Lecture 2

Integer and String Value

Simple examples of values

What is the characteristic of integer and string value?

           All expressions in Python have a type. Sometimes it is denoted as its class. We can use a built-in function type to reveal the type of any Python expressions

Value Transformation

Value Addition

How to operate on those values correctly?

Variables

How to have the computer remember the value?

 Assignment

The del keyword stands for delete. The del removes a variable's definition from the current interpreter session or an executing Python program

Identifier

      Python has strict rules for variable names. A variable name is one example of an identifier. An identifier is a word used to name things. One of the things an identifier can name is a variable. We will see other identifiers such as functions, class, and methods in the future

The details of rules:

• An identifier must contain at least one character
• The first character of an identifier must be an alphabetic letter (upper or lower case) or underscore
• The remaining characters (if any) may be alphabetic letters (upper or lower case), underscores, or digits
• You can not use any reserved word to define your identifier

Reserved word

Note that Python is case sensitive.

What about built-in functions such as print, type, int, str?

Identifier and Build-in functions

Floating-point Number

Round and Truncation

Control Code

   In addition to "normal‘’ characters such as alphabets, digits, punctuations, etc. We may embed special characters known as control codes. The backslash(\) signifies that the character that follows it is a control code, not a literal letter. The backslash(\) is known as the escape symbol.

   For example, '\n' means the newline control code, '\t' stands for tab, '\b' for backspace, '\a' equals to alert control code.

Control Code Example

User Input

How to input values into the computer and have it output those values?

Controlling print function

How can we control the format of printing?

String Format

String Format

String Format

String Format

Muli-line String

How can we have a string with multiple lines?

Expression

What is an expression?

    An expression in a programming language is a syntactic entity that may be evaluated to determine its value. It is a combination of one or more constants, variables, functions, and operators that the programming language interprets and computes to produce another value. This process, for mathematical expressions, is called evaluation

Commonly Used Python Operators

Commonly Used Python Operators

Imprecision

Do you know how the computer store floating-point number?

What issue it may cause?

Is there any solution?

     Floating-point numbers are not real numbers, so the result of 1.0/3.0 cannot be represented exactly without infinite precession. In the decimal (base 10) number system, one-third is repeating fraction, so it has an infinite number of digits. Even simple nonrepeating decimal numbers can be a problem.

Mixed Expression

Can we operate with int and float together?

Operator Precedence and Associativity

Arithmetic Example

More Arithmetic Operators

Swap

Can we swap without the temporary variable?

Errors

Syntax Errors: When the interpreter detects an invalid program statement during the translation phase, it will terminate the program's execution and report syntax errors.

Run-time Errors: A syntactically correct Python program still can have problems. Some language errors happen only in certain run-time case.

Logic Errors: This is the error the interpreter can not detect. The errors happen because the logic written by programers is wrong. Such as dead-lock or infinite loop.

Syntex Errors

Run-time Errors