CPSC120A
Fundamentals of Computer Science

Day 20 Notes

Encryption

Reading Questions

Quiz

Ord & Chr

Encryption requires performing mathematical operations on text
Can’t perform mathematical operations on strings
For example, 'a' + 3 produces an error
But each character in a string is represented as a number in memory
Can convert a character to is numerical representation
Then perform mathematical operations
```
num = ord('a')
num = num + 3)
```

Then convert back to character

num = ord('a')
num = num + 3)
print(chr(num))

Can also use this to convert letters to numbers in the range 0-25
```
num = ord('c') - ord('a')
print(num)
```

String Module

The string module has a few variables that make classifying characters easy
The variable string.ascii_lowercase is "abcdefghijklmnopqrstuvwxyz"
The variable string.ascii_uppercase is "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
The variable string.digits is "0123456789"

So to test if a letter is upper case is:

import string
print('A' in string.ascii_uppercase)

Can also use this to convert letters to numbers in the range 0-25 using the find method
```
import string
print(string.ascii_uppercase.find('C'))
```

Lab