CS 111 w20 lecture 15 outline

1 Polls

s = "abc"
s = s[:1] + "MOOO" + s[1:]
print(s)

data = [[3, 1, 6],
        [2, 8, 9],
        [5, 5, 5]]
print(data[:2][-1][::-1][0])

2 2D Data

index multiple times to get to individual elements in a nested list
row-major order
don't really behave like a grid with rows and columns (see trying to slice a column)
- a module called numpy provides a very useful variation on a list called an array

3 Image data

how to represent color?
- printing on paper: CMYK (cyan, magenta, yellow, black)

digital: RGB (red, green, blue)
- on screens, tightly packed red, green, and blue elements light with different intensities

many different geometries are possible

Samsung Galaxy Note 4

Samsung Galaxy Nexus

Apple iPhone 5

in image files, divided up into grid, each cell is called a pixel, each pixel has a mix of red, green, and blue
- Color picker: https://www.google.com/search?q=color+picker

3.1 Represent an Image in Python (2x2 example)

3D list!

[[[1, 0, 0], [0, 1, 0]],
 [[0, 0, 1], [1, 0, 0]]]

4 Working With Images

import matplotlib.pyplot as plt
image = plt.imread("beach_portrait.png")

image data as an array
- what you see when you print
- 0 to 1 in RGB
- shape, size
- len, loops the same
- differences: fixed size, only one kind of data
array indexing
- row, column
- slicing along multiple dimensions
broadcast assignment

4.1 Fruit Ninja Redux

crop the image by 10 pixels on each side
slice out 10-by-10 box in bottom right corner
black out every other column