Syntax

English example: "good morning hello you" is not syntactically valid. "good morning to you" is syntactically valid".

Programming language example: "hello"3 is not syntactically valid. 2*3 is syntactically valid.

Semantics

The meaning of the program. What is supposed to happen when the program executes.

Objects

Programs manipulate data objects
Objects have a type that defines the kinds of things programs can do to them:
- Example: Can add two numbers, but not a string and a number.
Objects are:
- Scalar (cannot be subdivided)
- Non-scalar (have internal structure that can be accessed)
Scalar Objects
- int : represent integers, e.g., 3
- float : represent real numbers, e.g., 1.23
- bool : represent boolean values, e.g., True and False
- NoneType : special and has one value, None
- can use type() to see the type of an object
```
>>> type(5)
int
>>> type(3.0)
float
```
Type conversions can convert object of one type to another
- float(1) converts integer 3 to float 3.0
- int(3.9) truncates float 3.9 to integer 3

Expressions

Combine objects and operators to form expressions:

an expression has a value which has a type.
syntax for simple expression: <object> <operator> <object>.

Operators on ints and floats

i+j sum
i-j difference
i*j product
i/j division
i%j remainder when i is divided by j
i**j i to the power of j.

Parentheses are used to tell Python to do these operations first. operator precedence without parentheses:

**
*
/
+ and - executed left to write, as appear in expression.

Equal sign is an assignment of a value to a variable

pi = 3.14159

Value is stored in computer memory. An assignment binds name to value. Retrive value associated with name or variable by invoking the name, by typing pi.

Abstracting expressions: give names to values of expressions, to reuse names instead of values. Example:

pi = 3.14159
radius = 2.2
area = pi * (radius ** 2)

Programming vs. Math: In programming, you do not "solve for x"

pi = 3.14159
radius = 2.2
# area of circle
area = pi*(radius**2)
radius = radius + 1

Notice that in an assignment, the expression on the right is evaluated to a value, and a variable name is on the left. radius += 1 is equivalent to radius = radius + 1.

Changing bindings: can re-bind variable names using new assignment statements. Previous value may still be stored in memory but lost the handle for it. e.g., radius -> 2.2 now becomes radius -> 3.2.