Introduction to C

Creating your first C program

Writing C Programs The programmer uses a text editor to

create or modify files containing C code.

C code is also called source code

A file containing source code is called a source file

After a source file has been created, the programmer must invoke the C compiler.

Invoking the Compiler Example

gcc pgm.c

where pgm.c is the source file that contains a program

The Result : a.out

If there are no errors in pgm.c, this command produces an executable file, one that can be run or executed.Both the cc and the gcc compilers name the

executable file a.outTo execute the program, type ./a.out at the

Unix prompt. Although we call this “compiling a program”,

what actually happens is more complicated.

3 Stages of Compilation

1 Preprocessor - modifies the source codeHandles preprocessor directives

Strips comments and whitespace from the code

3 Stages of Compilation

2 Compiler - translates the modified source code into object codeParser - checks for errors

Code Generator - makes the object code

Optimizer - may change the code to be more efficient

3 Stages of Compilation

3 Linker - combines the object code of our program with other object code to produce the executable file.

The other object code can come from:The Run-Time Library - a collection of object

code with an index so that the linker can find the appropriate code.

other object filesother libraries

Compilation Diagram

Editor

Compiler Preprocessor

ParserCode Generator Optimizer

Linker

<

Source File myprog.c

Object File myprog.oOther Obj’s Run-time library Other libraries

Executable file a.out

An algorithm for writing code

Write the algorithm Write the code using vi (pico, emacs) Try to compile the code While there are still syntax errors Fix errors Try to compile the code Test the program Fix any semantic errors Compile the code Test the program

Incremental Approach to Writing Code

Tips about writing code. Write your code in incomplete but working

pieces. For instance: For your project

Don’t write the whole program at once.Just write enough that you display the prompt to

the user on the screen.Get that part working first.Next write the part that gets the value from the

user, and then just print it out.

Incremental Approach to Writing Code (continued)

Get that working. Next change the code so that you use the

value in a calculation and print out the answer.

Get that working. Make program modifications:

○ perhaps additional instructions to the user○ a displayed program description for the user○ add more comments.

Get the final version working.

Structure of a ‘C’ program

pre-processor directives

global declarations

function prototypes

main()

{

local variables to function main ;

statements associated with function main ;

}

f1()

{

local variables to function 1 ;

statements associated with function 1 ;

}

f2()

{

local variables to function f2 ;

statements associated with function 2 ;

}.

A Simple C Program

/* Filename: hello.c Author: Brian Kernighan & Dennis Ritchie Date written: ?/?/1978 Description: This program prints the greeting

“Hello, World!” */

#include <stdio.h>

main ( ) { printf (“Hello, World!\n”) ; }

Anatomy of a C Program

program header comment

preprocessor directives

main ( ) { statement(s) }

The Program Header Comment

All comments must begin with the characters /* and end with the characters */

The program header comment always comes first

The program header comment should include the filename, author, date written and a description of the program

Preprocessor Directive

Lines that begin with a # are called preprocessor directives

The #include <stdio.h> directive causes the preprocessor to include a copy of the standard input/output header file stdio.h at this point in the code.

This header file was included because it contains information about the printf ( ) function that’s used in this program.

main ( )

Every program has a function called main, where execution begins

The parenthesis following main indicate to the compiler that it is a function.

Left Brace

A left curly brace -- { -- begins the body of every function. A corresponding right curly brace must end the function.

The style is to place these braces on separate lines in column 1.

printf (“Hello, World!\n”) ; This is the function printf ( ) being called

with a single argument, namely the string “Hello, World!\n”

Even though a string may contain many characters, the string itself should be thought of as a single quantity.

Notice that this line ends with a semicolon. All statements in C end with a semicolon.

Right Brace

This right curly brace -- } --matches the left curly brace above.

It ends the function main ( ).

Common variable types

• int - stores integers (-32767 to +32768)

• unsigned int – 0 to 65535

• char – holds 1 byte of data (-127 to 128)

• unsigned char – holds 1 byte (0 to +255)

• long – usually double int (signed)

• unsigned long – positive double int

• float – floating point variable

• double – twice of a floating point variable

• Note: local, global and static variables

scanf

scanf (“conversion specifier”,variable);

scanf (“%d”,&i);

Usual variable type Display:

%c char single character

%d (%i) int signed integer

%f float or double signed decimal

printf

printf (“%d”,i);

Usual variable type Display

%c char single character

%d int

%i signed integer

%e (%E) float or double exponential format

%f float or double signed decimal

%g (%G) float or double use %f or %e as required

%o int unsigned octal value

%p pointer address stored in pointer

%s array of char sequence of characters

%u int unsigned decimal

%x (%X) int unsigned hex value

Operators

+ addition- subtraction* multiplication/ division% mod or remainder (e.g., 2%3 is 2), also called 'modulo'<< left-shift (e.g., i<<j is i shifted to the left by j bits)>> right-shift& bitwise AND| bitwise OR^ bitwise exclusive-OR&& logical AND (returns 1 if both operands are non-zero; else 0)|| logical OR (returns 1 if either operand is non-zero; else 0)< less than (e.g., i<j returns 1 if i is less than j)> greater than<= less than or equal>= greater than or equal== equals!= does not equal

Operators (contd.)

Increment and Decrement Operators

• ++ Increment operator

• -- Decrement Operator

• k++ or k-- (Post-increment/decrement)

k = 5;

x = k++; // sets x to 5, then increments k to 6

• ++k or --k (Pre-increment/decrement)

k = 5;

x = ++k; // increments k to 6 and then sets x to the

// resulting value, i.e., to 6

Example – total cost calculator

#include <stdio.h>

main()

{

float cost =0.0;

float tax = 0.08;

float totalCost = 0.0;

printf(“Enter the cost of the item:”);

scanf(“%f”, &cost);

totalCost = cost *tax + cost;

printf(“Total cost is : %f\n”, totalCost);

}

Exercises

Extra Credit – Write a volume and surface area calculator

Functions and Prototypes

• Building blocks of code

• Group relevant and repetitive actions into

functions

• Variables are usually local (they exist only inside the function)

type FunctionName(type declared parameter list)

{

statements that make up the function

}

Example function

#include <stdio.h>

int add1(int);

void main()

{

int x;

x=5;

x=add1(x);

printf("%d“,x);

}

int add1(int i)

{

int y;

y=i+1;

return (y);

}

Returns an integer result

Expects an integer parameter

Conditional clauses

if (expression) statement

else statement

if (x==1)

{

printf(“Hello”);

printf (“hi\n”);

}

else

{

printf(“World’);

}

Conditional Loops

while (expression) statement

while (x<1000) x++;

while (x<1000)

{

x++;

y=y+10;

}

Conditional Loops

do statement while (expression)

x = 0;

do

{

x++;

} while (x<1000);

Unconditional Loops

for( initialization; expression; increment )

statement

for (i=0;i<100;i++)

{

printf("Hi.");

}

The switch statement

The C switch allows multiple choice of a selection of items

at one level of a conditional where it is a far neater way

of writing multiple if statements

switch (expression)

{ case item1:

statement1;

break;

case item2:

statement2;

break;

case itemn:

statementn;

break;

default:

statement;

break;

}

Arrays

Declaration:int x[10];

double y[15];

x[5]=10;

char c[15];

Note arrays start with 0 element

Pointers

• A pointer in C is the address of something

• The unary operator `&' is used to produce

the address of an object

int a, *b, c; // b is a pointer

b = &a; // Store in b the address of a

c = *b; // Store in c the value at

// address b (i.e., a)

• Pointers to pointers

• Pointers to functions

Character Pointers & Arrays

char *y;

char x[100];

y = &x[0];

y = x; // Does the same as the line above

*(y+1) gives x[1]

*(y+i) gives x[i]

Structures

• User defined ‘data type’

struct emprec

{

char name[25];

int age;

int pay;

};

struct emprec employee;

employee.age=32;

Nice to have

• Add plenty of comments /* */ or //

• Good layout.

main(){printf("Hello World\n");}

• Use meaningful variable names

• Use brackets to avoid confusion

a=(10.0 + 2.0) * (5.0 - 6.0) / 2.0

• Initialize your variables

Good Programming Practices

C programming standards are available on the Web

These standards include:Naming conventionsUse of whitespaceUse of BracesComments

Examples of Comment Styles /* a comment */ /*** another comment ***/ /*****/ /*A comment can be written in this * fashion to set it off from the * surrounding code. */

More Comments

/*******************************************\ * If you wish, you can put comments * * in a box. This is typically used for * * program header comments and for * * function header comments * \*******************************************/

Use of Whitespace

Use blank lines to separate major parts of a source file or function

Separate declarations from executable statements with a blank line

Preprocessor directives, main(), and the braces are in column 1

Use of Whitespace(continued)

All executable statements are indented one tab stop. How deep should my tabs be ?

Either 3 or 4 spaces. Choose whichever you like and use that same number consistently throughout your program.

2 spaces are not enough for good readability, more than 4 causes the indentation to be too deep.