Chapter: Computer Programming

C Programming Basics

Problem formulation – Problem Solving - Introduction to ‘ C’ programming –fundamentals – structure of a ‘C’ program – compilation and linking processes – Constants, Variables – Data Types –Expressions using operators in ‘C’ – Managing Input and Output operations – Decision Making and Branching – Looping statements – solving simple scientific and statistical problems.

C PROGRAMMING BASICS

INTRODUCTION TO C

As a programming language, C is rather like Pascal or Fortran.. Values are stored in variables. Programs are structured by defining and calling functions. Program flow is controlled using loops, if statements and function calls. Input and output can be directed to the terminal or to files. Related data can be stored together in arrays or structures.

Of the three languages, C allows the most precise control of input and output. C is also rather more terse than Fortran or Pascal. This can result in short efficient programs, where the programmer has made wise use of C's range of powerful operators. It also allows the programmer to produce programs which are impossible to understand. Programmers who are familiar with the use of pointers (or indirect addressing, to use the correct term) will welcome the ease of use compared with some other languages. Undisciplined use of pointers can lead to errors which are very hard to trace. This course only deals with the simplest applications of pointers.

A Simple Program

The following program is written in the C programming language.

#include <stdio.h> main()

{

printf("Programming in C is easy.\n");

}

BASIC STRUCTURE OF C PROGRAMS

C programs are essentially constructed in the following manner, as a number of well defined sections.

/* HEADER SECTION */ /* Contains name, author, revision number*/

/* INCLUDE SECTION */

/* contains #include statements */

/* CONSTANTS AND TYPES SECTION */

/* contains types and #defines

/* GLOBAL VARIABLES SECTION */

/* any global variables declared here

/* FUNCTIONS SECTION */

/* user defined functions

/* main() SECTION */

intmain()

{

}

CONSTANTS

A constant is an entity that doesn’t change whereas a variable is an entity that may change.

Types of C Constants

C constants can be divided into two major categories:

(a)Primary Constants

(b) Secondary Constants

Rules for Constructing Integer Constants

(a) An integer constant must have at least one digit.

(b) It must not have a decimal point.

(d) If no sign precedes an integer constant it is assumed to be positive.

(e) No commas or blanks are allowed within an integer constant.

(f) The allowable range for integer constants is -32768 to 32767.

Truly speaking the range of an Integer constant depends upon the compiler. For a 16-bit compiler like Turbo C or Turbo C++ the range is –32768 to 32767. For a 32-bit compiler the range would be even greater. Question like what exactly do you mean by a 16-bit or a 32-bit compiler, what range of an Integer constant has to do with the type of compiler and such questions are discussed in detail in Chapter 16. Till that time it would be assumed that we are working with a 16-bit compiler.

Ex.: 426 +782 -8000 -7605

Rules for Constructing Real Constants

Real constants are often called Floating Point constants. The real constants could be written in two forms—Fractional form and Exponential form.

Following rules must be observed while constructing real constants expressed in fractional form: (a)A real constant must have at least one digit.

(b)It must have a decimal point.

(c)It could be either positive or negative. (d)Default sign is positive.

(e)No commas or blanks are allowed within a real constant. Ex.: +325.34

426.0 -32.76 -48.5792

The exponential form of representation of real constants is usually used if the value of the constant is either too small or too large. It however doesn’t restrict us in any way from using exponential form of representation for other real constants.

Rules for Constructing Character Constants

A character constant is a single alphabet, a single digit or a single special symbol enclosed within single inverted commas. Both the inverted commas should point to the left. For example, ’A’ is a valid character constant whereas ‘A’ is not.

The maximum length of a character constant can be 1 character. Ex.: 'A'

'I'

'5'

'='

VARIABLES

User defined variables must be declared before they can be used in a program. Variables must begin with a character or underscore, and may be followed by any combination of characters, underscores, or the digits 0 - 9.

LOCAL AND GLOBAL VARIABLES

Local

These variables only exist inside the specific function that creates them. They are unknown to other functions and to the main program. As such, they are normally implemented using a stack. Local variables cease to exist once the function that created them is completed. They are recreated each time a function is executed or called.

Global

These variables can be accessed (ie known) by any function comprising the program. They are implemented by associating memory locations with variable names. They do not get recreated if the function is recalled.

Defining Global Variables

/* Demonstrating Global variables */

Example:

#include <stdio.h>

int add_numbers( void ); /* ANSI function prototype */

/* These are global variables and can be accessed by functions from this point on */ int value1, value2, value3;

int add_numbers( void )

{

auto int result;

result = value1 + value2 + value3; return result;

}

main()

{

auto int result;

result = add_numbers();

printf("The sum of %d + %d + %d is %d\n", value1, value2, value3, final_result);

}

The scope of global variables can be restricted by carefully placing the declaration. They are visible from the declaration until the end of the current source file.

Example: #include <stdio.h>

void no_access( void ); /* ANSI function prototype */ void all_access( void ); static int n2;/* n2 is known from this point onwards */

void no_access( void )

{

n1 = 10; /* illegal, n1 not yet known */ n2 = 5; /* valid */

}

static int n1; /* n1 is known from this point onwards */

void all_access( void )

{

n1 = 10; /* valid */

n2 = 3; /* valid */

}

AUTOMATIC AND STATIC VARIABLES

C programs have a number of segments (or areas) where data is located. These segments are typically, _DATA Static data _BSS Uninitialized static data, zeroed out before call to main() _STACK Automatic data, resides on stack frame, thus local to functions _CONST Constant data, using the ANSI C keyword const

The use of the appropriate keyword allows correct placement of the variable onto the desired data segment.

Example:

/* example program illustrates difference between static and automatic variables */ #include <stdio.h>

void demo( void ); /* ANSI function prototypes */ void demo( void )

{

auto int avar = 0; static int svar = 0;

printf("auto = %d, static = %d\n", avar, svar); ++avar; ++svar;

}

main()

{

int i;

while( i < 3 ) { demo();

i++;

}

Automatic and Static Variables

Example:

/* example program illustrates difference between static and automatic variables */ #include <stdio.h>

void demo( void ); /* ANSI function prototypes */

void demo( void ) {

auto int avar = 0; static int svar = 0; printf("auto = %d, static = %d\n", avar, svar); ++avar; ++svar;

}

main()

{

int i;

while( i < 3 ) { demo();

i++;

}

Program output

auto = 0, static = 0

auto = 0, static = 1

auto = 0, static = 2

The basic format for declaring variables is

data_type var, var, ... ;

where data_type is one of the four basic types, an integer, character, float, or double type.

Static variables are created and initialized once, on the first call to the function. Subsequent calls to the function do not recreate or re-initialize the static variable. When the function terminates, the variable still exists on the _DATA segment, but cannot be accessed by outside functions. Automatic variables are the opposite. They are created and re-initialized on each entry to the function. They disappear (are de-allocated) when the function terminates. They are created on the _STACK segment.

DATA TYPES

The four basic data types are

INTEGER

These are whole numbers, both positive and negative. Unsigned integers (positive values only) are supported. In addition, there are short and long integers.

The keyword used to define integers is,

int

An example of an integer value is 32. An example of declaring an integer variable called sum is, int sum;

sum = 20;

FLOATING POINT

These are numbers which contain fractional parts, both positive and negative. The keyword used to define float variables is,

float

An example of a float value is 34.12. An example of declaring a float variable called money is,

float money; money = 0.12;

DOUBLE

These are exponetional numbers, both positive and negative. The keyword used to define double variables

is,

double

An example of a double value is 3.0E2. An example of declaring a double variable called big is,

double big; big = 312E+7;

CHARACTER

These are single characters. The keyword used to define character variables is,

char

An example of a character value is the letter A. An example of declaring a character variable called letter

is,

Char letter; letter = 'A';

Sample program illustrating each data type

Example:

#include < stdio.h > main()

{

int sum;

float money;

char letter;

double pi;

sum = 10; /* assign integer value */

money = 2.21; /* assign float value */

letter = 'A'; /* assign character value */

pi = 2.01E6; /* assign a double value */

printf("value of sum = %d\n", sum );

printf("value of money = %f\n", money ); printf("value of letter = %c\n", letter ); printf("value of pi = %e\n", pi );

}

Sample program output value of sum = 10

value of money = 2.210000 value of letter = A

value of pi = 2.010000e+06

INITIALISING DATA VARIABLES AT DECLARATION TIME

In C variables may be initialised with a value when they are declared. Consider the following declaration, which declares an integer variable count which is initialised to 10.

int count = 10;

SIMPLE ASSIGNMENT OF VALUES TO VARIABLES

The = operator is used to assign values to data variables. Consider the following statement, which assigns the value 32 an integer variable count, and the letter A to the character variable letter

count = 32; letter = 'A'

Variable Formatters

%d decimal integer

%c character

%s string or character array

%f float

HEADER FILES

Header files contain definitions of functions and variables which can be incorporated into any C program by using the pre-processor #include statement. Standard header files are provided with each compiler, and cover a range of areas, string handling, mathematical, data conversion, printing and reading of variables.

To use any of the standard functions, the appropriate header file should be included. This is done at the beginning of the C source file. For example, to use the function printf() in a program, the line

#include <stdio.h>

should be at the beginning of the source file, because the definition for printf() is found in the file stdio.h All header files have the extension .h and generally reside in the /include subdirectory.

#include <stdio.h> #include "mydecls.h"

The use of angle brackets <> informs the compiler to search the compilers include directory for the specified file. The use of the double quotes "" around the filename inform the compiler to search in the current directory for the specified file.

OPERATORS AND EXPRESSIONS

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

ARITHMETIC OPERATORS:

The symbols of the arithmetic operators are:-

Operation Operator Comment Value of Sum before Value of sum

after

Multiply * sum = sum * 2; 4 8

Divide / sum = sum / 2; 4 2

Addition + sum = sum + 2; 4 6

Subtraction - sum = sum -2; 4 2

Increment ++ ++sum; 4 5

Decrement -- --sum; 4 3

Modulus % sum = sum % 3; 4 1

Example: #include <stdio.h> main()

{

int sum = 50; float modulus;

modulus = sum % 10;

printf("The %% of %d by 10 is %f\n", sum, modulus);

}

PRE/POST INCREMENT/DECREMENT OPERATORS

PRE means do the operation first followed by any assignment operation. POST means do the operation after any assignment operation. Consider the following statements

++count; /* PRE Increment, means add one to count */

count++; /* POST Increment, means add one to count */

Example:

#include <stdio.h> main()

{

int count = 0, loop;

loop = ++count; /* same as count = count + 1; loop = count; */ printf("loop = %d, count = %d\n", loop, count);

loop = count++; /* same as loop = count; count = count + 1; */ printf("loop = %d, count = %d\n", loop, count);

}

If the operator precedes (is on the left hand side) of the variable, the operation is performed first, so the statement loop = ++count;

really means increment count first, then assign the new value of count to loop.

THE RELATIONAL OPERATORS

These allow the comparison of two or more variables.

== equal to != not equal

< less than

<= less than or equal to

> greater than

>= greater than or equal to

Example:

#include <stdio.h>

main() /* Program introduces the for statement, counts to ten */

{

int count;

for( count = 1; count <= 10; count = count + 1 ) printf("%d ", count );

printf("\n");

}

RELATIONALS (AND, NOT, OR, EOR)

Combining more than one condition

These allow the testing of more than one condition as part of selection statements. The symbols are

LOGICAL AND &&

Logical and requires all conditions to evaluate as TRUE (non-zero).

LOGICAL OR ||

Logical or will be executed if any ONE of the conditions is TRUE (non-zero).

LOGICAL NOT !

logical not negates (changes from TRUE to FALSE, vsvs) a condition.

LOGICAL EOR ^

Logical eor will be excuted if either condition is TRUE, but NOT if they are all true.

Example:

The following program uses an if statement with logical AND to validate the users input to be in the range 1-10.

#include <stdio.h> main()

{

int number; int valid = 0;

while( valid == 0 )

{

printf("Enter a number between 1 and 10 -->"); scanf("%d", &number); if( (number < 1 ) || (number > 10) )

{

printf("Number is outside range 1-10. Please re-enter\n"); valid = 0;

}

else valid = 1;

}

printf("The number is %d\n", number );

}

Example:

NEGATION

#include <stdio.h> main()

{

int flag = 0; if( ! flag )

{

printf("The flag is not set.\n"); flag = ! flag;

}

printf("The value of flag is %d\n", flag);

}

Example:

Consider where a value is to be inputted from the user, and checked for validity to be within a certain range, lets say between the integer values 1 and 100.

#include <stdio.h> main()

{

int number; int valid = 0;

while( valid == 0 ) {

printf("Enter a number between 1 and 100"); scanf("%d", &number ); if( (number < 1) || (number > 100) )

printf("Number is outside legal range\n"); else

valid = 1;

}

printf("Number is %d\n", number );

}

THE CONDITIONAL EXPRESSION OPERATOR or TERNARY OPERATOR

This conditional expression operator takes THREE operators. The two symbols used to denote this operator are the ? and the :. The first operand is placed before the ?, the second operand between the ? and the :, and the third after the :. The general format is,

condition ? expression1 : expression2

If the result of condition is TRUE ( non-zero ), expression1 is evaluated and the result of the evaluation becomes the result of the operation. If the condition is FALSE (zero), then expression2 is evaluated and its result becomes the result of the operation. An example will help,

s = ( x < 0 ) ? -1 : x * x;

If x is less than zero then s = -1

If x is greater than zero then s = x * x

Example:

#include <stdio.h> main()

{

int input;

printf("I will tell you if the number is positive, negative or zero!"\n"); printf("please enter your number now--->");

scanf("%d", &input );

(input < 0) ? printf("negative\n") : ((input > 0) ? printf("positive\n") : printf("zero\n"));

}

BIT OPERATIONS

C has the advantage of direct bit manipulation and the operations available are,

Operation Operator Comment Value of Sum before Value of

sum after

AND & sum = sum & 2; 4 0

OR | sum = sum | 2; 4 6

Exclusive OR ^ sum = sum ^ 2; 4 6

1's Complement ~ sum = ~sum; 4 -5

Left Shift << sum = sum << 2; 4 16

Right Shift >> sum = sum >> 2; 4 0

Example:

/* Example program illustrating << and >> */

#include <stdio.h> main()

{

int n1 = 10, n2 = 20, i = 0;

i = n2 << 4; /* n2 shifted left four times */ printf("%d\n", i);

i = n1 >> 5; /* n1 shifted right five times */ printf("%d\n", i);

}

Example:

/* Example program using EOR operator */ #include <stdio.h>

main()

{

int value1 = 2, value2 = 4; value1 ^= value2;

value2 ^= value1; value1 ^= value2;

printf("Value1 = %d, Value2 = %d\n", value1, value2);

}

Example:

/* Example program using AND operator */ #include <stdio.h>

main()

{

int loop;

for( loop = 'A'; loop <= 'Z'; loop++ )

printf("Loop = %c, AND 0xdf = %c\n", loop, loop & 0xdf);

}

MANAGING INPUT AND OUTPUT OPERATORS

Printf ():

printf() is actually a function (procedure) in C that is used for printing variables and text. Where text appears in double quotes "", it is printed without modification. There are some exceptions however. This has to do with the \ and % characters. These characters are modifier's, and for the present the \ followed by the n character represents a newline character.

Example:

#include <stdio.h> main()

{

printf("Programming in C is easy.\n"); printf("And so is Pascal.\n");

}

@ Programming in C is easy.

And so is Pascal.

FORMATTERS for printf are, Cursor Control Formatters

Scanf ():

Scanf () is a function in C which allows the programmer to accept input from a keyboard.

Example: #include <stdio.h>

main() /* program which introduces keyboard input */

{

int number;

printf("Type in a number \n");

scanf("%d", &number);

printf("The number you typed was %d\n", number);

}

FORMATTERS FOR scanf()

The following characters, after the % character, in a scanf argument, have the following effect.

d read a decimal integer

o read an octal value

x read a hexadecimal value

h read a short integer

l read a long integer

f read a float value

e read a double value

c read a single character

s read a sequence of characters

[...] Read a character string. The characters inside the brackets

Accepting Single Characters From The Keyboard

Getchar, Putchar

getchar() gets a single character from the keyboard, and putchar() writes a single character from the keyboard.

Example:

The following program illustrates this,

#include <stdio.h> main()

{

int i; int ch;

for( i = 1; i<= 5; ++i )

{

ch = getchar(); putchar(ch);

}

The program reads five characters (one for each iteration of the for loop) from the keyboard. Note that getchar() gets a single character from the keyboard, and putchar() writes a single character (in this case, ch) to the console screen.

DECISION MAKING

IF STATEMENTS

The if statements allows branching (decision making) depending upon the value or state of variables. This allows statements to be executed or skipped, depending upon decisions. The basic format is,

if( expression ) program statement;

Example:

if( students < 65 ) ++student_count;

In the above example, the variable student_count is incremented by one only if the value of the integer variable students is less than 65.

The following program uses an if statement to validate the users input to be in the range 1-10.

Example:

#include <stdio.h> main()

{

int number; int valid = 0;

while( valid == 0 )

{

printf("Enter a number between 1 and 10 -->"); scanf("%d", &number);

/* assume number is valid */ valid = 1; if( number < 1 )

{

printf("Number is below 1. Please re-enter\n"); valid = 0;

}

if( number > 10 )

{

printf("Number is above 10. Please re-enter\n"); valid = 0;

}

printf("The number is %d\n", number );

}

IF ELSE

The general format for these are,

if( condition 1 ) statement1; elseif(condition2) statement2; elseif(condition3) statement3;

else statement4;

The else clause allows action to be taken where the condition evaluates as false (zero).

The following program uses an if else statement to validate the users input to be in the range 1-10.

Example: #include <stdio.h> main()

{

int number; int valid = 0;

while( valid == 0 )

{

printf("Enter a number between 1 and 10 -->"); scanf("%d", &number);

if( number < 1 )

{

printf("Number is below 1. Please re-enter\n"); valid = 0;

}

else if( number > 10 )

{

printf("Number is above 10. Please re-enter\n"); valid = 0;

}

else valid = 1;

}

printf("The number is %d\n", number );

}

This program is slightly different from the previous example in that an else clause is used to set the variable valid to 1. In this program, the logic should be easier to follow.

NESTED IF ELSE

/* Illustates nested if else and multiple arguments to the scanf function. */

Example:

#include <stdio.h> main()

{

int invalid_operator = 0; char operator;

float number1, number2, result;

printf("Enter two numbers and an operator in the format\n"); printf(" number1 operator number2\n");

scanf("%f %c %f", &number1, &operator, &number2); if(operator == '*')

result = number1 * number2; elseif(operator=='/') result=number1/number2; elseif(operator=='+') result=number1+number2; elseif(operator=='-') result=number1-number2; else

invalid_operator = 1;

if( invalid_operator != 1 )

printf("%f %c %f is %f\n", number1, operator, number2, result );

else

printf("Invalid operator.\n");

}

BRANCHING AND LOOPING

ITERATION, FOR LOOPS

The basic format of the for statement is,

for(start condition; continue condition; re-evaulation ) program statement;

Example:

/*sample program using a for statement */

#include <stdio.h>

main()/* Program introduces the for statement, counts to ten */

{

int count;

for( count = 1; count <= 10; count = count + 1 ) printf("%d ", count );

printf("\n");

}

The program declares an integer variable count. The first part of the for statement for( count = 1; initialises the value of count to 1. The for loop continues whilst the condition count <= 10; evaluates as TRUE. As the variable count has just been initialised to 1, this condition is TRUE and so the program statement printf("%d ", count ); is executed, which prints the value of count to the screen, followed by a space character. Next, the remaining statement of the for is executed

count = count + 1 );

which adds one to the current value of count. Control now passes back to the conditional test, count <= 10;

which evaluates as true, so the program statement printf("%d ", count );

is executed. Count is incremented again, the condition re-evaluated etc, until count reaches a value of 11.

When this occurs, the conditional test

count <= 10;

evaluates as FALSE, and the for loop terminates, and program control passes to the statement

printf("\n");

which prints a newline, and then the program terminates, as there are no more statements left to execute.

THE WHILE STATEMENT

The while provides a mechanism for repeating C statements whilst a condition is true. Its format is,

while( condition ) program statement;

Somewhere within the body of the while loop a statement must alter the value of the condition to allow the loop to finish.

Example:

/* Sample program including while */

#include <stdio.h>

main()

{

int loop = 0;

while( loop <= 10 ) {

printf("%d\n", loop);

++loop;

}

The above program uses a while loop to repeat the statements

printf("%d\n", loop); ++loop;

whilst the value of the variable loop is less than or equal to 10.

Note how the variable upon which the while is dependant is initialised prior to the while statement (in this case the previous line), and also that the value of the variable is altered within the loop, so that eventually the conditional test will succeed and the while loop will terminate. This program is functionally equivalent to the earlier for program which counted to ten.

THE DO WHILE STATEMENT

The do { } while statement allows a loop to continue whilst a condition evaluates as TRUE (non-zero). The loop is executed as least once.

Example:

/* Demonstration of DO...WHILE*/

#include <stdio.h> main()

{

int value, r_digit;

printf("Enter the number to be reversed.\n"); scanf("%d", &value);

{

r_digit = value % 10; printf("%d",r_digit);value=value/10;

}

while(value!=0);

printf("\n");

}

The above program reverses a number that is entered by the user. It does this by using the modulus % operator to extract the right most digit into the variable r_digit. The original number is then divided by 10, and the operation repeated whilst the number is not equal to 0.

SWITCH CASE:

The switch case statement is a better way of writing a program when a series of if elses occurs. The general format for this is,

switch(expression)

{

case value1: Program statement; program statement; break;

case valuen: program statement; break;

default:

break;

}

The keyword break must be included at the end of each case statement. The default clause is optional, and isexecuted if the cases are not met. The right brace at the end signifies the end of the case selections.

Example:

#include <stdio.h> main()

{

int menu, numb1, numb2, total; printf("enter in two numbers -->"); scanf("%d %d", &numb1, &numb2 ); printf("enter in choice\n"); printf("1=addition\n"); printf("2=subtraction\n");

scanf("%d",&menu); switch( menu )

{

case 1: total = numb1 + numb2; break; case 2: total = numb1 - numb2; break;

default:printf("Invalidoptionselected\n");

}

if( menu == 1 )

printf("%d plus %d is %d\n", numb1, numb2, total ); else if( menu == 2 ) printf("%d minus %d is %d\n", numb1, numb2, total );

}

The above program uses a switch statement to validate and select upon the users input choice, simulating a simple menu of choices.

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Computer Programming : C Programming Basics |