l2 - basic c elements_4

8/9/2019 L2 - Basic C Elements_4

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Basic C Elements

The building blocks of C….

Lecture Overview

Overview of

C lan ua e

Run Cycle

Basic C Elements

[ CS1010E AY1112S1 Le cture 2 ]2

C Language: A brief history

C Programming Language

Ori inall desi ned b 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

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)

Hi h Level Pro rammin Lan ua e Follow a set of rules (grammar) to express algorithm

General purpose language

mpera ve anguage



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Run Cycle for Compiled Language

Run Cycle:

The rocess of wri tin com i lin and

executing a program

Writing

Writing

• Tool: Editor

Compiling

Compiling

• Tool: Compiler

Executing

Executing

• Tool: None

• Produce:Source Code

• Produce:Executable

• Produce:Result

Compilation Error

Runtime Error


Environment and Tools in this Course Environment:

Cygwin: Emulate Linux (Unix-like) under

Windows

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


Run Cycle: Writing a Program

Use an editor to write a program following C

syntax

editor under

cygwin7

[ CS1010E AY1112S1 Le cture 2 ]

Run Cycle: Compiling the Source Code

Use a compiler to translate C Source Codeinto Machine Code Source code with syntax errors will fail to compile

Known as Compilation Errors

s ng 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

May terminate successfully but give the wrong result Logic Error

A successful

run o e an

executable


Run Cycle: A reminder

Run Cycle happens in the “ Implementation”

step of the problem solving process

You should have designed the before you attemptto write the program

Analysis

DesignWriting

Writing

• Tool: Editor

Compiling

Compiling

• Tool: Com i ler

Executing

Executing

• Tool: Nonemp emen a on

Testing

• Produce:Source Code

• Produce:Executable

• 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 Preprocessor

int main( )

rec ve

printf("Hello World!\n");Mainfunction function

return 0;

}

body

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C Elements: Preprocessor Directive Specific instructions for the preprocesor

(part of the compiler)

Y N

T A X

#directive_name [additional information]

#include XXXX

U S A G E Meaning:

Add the f ile “ XXXX” fo r compi lat ion

#include

We will introduce other directives when we

encounter them in future12



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C Elements: Preprocessor Directive Another commonly used directive

#define Name Value

G E

Meaning:

Substitute all occurrences of Name in the source

U S 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 them in 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)

A X

int main( )

{

0 or more statements

S Y N

return 0;

}

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

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

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C Elements: Statements

Statements form the major element of a C

program:

Basically represent a “step” in algorithm

In this course, we categorise the C

statements as:

1. Declaration Statements

2. Assignment Statements

3. Function Call Statements

4. Control Structure (Control Flow) Statements

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C Statement: Function Call Statement Function Call Statement:

Use (invoke) a function

Is a well defined program unit that performs a task

can be predefined (already written) or user defined more in lecture 3

X

S Y N T function_name ( [function_arguments] );

Function arguments:

Information needed by the function to perform

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The printf() function: Brief look

The printf( ) function is a:

redefined function rovided b 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)

Y N T A X

printf ( " message_to_be_shown " );

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C Statement: Control-Flow Statement

Control flow statement changes the defaultsequential execution of the program

There are a number of control flow statements we have a return statement in the sample

T A X

return value

S Y

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

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Programming Style: The role of Spaces White spaces:

space, empty line, tab

Generally ignored by compiler, as long as: You place them between elements in the program They are not part of a message (i.e. in between “ ” )

We use them to im rove ro ram readabilit

include

int main( ){

printf(“Hello World!\n”);return 0;}

,

but a lot harder to read.

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Ex ressions


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Problem: Area and Circumference of a Circle

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

c rcum erence o e correspon ng c rc e

1. Get the radius R from user

2. Area PI * R * R

3. Circumference 2 * PI * R

. r n e resu rea an rcum erence

,implement it at this point……

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C Statement: Declaration Statement Variable:

Storage for temporary result (data) during execution

A C variable is specified by: Identifier: Name of the variable

Data Type: Type of value stored in the variable

variable is like a “box” to store data

12345result

Identifier

Actual Value

what kind of value you

can find in the box22


C Statement: Declaration Statement Declaration Statement

Specify the information of a variable to the compiler

Variable must be declared before usage

datatype identifier ;

S Y N T A datatype identifier1, identifier2, ...;

datatype identifier = initial_value;

Data Type Desc ri pti on

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 onl once in a function

Must follow naming rule

Identifier Naming rule:

,

Cannot begin with a digit

meaning 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 ou fi ure out the meanin ?

double area;

,

double area = 0.0;

double area = 0.0, circumference = 1.0;

double 1area;

double area_of_circle;

double return;

double printf;

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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 o e o ow ng s y es cons s en y:

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 thedeclaration statements together before otherypes o s a emen s

int main( )

Y N T A X

{

[0 or more declaration statements]

[0 or more other statements]

Sreturn 0;

}

ou can eas y n ou a e var a e use

in a function at the top part of function in this

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C Element: Assignment Statement

Assignment Statement:

The main way to change the value stored in a

variable

X variable = value;

S Y N T

variable = expression;

Execution:

1. Evaluate calculate the Ri ht 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;

The expression can be quite complicated:

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

Let’s look at the expression in depth:

pera or

Precedence, Associative Rule and Bracket29


Assignment Statement: Operators

Binary Operators:

Works on two ar uments: ar 1 o ar 2

Example: +, -

* (multiplication)

/ (dividision)

mo u o, rema n er, on y or w o e num ers

Unary Operators:

or s on one argumen : op arg Example:

- (negative): -7.14

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Assignment Statement: Operators

Operators are ordered by precedence:

- -, - , -

% 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

Unar O erator: Ri ht Associative

x- - y x – ( - y)31


Assignment Statement: Brackets

Precedence and Associative rule can be

trick :

It is best to avoid them

You can either:

Break a com licated ex ression 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. . 123 123.0

Unsafe Conversion:

e.g. 123.45 123

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I O

scanf(): Predefined Function for Input

D E R #include

A X

H E

use the function

S Y N T scanf( "format_string" [, input_list] );

E x a m p l

scanf( "%d ", &integerVar );

Placeholder to

indicate whatdatatype to expect

Variable to store the

value entered by user.

from user .

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scanf(): Predefined Function for Input

Placeholder:

%d for inte er 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:

p l e

" "

E x a

, ,

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10/12

printf(): Predefined Function for Output

D E R #include

A X

H

E

S Y N T printf( "format_string " [, print_list] );

e

E x a m p l

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

Note the usage of placeholder in the format

s r n g

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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

p l e

" " *

E x a

, , , . .

Value stored inintVar will be

printed

esu o s

expression 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 rint out ‘”’ the double uote character

Try it out:

x a m

p l e

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

E

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1. Get the radius R from user

2. Area PI * R * R

3. Circumference

2 * PI * R.


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From Algorithm to Code

Convert the "Circle algorithm" to code:#include

.

int main( )

{

, ,

printf ( “Enter Radius = " );

scanf ( "%lf", &radius );

area = PI * radius * radius;

circumference = 2 * PI * radius;

rintf “The area is %lf Circumference is %lf n" , ,

area, circumference);

return 0;

}

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C Elements: Comments

You can add descriptive messages in the

code for ex lanation and extra information

A X

// a single line comment

S Y N * a multiple

lines comment

*/

Comments are for humans only ignored by compiler

You can also comment off a chunk of actual codeto hide them from compilation

program

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Programming Style: Useful Comments

We expect you to write useful comments in

our code:

Explain a piece of complicated logic

Ex lain the ur ose of a function more on thislater)

Don’t overdo it:

There is no need to comment on every line of

code

Excessive comment is actually bad programmings y e

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2. Another Exam le Pro ram

standard header ile preprocessor

directives

constant

comments

reserved

words

var a es

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 ex ressed

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

s

Declaration Statements

Assignment Statements

E l e m e n t

- arithmetic expressions

Control Flow Statements

- return statement

Function Call statements

g S t y l e

Use white space for readability

Place declarations before other type of

g r a m m i n

statements

Use meaningful identifiers

P r o

Use comments

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