedded into a string by enclosing in {} and preceding the string with the letter f like so:
```
print(f"the light sensor reads {cpx.light} in some unknown units")
```
When using f-strings, the precision of numerical variables can be specified using the notation :.nf where n is the number of significant figures after the decimal point.
```
print(f"the light sensor reads {cpx.light:.1f} in some unknown units")
```
Write a program that prints the temperature to 2 decimal places at 1 second intervals

Loops and counter variables

Counter variables are used inside a loop to increment each time the loop is run.

A simple counter variable

k = 0
while True:
    print(f"The value of k is {k}")
    k = k + 1

In this example, k is a counter variable.

Counter variables can be conditional:

k = 0
while True:
  if cpx.light > 200:
    k = k + 1
  print(f"The value of k is {k}")

The `break` keyword terminates a loop

The break keyword terminates the lowest-level loop in which it is found, immediately.

Compare the following

Option A

k = 0
while True:
    k += 1
    print(f"iteration {k}")
    if k == 9:
        break
        print("breaking")

Option B

k = 0
while True:
    k += 1
    print(f"iteration {k}")
    if k == 9:
        print("breaking")
        break

When using nested loops, break terminates the innermost loop only.

from adafruit_circuitplayground.express import cpx
import time
k = 0
while True:
    time.sleep(1)
    k += 1
    print(f"Outer iteration {k}")
    if k % 3 == 0: # k ÷ 3 has remainder 0
        m = 0
        while True:
            time.sleep(1)
            m += 1
            print(f"Outer iteration {k}, inner iteration {m} ")
            if cpx.touch_A2:
                print("terminating")
                break

Modify this so that if pin A5 is touched, the outer loop terminates

The `continue` keyword

The continue keyword immediately exits the current iteration of a loop and moves on to the next one.

Run and compare the following code

Option A

from adafruit_circuitplayground.express import cpx
import time
k = 0
while True:
    time.sleep(1)
    k += 1
    print(f"Outer iteration {k}")
    if k % 3 == 0: # k ÷ 3 has remainder 0
        m = 0
        while True:
            time.sleep(1)
            if cpx.touch_A2:
                print("skip this iteration")
                continue
            m += 1
            print(f"Outer iteration {k}, inner iteration {m} ")

Option B

from adafruit_circuitplayground.express import cpx
import time
k = 0
while True:
    time.sleep(1)
    k += 1
    print(f"Outer iteration {k}")
    if k % 3 == 0: # k ÷ 3 has remainder 0
        m = 0
        while True:
            time.sleep(1)
            m += 1
            print(f"Outer iteration {k}, inner iteration {m} ")
            if cpx.touch_A2:
                print("skip this iteration")
                continue

In Option A, the inner loop is terminated upon touching A2 before the print statement, so line 16 is not executed if A2 is touched.
In Option B, the inner loop is terminated upon touching A2 after the print statement, so line 13 is executed even if A2 is touched.

The `pass` keyword

pass is used inside a loop (if, while, for, etc.) as a placeholder that doesn’t do anything.

while True:
  pass

The `for` loop

A for loop runs a specified number of times.

for i in range(5):
    print("Run once")

The iterating variable in a for loop has a different value each time the loop is run — like a built-in counter variable that you don’t need to increment.
```
for i in range(5):
    print(f"Run once. This time, the value of i is {i}")
```

A for loop iterates over an iterable object. These include:

List

for i in [2,99,3.5,"hello"]:
    print(i)

Tuple

for i in (2,99,3.5,"hello"):
    print(i)

Range
```
for i in range(2,15,3):
    print(i)
```
String
```
for i in "hello":
    print(i)
```

Like any other block of code, you can nest a for loop inside another for loop.
- A common use of nested loops in engineering is a ‘grid’ of variables
```
  for i in range(3):
      for j in range(5):
          print(f"i = {i} and j = {j}")
```

More features of the Circuit Playground Express

Switches and buttons:
- Button A cpx.button_a
- Button B cpx.button_b
- Sliding switch cpx.switch
Start and stop tone:
- cpx.start_tone(440) starts playing the tone 440 Hz.
- cpx.stop_tone() terminates whichever tone is being played.
The function cpx.pixels.fill() takes as argument a 3-element tuple and fills all pixels with that color.
- You can use this with cpx.pixels.fill((0,0,0)) to switch off all neopixels.
See the documentation for the Adafruit CircuitPlayground Library to see examples of how to use all of its features.

Conditional LEDs program

Write a program and save to code.py. Your program should:

Light up a blue pixel for as long as button A is pressed.
Light up a yellow pixel for as long as button B is pressed.
If a button is not being pressed, the corresponding light should be off
Print a statement about which button is currently pressed every x seconds, where x is a small number less than one.

Announcements

Running code on your Circuit Playground Express

Importing the time package

Objects in Python

if statements

if else and elif

Multiple elifs in one block

Nesting if statements