data-science-notes/python.md

# Python

## Lesson Objectives

1. Print a message
1. Add a comment
1. Create a variable and assign it a value
1. Explain the different data types
1. Perform calculations with variables
1. Use string operations
1. Create a list
1. Access an element of a list
1. Perform a set of commands depending on a situation
1. Get user input
1. Repeatedly perform a set of commands
1. Use a for loop
1. Create a class for an object

## Print a message

You can print a message to the user

```python
print "hello!"
```

## Add a comment

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

```python
# this will not be executed
```

## Create a variable and assign it a value

```python
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

```python
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

```python
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

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

## Create a list

You can create lists of things

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

You can even have lists of lists

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

You can conceptualize a list of lists however you want

### ACTIVITY

How would you change the previous example so that each inner list is a column?

## Access an element of a list

Lists have elements stored at numerical indexes, starting at 0

```python
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

```python
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:

```python
a = 'oh hai!'
if a == 'oh hai!':
    print 'this works'
```

## Get user input

You can get user input from the command like so:

```python
user_input = raw_input("Please enter something: ")
print "you entered: " + user_input
```

### ACTIVITY

Write a program that models this flow chart:

![where should I post that?](http://socialnewsdaily.com/wp-content/uploads/2013/04/where-do-i-post-it.jpg)

## Repeatedly perform a set of commands

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

### ACTIVITIES

1. Write a program that models this flow chart:

    ![pictionary](http://pics.blameitonthevoices.com/032011/how_to_play_pictionary.jpg)

1. Given the following list [70, 95, 97, 55, 3, 24, 89, 97, 84, 11]
    - Write a program that loops through each value in the list and prints it
    - Write a program that loops through each value in the list and adds them all together
    - Write a program that loops through each value in the list and prints the average
    - Write a program that loops through each value in the list and prints the minimum
    - Write a program that loops through each value in the list and prints the maximum
1. Combine all the programs from the previous step into one program that asks the user what operation they would like to do
1. Alter the last program so that it performs the operations for only numbers that are greater than a number specified by the user

## Use a for loop

The process of looping through an array can be simplified with a `for` loop:

```python
foods = ['hot dogs', 'beer', 'bald eagles']
for food in foods:
      print(food)
```

### ACTIVITIES

Simplify the last set of activities using a `for` loop

## Create a class for an object

You can use a `class` or blueprint for objects that you'll use

```python
class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def greet(self):
    print("Hello, my name is " + self.name + ". My age is " + str(self.age))

me = Person("Matt", 36)
me.greet()
sally = Person("Sally", 53)
sally.greet()
```

- `__init__` is a function that gets called when a new object is created.
- `self` is the object that's created