Lecture 06

E21 Computer Engineering Fundamentals

Announcements

• Test 1 is on Tuesday, Sep 22 8:30 — 8:55 AM in SCI 199
  • Based on Lectures 1 through 4 and HW 1 and 2
  • Handwritten, closed-book, closed-notes
  • No calculators allowed (future tests might allow them)

Precision + Accuracy of Accelerometer

Characterize the precision and accuracy of the on-board accelerometer. Find true value here.

Download statisticalfunctions.py and store on CIRCUITPY.

• Use the following skeleton code

  from adafruit_circuitplayground.express import cpx
  from statisticalfunctions import stdev
  import time
  
  def magnitude(a,b,c):
      return (a**2 + b**2 + c**2)**(1/2)
  
  # Create an empty list that will store the readings
  readings = []
  
  # Time delay between measurements
  delay = 0.1
  
  # Collect data until button b is pressed
  while True:
      # Your code here
  
  true_value = 9.806
  # Your code here

Your program should print, for example:

After 99 readings, the accelerometer reports 9.417 m/s^2 with standard deviation 0.254 and a spread of 17.4%

Dictionaries in Python

• A dictionary is a collection of key-value pairs.

• For example, we can define a dictionary with RGB values for common colors.

  colors = {
      "red": (50,0,0),
      "green": (0,50,0),
      "blue": (0,0,50)
      }

• To use this dictionary,

Constructing dictionaries

There are two equivalent approaches:

1. Curly braces followed by key: value pairs separated by commas:

   colors = {"red": (50,0,0),"green": (0,50,0),"blue": (0,0,50)}

2. The dict function

   colors = dict([('red', (50,0,0)), ('green', (0,50,0)), ('blue', (0,0,50))])

Note

Python allows optional new lines to make code more readable:

• Code all in one line:

  colors = dict([('red', (50,0,0)), ('green', (0,50,0)), ('blue', (0,0,50))])

• Code separated by new lines

  colors = dict(
      [
          ('red', (50,0,0)),
          ('green', (0,50,0)),
          ('blue', (0,0,50))
      ])

Writing a function to control NeoPixels

Write a function for the Circuit Playground Express that:

• takes three arguments:

  • a string denoting the color (choose 5 colors from https://rgbcolorpicker.com/)

  • a number from 0 to 9 denoting which pixel to switch on.

  • a number from 0 to 1 denoting the brightness

    cpx.pixels.brightness = 0.5 # sets the brightness to half

• makes use of a dictionary

from adafruit_circuitplayground.express import cpx

# Color definitions

mycolors = {
    "red":    (255,0,0),
    "green":  (0,255,0),
    "blue":   (0,0,255),
    "orange": (255,160,0),
    "teal":   (0,255,236)
    }

def lightup(colorname,pixelnum,b):
    color_tuple = mycolors[colorname]
    cpx.pixels[pixelnum] = color_tuple
    cpx.pixels.brightness=b
    return None

Debugging code using Thonny Code Editor

By pressing the button, Thonny enters debugging mode

More about Python lists + tuples

• A Python iterable, such as a list or tuple, can contain another iterable
• Python allows lists of lists, etc; MATLAB does not.

• Python allows chaining multiple [] commands:

>>> from adafruit_circuitplayground.express import cpx
>>> cpx.pixels
[(0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0)]
>>> cpx.pixels[4] = (45,50,2)
>>> cpx.pixels
[(0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (45, 50, 2), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0), (0, 0, 0)]
>>> cpx.pixels[4][1]
50
>>> 

Reviewing lists vs tuples

Why does this work:

cpx.pixels[6] = (40,0,0)

But this does not?

cpx.pixels[6][0] = 40

The Nested for loop over a list\(^*\) of lists\(^*\)

\(^*\) or any iterables (list, range, tuple)

• Consider the list of lists:

  a = [[5,6,7],
       [1,2,9],
       [4,8,3]]

• Write a for loop that prints every element in a

• Solution

  for j in a:
    print(j)

• Write a pair of nested for loops that print every element inside every element in a with the order: 5, 6, 7, …

• Solution

  for thing in a:
      for thing2 in thing:
          print(thing2)

Nested for loops using indices

We can print every element in a using

  for j in a:
    print(j)

• Another approach is to use the index:

  for k in range(len(a)):
    print(a[k])

• Use the indices to print every element inside every element in a:

• Solution

  for k in range(len(a)):
      element = a[k]
      for m in range(len(element)):
          entry = element[m]
          print(entry)

• Print every element inside every element in a without using indices, i.e. no [] notation.

• Solution

  for thing in a:
      for entry in thing:
          print(entry)

Two-level loops for raster scanning in TFWR

• ‘Raster scan’ refers to a systematic ‘sweeping’ of a two-dimensional rectangular area.

• Harvest grass with your drone using East-then-North raster scanning in three different ways:

  

  1. Without loops
  2. Using one for loop
  3. Using two for loops