l2 - basic c elements

8/9/2019 L2 - Basic C Elements

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Lecture Overview

Overview of

C language

Run Cycle

Basic C Elements

Various Statements

Programming Styles

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C Language: A brief history

C Programming Language

Originally designed by Dennis Ritchie in 1972

The 1978 book “The C Programming Language”

by Brian Kernighan and Dennis Ritchie

Gave the first specification of the language Standardized:

in 1990: ANSI C ( ISO C, C89/C90 )

The most widespread version

Used in this course

Evolved:

in 1999: C99

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C Language: Characteristics

C Programming language is a:

Compiled language Program written in C (source code) must be compiled

into machine code (executable)

Using C Compiler

High Level Programming Language Follow a set of rules (grammar) to express algorithm

General purpose language

Imperative language



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Run Cycle for Compiled Language

Run Cycle:

The process of writing, compiling and

executing a program

Writing

• Tool: Editor

• Produce:Source Code

Compiling

• Tool: Compiler

• Produce:Executable

Executing

• Tool: None

• Produce:Result

Compilation Error

Runtime Error

Logic Error



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Environment and Tools in this Course Environment:

Cygwin: Emulate Linux (Unix-like) underWindows

Tools: Editor: vim, nano, etc (You only need one)

Compiler: gcc (GNU C Compiler)

Comments and Rationale:

Tools look primitive but very flexible and powerful

Explicitly show the run cycle step by step



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Run Cycle: Writing a Program

Use an editor to write a program following C

syntax

Using vim

editor under

cygwin7

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Run Cycle: Compiling the Source Code

Use a compiler to translate C Source Code

into Machine Code Source code with syntax errors will fail to compile

Known as Compilation Errors

Using gcc

compiler

under cygwin



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Run Cycle: Executing the Executable The executable can be directly executed

May terminate unexpectedly due to errors Runtime Error

May terminate successfully but give the wrong result Logic Error

A successful

run of the an

executable



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Run Cycle: A reminder

Run Cycle happens in the “Implementation”

step of the problem solving process

You should have designed the before you attempt

to write the program

Analysis

Design

Implementation

Testing

Writing

• Tool: Editor

• Produce:Source Code

Compiling

•Tool:Compiler

•Produce:Executable

Executing

•Tool: None

•Produce:Result

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Your very first C Program

The famous “Hello World!” program

Simple program to show a message on screen

#include

int main( )

{

printf("Hello World!\n");

return 0;

}

Preprocessor

Directive

Main

function functionbody

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C Elements: Preprocessor Directive Specific instructions for the preprocesor

(part of the compiler)

S Y N T A X

#directive_name [additional information]

U S A G E

#include XXXX

Meaning:

Add the file “ XXXX” for compilation

Example:#include

We will introduce other directives when we

encounter them in future12



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C Elements: Preprocessor Directive Another commonly used directive

U

S A G E

#define Name Value

Meaning:

Substitute all occurrences of Name in the source

code with Value

Example:

#define MAXIMUM 5000

This directive is useful when: There is a constant value used multiple time in the code

Allow easy modification in future

We will introduce other directives when we encounter themin future

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C Elements: The main Function The main function is the starting point of

your program Execution starts from here

a special case of function (lecture 3)

S Y N T A X

int main( ){

[0 or more statements]

return 0;

}

Caution: C is case sensit ive, e.g. “Main” is not the same as “main”

Pay attention to the braces: “( )” and “{ }”

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C Statement: Function Call Statement Function Call Statement:

Use (invoke) a function A Function:

Is a well defined program unit that performs a task

can be predefined (already written) or user defined

more in lecture 3

S Y N T A X

function_name ( [function_arguments] );

Function arguments:

Information needed by the function to performthe task

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The printf() function: Brief look

The printf( ) function is a:

predefined function provided by C standard

library

defined in the header file

This is why we have the “#include ” directive Purpose:

prepare and show a message on the output device

(usually your screen)

S Y N T A X

printf ( " message_to_be_shown " );

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C Statement: Control-Flow Statement

Control flow statement changes the default

sequential execution of the program There are a number of control flow statements

we have a return statement in the sampleprogram

Return Statement:

S Y N T A X

return value;

Terminates the current function Give (return) the value to the caller (user) of this function

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Variables and Arithmetic

Expressions


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Problem: Area and Circumference of a Circle

Problem Description: Given the radius R, calculate the area and

circumference of the corresponding circle

A simple algorithm:

However, our C knowledge is not enough toimplement it at this point……

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1. Get the radius R from user

2. Area PI * R * R

3. Circumference 2 * PI * R

4. Print the result Area and Circumference



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C Statement: Declaration Statement Declaration Statement

Specify the information of a variable to the compiler Variable must be declared before usage

S Y N T

A X

datatype identifier ;

datatype identifier1, identifier2, ...;

datatype identifier = initial_value;

Data Type Description

int Integer (whole number) with no fractional part

double Real number with fractional part

Other To be covered later in the course

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C Statement: Declaration Statement

Identifier :

Can be declared only once in a function

Must follow naming rule

Identifier Naming rule:

Consists of only letters, digits and underscores

Cannot begin with a digit

Cannot use keywords (words with specialmeaning in C)

E.g. return, int, double, … …

Should not redefine identifiers defined in C library

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C Statement: Declaration Statement Examples:

Check for validity Can you figure out the meaning?

double area;

double area, circumference;

double area = 0.0;

double area = 0.0, circumference = 1.0;

double 1area;

double area_of_circle;

double printf;

double return;

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Programming Style: Meaningful Identifiers

Additional requirement when naming

identifier for readability: The name should be short and meaningful

Identifier with multiple words should be avoided

If you must use identifier with multiple words, use

one of the following styles consistently:

Separate word with “_” (underscore)

E.g. radius_of_circle Capitalise the first letter of each word beyond the first:

E.g. radiusOfCircle

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Programming Style: Declare Together!

In a function, you should place all the

declaration statements together before othertypes of statements

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S Y N T A X

int main( )

{

[0 or more declaration statements][0 or more other statements]

return 0;

}

You can easily find out all the variable used

in a function at the top part of function in this

way



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C Element: Assignment Statement

Assignment Statement:

The main way to change the value stored in a

variable

S Y N

T A X variable = value;

variable = expression;

Execution:

1. Evaluate (calculate) the Right Hand Side(R.H.S) to a single value V

2. Place V in the variable on L.H.S.

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C Element: Assignment Statement

Simple Examples:

Assume variables are declared

area = 1.23;

result = result + 4;

result = x + y;

The expression can be quite complicated:

answer = (x + y) * 5 / 1.23;

Let’s look at the expression in depth:

Operator

Precedence, Associative Rule and Bracket 29[ CS1010E AY1112S1 Le cture 2 ]


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Assignment Statement: Operators

Operators are ordered by precedence:

+, - Unary +, -

% Remainder

*, / Multiplication, Division

+, - Addition, Subtraction

Operators with the same precedence are

ordered by associative rule

Binary Operator: Left Associative

a + b + c ( a + b ) + c

Unary Operator: Right Associative

x- - y x – ( - y)31



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Assignment Statement: Brackets

Precedence and Associative rule can be

tricky:

It is best to avoid them

You can either: Break a complicated expression into several

statements

OR, use brackets to avoid logical error

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Assignment Statement: Conversion Datatype conversion will happen automatically

during evaluation and assignment

Example: intVar = 1 / 2;

doubleVar = 1 / 2;

doubleVar = 1 / 2.0;

Safe Conversion: No information is lost during conversion

e.g. 123 123.0

Unsafe Conversion: Information is lost during conversion

e.g. 123.45 123

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Simple Input and Output

(I/O)


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scanf(): Predefined Function for Input

S Y N T A X

scanf( "format_string" [, input_list] );

H E A D E R #include

You need to include the above header file in order to

use the function

E x a m p l e

scanf( "%d ", &integerVar );

Placeholder to

indicate what

datatype to expect

from user

Variable to store the

value entered by user.

Note the (&) symbol.

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scanf(): Predefined Function for Input

Placeholder:

%d for integer value

%lf for double floating point value

Variable:

Must match the datatype specified by the placeholder Need a “&” symbol in front

Will be explained in lecture 6

A single scanf() can read in multiple values:

E x a m p l e

scanf( "%d%d ", &intVar1, &intVar2 );

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printf(): Predefined Function for Output

Note the usage of placeholder in the format

string

S Y

N T A X

printf( "format_string " [, print_list] );

H E A D E R #include

E x a m p l e

printf( "Answer is %d ", intVar );

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By using placeholder, we can show

computed values to the user: Value stored in a variable

Value evaluated by an expression

E x a m p l e

printf( "%d, %lf", intVar, 3.14 * 1.23);

Value stored inintVar will be

printed

Result of thisexpression will be

printed

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Special Control Characters:

also known as escape sequence

mainly used to control the movement of cursor

\n go to newline

\t jump to next tab

\\ to print out ‘\’

\” to print out ‘”’ (the double quote character)

Try it out:

E x a m p l e

printf( "To print \" you need to \\\"\n");

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From Algorithm to Code

Convert the "Circle algorithm" to code:#include #define PI 3.14159

int main( )

{

double radius, area, circumference;

printf ( “Enter Radius = " );

scanf ( "%lf", &radius );

area = PI * radius * radius;

circumference = 2 * PI * radius;

printf ( “The area is %lf, Circumference is %lf\n",area, circumference);

return 0;

}

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Programming Style: Useful Comments

We expect you to write useful comments in

your code:

Explain a piece of complicated logic

Explain the purpose of a function (more on this

later)

Don’t overdo it:

There is no need to comment on every line of

code

Excessive comment is actually bad programming

style

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2. Another Example Program

preprocessor

directives

standard header file

comments

constant

reserved

words

variables

standard identifiers

special

symbols

punctuations

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Problem 1: What time is it?

Problem statement:

Given a start time ST and end time ET, expressed

by the hour (0 to 23) and minute (0 to 59)

Show the total elapsed time (hour and minute)

between start time and end time

For simplicity:

You can assume that ST and ET are in the same

day, so ST is always smaller than ET

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Summary

C

E l e m e n t s

Declaration Statements

Assignment Statements

- arithmetic expressions

Control Flow Statements

- return statement

Function Call statements

P r o g r a m m i n g S t y l e

Use white space for readability

Place declarations before other type of

statements

Use meaningful identifiers

Use comments

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