# 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. Repeatedly perform a set of commands

## 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 an list of lists however you want

```python
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

```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 'works'
```

## Repeatedly perform a set of commands

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