CPSC120B - Day 28 Notes

To read from a file use built-in function to open it
```
file = open('data.txt', 'r')
```
The first parameter is the file name
The second is that we are going to read from the file (the os cares because depending on what you are doing it can be more or less efficient)
The file object is iterable
```
for line in file:
    print(line)
```
Note the extra blank lines that are printed

Try printing just the last character of each line

for line in file:
    print('"' + line[-1] + '"')

This is where strip is useful

for line in file:
    print(line.strip())

Also should close it when you are done

for line in file:
    print(line.strip())
file.close()

This releases system resources, if you forget your program will work but will waste memory

Can also get the files contents as a string using read
```
text = file.read()
```
Or can get it as a list of strings by line
```
list_of_text =  file.readlines()
```

Or read one line at a time

line_of_text = file.readline()
another_line_of_text = file.readline()

readline returns empty string when done reading the file, so can use it in a while loop

line = file.readline()
while len(line) != 0:
    line = file.readline()
    print(line.strip())

Writing is just as simple as reading
Just like read, need to open the file
```
file = open(‘data.txt’, ‘w’)
```
The ‘w’ parameter means write mode
Note, if the file does not exist, it is created
If it does exist, it overwrites it with a new file
To not overwrite, but to add to the end, open it in append mode
```
file = open(‘data.txt’, ‘a’)
```
To actually write, use the write method
```
file.write('text to write')
```
Note that it doesn’t write new line characters

Use the special text ‘’ to write new lines

file.write('text to write\n')
file.write(text + '\n')

Again, use close when done writing to save system resourses

file = open(‘data’, ‘w’)
for i in range(len(list_of_text)):
    file.write(list_of_text[i] + '\n')
file.close()

Text to write to a file is stored in a buffer to make it more efficient
If a file is not closed and program ends prematurely, the text may not be written to the file

LCD screens are made up of lots of red, green, and blue lights
Different combinations of red, green, and blue are interpreted by our brains as different colors
Use numbers to encode brightness, often 0-255 or 0.0-1.0
Example with an online color picker
To display an image on the screen, need to set the brightness of all of the lights that make up the screen, so many, many numbers
The PPM file format was originally created to solve operating system compatibility issuse when sending images over email.
The PPM file solved this by sending images as text
Encoding images as text is really inefficient, but it makes them easy for us to work with
PPM images start with the following lines:
```
P3
2 2
255
```
The ‘P3’ is called the magic number, its a number when it is converted to binary
It tells programs reading the file that it is a PPM file
The ‘2 2’ is the dimensions of the image, width then height
255 is the range of red, green, and blue, values, 0 - 255, the 0 is implied
After that, just specify red, green, blue, for each pixel in row order
```
P3
2 2
255
255
0
0
0
255
0
0
0
255
0
0
0
```
Can view the image in Ububu by double clicking the image
Need to zoom in because it is so small
Gimp can read and write PPM images
Find an image on google, open in Gimp, export to PPM, choose ASCII
Note that Gimp puts a comment at the top of the file, either need to delete this line in a text editor or adjust your code to skip the line

CPSC120B
Fundamentals of Computer Science