Lecture 3 basic syntax and semantics

1 of 34Module 3 : Basic syntax and semantics Introduction to Computational Thinking Module 3 : Basic syntax and semantics Asst Prof Chi‐Wing FU, Philip Office: N4‐02c‐104 email: cwfu[at]ntu.edu.sg

2 of 34Module 3 : Basic syntax and semantics Topics • Semantics and Syntax • Let’s learn with a simple example • More on Python Syntax

3 of 34Module 3 : Basic syntax and semantics Semantics and Syntax When writing programs, you have to take care of • Semantics – Meaning of your program • Syntax – Specifying your algorithm using a programming language Problem Algorithm Program Run on Computational Thinking Programming Semantics Syntax

4 of 34Module 3 : Basic syntax and semantics Semantics and Syntax Just like communication with English: • The meaning (semantics) of the sentence, as well as • The grammar (syntax) of the sentence, so that others can understand, e.g., √ he has X he have √ we are X we is

5 of 34Module 3 : Basic syntax and semantics Semantics and Syntax • Different languages have different syntax; this applies to both spoken languages and computer programming languages • To make sure the Python shell can understand your Python program, your program has to follow the syntax of the Python language This module talks about basic Python syntax

6 of 34Module 3 : Basic syntax and semantics Topics • Semantics and Syntax • Let’s begin with a simple example • More on Python Syntax

7 of 34Module 3 : Basic syntax and semantics Code Listing 1.1 • To understand Python syntax, it is better to take a working Python program as an example:

8 of 34Module 3 : Basic syntax and semantics Terminology #1) Statement • Each line of code in a Python program is called a statement • Python interprets and runs statements one by one

9 of 34Module 3 : Basic syntax and semantics Statement Continuation • Python is sensitive to end of line in text files, which marks the end of a statement; in text editors, we press “enter” • The symbol is used to continue a statement with the next line so that two lines can be joined as a single statement (this is good for long statements… readability) Improve readability in a text editor

10 of 34Module 3 : Basic syntax and semantics Terminology #2) Comments • The pound sign # in Python indicates a comment • Anything after # is ignored for interpretation (in green) • Comments provide information to improve code readability Comment lines

11 of 34Module 3 : Basic syntax and semantics Terminology #3) Keywords • Python reserves certain words for specific purposes in the programming language itself, e.g., import, etc. (light blue) • You cannot use these words to define your own stuff; they are called the reserved words Light blue

12 of 34Module 3 : Basic syntax and semantics Terminology #4) Modules • A module is a Python file containing elements to help working on a certain problem, e.g., math (see above) -> math.pi • Modules are great resources provided by Python to perform various common tasks, e.g., database, network, etc. math module and dot operator

13 of 34Module 3 : Basic syntax and semantics Terminology #5) User Input • input is a built-in function provided by Python • Prints the message string on the screen and waits till the user types something (anything), ending with Enter • Returns a string (a sequence of characters) no matter what is given, even a number a function call to get input

14 of 34Module 3 : Basic syntax and semantics Terminology #6) Computation • Using the input radius, we can compute circumference and area • Note: = is not equal sign! It is an “assignment operator” in most programming languages to assign values to variables main computation variables

15 of 34Module 3 : Basic syntax and semantics Terminology #7) Print results • print is another built-in function provided by Python; it displays the related message and data on the Python shell screen (note: use comma to separate elements) • A single print() makes an empty line -> try several empty print()

16 of 34Module 3 : Basic syntax and semantics So altogether… this program? • There are three steps in the program… See the comments on top of the program!!!

17 of 34Module 3 : Basic syntax and semantics So you should know… Basics… • Statements and Statement Continuation • Comments with # • Keywords / Reserved words (e.g., import) • Modules (e.g., math) • Built-in functions (e.g., input and print) • Variable and Assignment operator = (more to come in next module)

18 of 34Module 3 : Basic syntax and semantics Topics • Semantics and Syntax • Let’s learn with a simple example • More on Python Syntax

19 of 34Module 3 : Basic syntax and semantics Let’s look at more syntax stuff • Comments • Whitespace • Indentation • Tokens: Keywords, Operators, Punctuators and Delimiters, Literals • Expressions • Interpreter Errors

20 of 34Module 3 : Basic syntax and semantics 1) Comments • Basics: Anything that follows # is ignored (by interpreter/compiler) on that statement • Though contributing nothing to the program execution, comments are important things to improve readability… • But… No universal rules for right style and number of comments

21 of 34Module 3 : Basic syntax and semantics 1) Comments • Useful guidelines: • Why philosophy: Good comments do not repeat the code or explain it. They should clarify the intention of the code and explain higher level concept • How philosophy: If your code contains a novel or noteworthy solution, add some comments to explain it

22 of 34Module 3 : Basic syntax and semantics 2) Whitespace • Purpose: to separate words in a statement • Python counts the following characters as white space: • Space, tab, return, etc. (see textbook) • For the most part, you can place “white space” (spaces) anywhere in your program; use it to make a program more readable, e.g., a = a + 1 + c instead of a=a+1+c math.asin( math.cos(a) + math.tan(b+c)*3 )

23 of 34Module 3 : Basic syntax and semantics • Leading whitespace at the beginning of a statement defines indentation, e.g., (will see more in module 6) 3) Indentation 1 level 2 levels Indentation:

24 of 34Module 3 : Basic syntax and semantics 3) Indentation • Purpose: - Python requires indentation for grouping, in particular for control flow: branching and looping (see module 6) - Make code more readable • Note: consistently use same number of spaces (see more in module 6)

25 of 34Module 3 : Basic syntax and semantics 4) Tokens • Tokens are special elements in a programming language (note: interpreter/compiler will first identify them when parsing each statement in a program, so that the interpreter/compiler can later understand the meaning of your statement) • In Python, we have four basic types of tokens: • Keywords • Operators • Punctuators and Delimiters • Literals

26 of 34Module 3 : Basic syntax and semantics and del from not while as elif global or with assert else if pass yield break except import print class in raise continue finally is return def for lambda try Keywords • Special words reserved in Python • Programmers (we) cannot use them to name things Note: “exec” removed in Python 3

27 of 34Module 3 : Basic syntax and semantics + - * ** / // % << >> & | ^ ~ < > <= >= == != Operators • Special characters (or sequence of characters) that imply certain operations, e.g., mathematical and logical. Note: < > removed in Python 3

28 of 34Module 3 : Basic syntax and semantics ‘ “ # _ ( ) [ ] { } @ , : . ` = ; += -= *= /= //= %= &= |= ^= >>= <<= **= Punctuators & Delimiters • Punctuators, also known as delimiters separate different types of elements in Python statements and expressions

29 of 34Module 3 : Basic syntax and semantics • Literals are fixed values used in a computer program, e.g., 123 and 3.14 are literals E.g., 1, 2, and 3 in the program above are literals How many numerical literals in program above? Literals

30 of 34Module 3 : Basic syntax and semantics 5) Expressions • Anything that produces/returns a value • Let’s say by combining values (e.g., literals, variables, etc.) and operations (e.g., operators, functions, etc.) • E.g., • 3.14 • 100 * 5 • result * 100 • 2 * math.pi * radius + float(input("input:")) • Note: • Interpreter ignores whitespaces (but helps readability) • In Python, statements do not return a value

31 of 34Module 3 : Basic syntax and semantics More: Side Effects and Returns • Make sure you get the difference: What is the difference between a side effect and a return? • 1 + 2 returns a value (it’s an expression). You can catch the return value. However, nothing else changed as a result • print("hello") doesn’t return anything, but something else, the side effect, did happen. Something printed on screen! • How about a=1+2?

32 of 34Module 3 : Basic syntax and semantics 6) Interpreter Errors • The interpreter translates Python code into machine language. The first stage of that process is determining whether it is valid or not • If the code is somehow malformed, Python cannot run your code and you get an interpreter error: Syntax error: Python cannot translate the code

33 of 34Module 3 : Basic syntax and semantics Take Home Messages • Semantics – Meaning of your program • Syntax – Specifying your algorithm using the programming language • This module is about terminologies and syntax: Statements, statement continuation, modules, comments, whitespace, indentation, tokens (keywords, operators, punctuators and delimiters, literals), functions, expression, interpreter errors • Side effects and returns (statement VS expression)

34 of 34Module 3 : Basic syntax and semantics Reading Assignment • Textbook Chapter 1: Beginnings 1.2 to 1.4

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Lecture 3 basic syntax and semantics