# Python Programming - Getting Started

###### tags: `Python`, `VPython`

## Variables and Objects

Variables can be created anywhere in a Python program, the equal sign (`=`) is used to assign a value to a variable. The type of the variable is determined by the assignment statement.

a = 3                      # an integer
b = -2.4                   # a floating-point number
c = "ABC"                  # a string
Earth = sphere(pos=vector(0,0,0), radius=6.4e6) # a 3D VPython object
values = [a, b, c, 100, "abc", Earth]           # a list of values

Basic VPython objects such as sphere() and box() have a set of "attributes" such as color, and you can define additional attributes such as mass or velocity. Other objects, such as vector(), have built-in attributes but you cannot create additional attributes.

The value of a variable or object can be changed. If a variable is not “defined” (assigned a value), trying to use it will give you an error.

Run the following code:

print(n)     # try to access an undefined variable
x = 10
y = 2
print(x + y)
z = x / y    # copying the value of ( x / y ) to z
print(z)
x = x * z    # copying the value of ( x * z ) to x
y = y - z    # copying the value of ( y - z ) to y
print(x, y)  # print multiple objects, output results are delimited by a space

## String

In programming, we call a text **string**.

* Character
* A unit of a text.
* A letter, a numerical digit, or a symbol.
* String
* A series of characters.
* For example, `"Hello"` consists of five characters that are `'H'`, `'e'`, `'l'`, `'l'`, and `'o'`.
* String representation
* Single quotes
* `'ABC'`
* `'123456890'`
* Double quotes
* `"ABC"`
* `"1234567890"`
* No different between single quotes and double quotes

## Operators

### String Operators

| Operator | Meaning | Example |
| -------- | -------------------- | ------- |
| + | String concatenation | |
| += | String appending | |

### Arithmetic Operators

| Operator | Meaning | Example |
| -------- | -------------------------------- | ------- |
| + | Addition | x + y |
| - | Subtraction | x - y |
| * | Multiplication | x * y |
| / | Division | x / y |
| % | Modulus | x % y |
| ** | Exponent | x ** y |
| // | Floor division(integer division) | x // y |

Run the following code:

x = 11
y = 7
z = x % y
print(z)     # 4
z = y ** 2
print(z)     # 49
z = 2 ** 0.5
print(z)     # 1.4142135623730951
z = x / y
print(z)     # 1.5714285714285714
z = x // y
print(z)     # 1

## Data Conversion

| Usage | Meaning |
| ------------- | ------------------------------------------------------ |
| int(object) | Convert an object to an integer |
| float(object) | Convert an object to a floating number (a real number) |
| str(object) | Convert an object to a string |

## Reference

[Complete Documentation for VPython](https://www.glowscript.org/docs/VPythonDocs/index.html)

[The Python Tutorial](https://docs.python.org/3/tutorial/index.html)