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# Basic Types<br>and Operations

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CS 65 // 2021-02-09

## Extra Credit Activities
- Songs for A New World
- Basketball game tomorrow at 6

## Assignment 2
- Due Thursday before class
- 
 Any questions?

# Questions
## ...about anything?

# TYPES

## Types in Programming Languages
- Basic values in algorithms usually have some **type**
    + e.g. bread, PB, preserves, in our PB&J algorithm
- A type can help us know **how to use** the values

## Five Questions
...to ask when encountering a new type

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1. What is its **name**?
2. What is its **purpose**?
3. How do I **express** values of it to the computer?
4. How does the computer **display** values of it to us?
5. What **operations** can be performed on it?

## Types in Python
`34`, `34.0`, and `"34"` have different *types* in Python

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+ 
 What is the type of `34`?
+ 
 What is the type of `34.0`?
+ 
 What is the type of `"34"`?

# INTEGER (int)

- **Purpose**:
    + Manipulating whole valued numbers
    + **Exact** number and can be arbitrary large or small
- **Expressed as**:
    + `34`
    + `-123`
- **Operations**:
    + `+`, `/`, `-`, `*`, ...

# FLOAT

- **Purpose**:
    + Manipulating fractional numbers
    + **Inexact** and accumulate errors over time
- **Expressed as**:
    + `34.0`
    + `-3.1415`
- **Operations**:
    + `+`, `/`, `-`, `*`, ...

# STRING (str)

- **Purpose**: 
    + Manipulating textual information
- **Expressed as**:
    + `"abc"`
    + `'abc'`
- **Operations**:
    + `print`, `input`, `+` (concatenate)

# EXPRESSIONS

## Expressions
- Expressions are any valid combination of basic values, operations, and variables that evaluate to a single value
    + `(5 + x) * 3`
- 
 Expressions are evaluated according to **order of operations** or **precedence**
    + Multiplication happens before addition, etc.

## Creating Variables
- To create (or modify) a variable, you can use an **assignment statement** which has the form:
    ```py
    # name = expression
    val = (3 + 4) / 2
    ```

- 
 First `(3 + 4) / 2` is evaluated to produce `3.5`
- 
 Then the variable `val` is created and points to `3.5`

## `input` and `print`
- When writing Python in **script mode**, it is useful to ask the user running the script a question or print a message to them
- 
 `input` is a function that requests keyboard input
- 
 `print` is a function that prints textual output

```py
print("Hello, there!")
name = input("What is your name? ")
print("The name you entered is", name)
```

# Self Checks

## A Complete Program

```py [|5|12|8|]
# A simple program illustrating chaotic behavior
#
# Author:
#   John Zelle, from Python Programming, Third Edition
def main():
    print("This program illustrates a chaotic function")
    x = float(input("Enter a number between 0 and 1: "))
    for i in range(10):
        x = 3.9 * x * (1 - x)
        print(x)

main()
```

# Lab Time