data-science-notes/python.md

Python

Lesson Objectives

1. Print a message
2. Add a comment
3. Create a variable and assign it a value
4. Explain the different data types
5. Perform calculations with variables
6. Use string operations
7. Create a list
8. Access an element of a list
9. Perform a set of commands depending on a situation
10. Repeatedly perform a set of commands

Print a message

You can print a message to the user

print "hello!"

Add a comment

• Comments let you summarize what you're doing
• They don't get executed

# this will not be executed

Create a variable and assign it a value

a = "hello"
print a ##print the value of the variable 'a'

Explain the different data types

There are lots of different types of data that you can use in python

• String (text)
• Integers (whole numbers)
• Float (decimal numbers)
• Booleans (True/False)

You can convert one data type to another

a = str(1) #a = "1"
b = int("5") #b = 5
c = float(4) #c = 4.0
d = int(5.7) #d = 5

Perform calculations with variables

a = 1
b = a + 1 #b = 2
c = b * 3 #c = 6
d = c - 1 #d = 5
e = float(d) / 2 #e = 2.5
f = d ** 2 #exponent: f = 25

Use string operations

a = "first string"
b = "second string"
c = a + " " + b

Create a list

You can create lists of things

a = [1, 5, "some string", True, 5.6]

You can even have lists of lists

a = [
    [1, 2, 3], #first row
    [4, 5, 6], #second row
    [7, 8, 9], #third row
    [10] #fourth row
]

You can conceptualize an list of lists however you want

a = [
    [1, 4, 7, 10], #first column
    [2, 5, 8], #second column
    [3, 6, 9], #third column
]

Access an element of a list

Lists have elements stored at numerical indexes, starting at 0

a = [1, 5, "some string", True, 5.6]
print a[0] #1
print a[1] #5
print a[4] #5.6

Perform a set of commands depending on a situation

a = 22
if a < 10:
    print "a is less than 10"
elif a == 10:
    print "a is 10"
else:
    print "a is greater than 10"

The conditions can be

• < less than

• greater than

• <= less than or equal to

• = greater than or equal to

• == an exact match

You can also compare strings:

a = 'oh hai!'
if a == 'oh hai!':
    print 'works'

Repeatedly perform a set of commands

a = 10
while a < 20:
    print "the value of a is currently: " + str(a)
    a = a + 1