Notes on Python for Informatics Chapter 2: Variables, expressions and statements

Source

There are different value types. 

• type: A category of values. The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str).
• value: One of the basic units of data, like a number or string, that a program manipulates.

The type of a value can be retrieved with the type() built-in function.
Examples:

• int - integer number i.e. 1, 2
• float - floating point number i.e. 1.0, 1.187694652
• str - string text i.e. "1", "Hello World!"
• bool - boolean (either true or false) i.e. True, False (note the case!)

Explanation:

• string: A type that represents sequences of characters.
• integer: A type that represents whole numbers.
• floating-point: A type that represents numbers with fractional parts.

Read more on Python's Built-in Types 
Read more on Python's Built-in Functions

Variables represent values.

• variable: A name that refers to a value.
• assignment: A statement that assigns a value to a variable.

Examples:

• x = 1
• x = 1.0
• x = "one"
• x = True

It is generally a good idea to use mnemonic variable names that refer to the content of the variable.

• mnemonic: A memory aid. We often give variables mnemonic names to help us remember what is stored in the variable.

Rules regarding variable names:

• They have to begin with a letter (exception: variable names can start with an underscore character but this for is generally used for library code.)
• They can contain letters (both upper and lowercase) and numbers and the underscore character (_)
• They are case sensitive (it is suggested to use only lowercase letters in variable names) - just like everything else in Python
• They can be arbitrarily long

The reserved words (keywords) of Python cannot be assigned as variable names.

• keyword: A reserved word that is used by the compiler to parse a program.

Read the full list of Python's keywords

Comments can be made on the program code.

• comment: Information in a program that is meant for other programmers (or anyone reading the source code) and has no effect on the execution of the program.

A comment starts with the # character, and lasts until the end of the line.
Examples:

• # This whole line is a comment
• x = 1 # The part before the "#" will run as a code, but the rest is a comment.

Statements

• statement: A section of code that represents a command or action.

Examples:

• x = 3.5 # assignment statement
• print x # print statement

Operators and Operands

• operator: A special symbol that represents a simple computation like addition, multiplication, or string concatenation. 
• operand: One of the values on which an operator operates.

Different operations can be run on differen types of values.
Examples of numeric type operators:

• + performs addition
• - performs subtraction
• * performs multiplication
• / performs division (always in Python 3) or floor division depending on the value type (in Python 2)
• // performs always floor division
• % performs division and results the remainder of the division
• ** performs exponentiation

Explanation:

• floor division: The operation that divides two numbers and chops off the fraction part.
• modulus operator: An operator, denoted with a percent sign (%), that works on integers and yields the remainder when one number is divided by another.

Examples of sequence type operators:

• + performs concatenation

• concatenate: To join two operands end-to-end.  I.e. "2" + "3" concatenates to "23"

Read more on numeric types and their operators.
Read more on sequence types and their operators.

Expressions

• expression: A combination of variables, operators, and values that represents a single result value. 

Examples:

• x # if x already has a value assigned
• x + 1 # if x already has a value assigned
• 1
• 20 + 32 * (x - 1) / (x * 60 ) + x * x / 60 * 5**2 - ( 5 + 9 ) * ( 15 - 7 )

• evaluate: To simplify an expression by performing the operations in order to yield a single value.

Examples, taken that x = 5

• x # will yield 5
• x + 1 # will yield 6
• 1 # will yield 1
• 20 + 32 * (x - 1) / (x * 60 ) + x * x / 60 * 5**2 - ( 5 + 9 ) * ( 15 - 7 ) # will yield -92 because of the rules of precedence.

• rules of precedence: The set of rules governing the order in which expressions involving multiple operators and operands are evaluated.

For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

• Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even if it doesn’t change the result.
• Exponentiation has the next highest precedence, so 2**1+1 is 3, not 4, and 3*1**3 is 3, not 27.
• Multiplication and Division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So 2*3-1 is 5, not 4, and 6+4/2 is 8, not 5.
• Operators with the same precedence are evaluated from left to right. So in the expression 5-3-1 is 1, not 3 because the 5-3 happens first and then 1 is subtracted from 2.

User input

The user can be prompted to input data through the keyboard with the raw_input() built-in function.