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

Topics
• Semantics and Syntax
• Let’s learn with a simple example
• More on Python Syntax

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

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

• 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

Topics
• Let’s begin with a simple example

Code Listing 1.1
• To understand Python syntax, it is better to
take a working Python program as an example:

Terminology #1) Statement
• Each line of code in a Python program is called a statement
• Python interprets and runs statements one by one

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

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

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

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

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

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

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()

So altogether… this program?
• There are three steps in the program…
See the comments on top of the program!!!

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)

Topics
• Let’s learn with a simple example

Let’s look at more syntax stuff
• Comments
• Whitespace
• Indentation
• Tokens: Keywords, Operators, Punctuators
and Delimiters, Literals
• Expressions
• Interpreter Errors

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

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

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 )

• 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:

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)

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

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

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

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

• 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

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

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?

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

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)

Reading Assignment
• Textbook
Chapter 1: Beginnings
1.2 to 1.4

Lecture 3 basic syntax and semantics

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