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An Introduction to C

Muhammed Al-Mulhem

Jan. 2008

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Goals of this tutorial

• To introduce the basics of imperative programming using the C programming language

• To prepare you for basic imperative programming

• To prepare you for better understanding of the other parts of the course

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Overview

• Basic types in C

• Some simple programs

• Format specifiers

• More example programs

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C Programs: General Form

preprocessor directives

int main(){

variable declarations;

statements;

}

Example:

#include <stdio.h>

int main(){

int x = 1;

printf(“Salaam Shabab\n”);

printf(“This is program %d\n”, x);

return 0;

}

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Basic Data Types (some)

• int  

• char            

• double 

• short

• Basic pointer types (associated with each basic type)

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Format Specifiers (some)

Variable type Conversion Specifiers        

char  %c

Int %d

Float %f

Double %lf

String %s

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Format Specifiers (some)

#include <stdio.h>int main(){ char ch = 's'; int i = 3; float f = 3; char *name = "ICS 410";

printf("Your values are:\n"); printf("A char: %c\n", ch); printf("A char: %d\n", ch); printf("An int: %d\n", i); printf("A float: %f\n", f); printf("A string: %s\n", name); printf("Mixed: %f\t%d\t%c\n", i,f,ch);

return 0;}

Your values are:

A char: s

A char: 115

An int: 3

A float: 3.000000

A string: ICS 313

Mixed: 0.000000 1074266112 s

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Operators

• Arithmetic• int i = i+1; i++; i--; i *= 2;• +, -, *, /, %,

• Relational and Logical• <, >, <=, >=, ==, !=• &&, ||, &, |, !

• Control structure• if ( ) { } else { }• while ( ) { }• do { } while ( );• for(i=1; i <= 100; i++) { }• switch ( ) {case 1: … }• continue; break;

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Example

#include <stdio.h>

#define DANGERLEVEL 5 /* like Java ‘final’ */

int main()

{

float level=1;

if (level <= DANGERLEVEL)

printf(“Low on gas!\n”);

else printf(“Good driver !\n”);

return 0;

}

Part - II

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Overview

• Introduction to pointers

• Arrays and pointers

• Strings in C

• Pointers as function parameters

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Pointers in C: An Introduction

• A pointer is an address in memory of some ordinary data

• There is a pointer type associated with every type

Example 1:

#include <stdio.h>int main(){ int x = 1; int *xp; xp = &x; printf(“x = %d\n”, x); printf(“*xp = %d\n”, *xp); return 0;}

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Pointers: A Pictorial View

float f;

float *fp;

? ?

f fp

4300 4304

?

any float

any address

? 4300

f fp

4300 4304

fp = &f;

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Pointers: A Pictorial View (cont’d)

*fp = 3.2; /* indirection operator */

f fp

4300 4304

3.2 4300

f fp

4300 4304

1.3 4300

float g=*fp; /* indirection: g is now 3.2 */

f = 1.3;

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Example 2:Dereferencing Pointers

#include <stdio.h> int main(){ int i=3, j=7, *ip;

ip = &i; *ip = 5; ip = &j; *ip += 11; i += *ip; printf("i = %d\n", i); printf("j = %d\n", j); printf("*ip = %d\n", *ip);

return 0;

}

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Example 3: Pointer Compatibility

#include <stdio.h> int main(){ int x = 3, *xp, *xp2; float f = 7, *fp; void *vp; /* generic pointer: no arithmetic no

dereferencing */

xp = &x; fp = &f; fp = xp; vp = xp; fp = vp; xp2 = vp; (*xp)++;

printf("x = %d\n", *xp); printf("*vp = %d\n", *fp); printf("*vp = %d\n", *xp2); return 0;}

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Arrays and Pointers

• Arrays in C are not objects: no methods, no ‘length’ field

• The name of an array is a constant pointer

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Example 4: Arrays and pointers

#include <stdio.h> int main(){ int i, j, *ip, myArray[5] = {2,3,5,7,11}; ip = &myArray[2]; for(i= 0; i<5; i++) printf("myArray[%d] = %d\n", i, *(myArray+i));

*(ip+1) = 4; *(ip-1) += 11; j = *(ip-2);

for(i=0; i<5; i++) printf("myArray[%d] = %d\n", i, *(myArray+i)); printf("j = %d\n", j); return 0;}

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Example 5: Dynamic Arrays

#include <stdio.h> int main(){ int i, j, size, *ip, *myArray;

printf("Enter array size:"); scanf("%d", &size); myArray = (int *)malloc(size*sizeof(int));

printf("Enter the %d elements (separated by space: ",size); for(i=0;i<size; i++){ scanf("%d", myArray+i); }

ip = &myArray[2]; *(ip+1) = 4; *(ip-1) += 11; j = *(ip-2);

for(i=0; i<5; i++) printf("myArray[%d] = %d\n", i, *(myArray+i));

printf("j = %d\n", j); return 0;

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Example 6: File I/O

#include <stdio.h>

int main(){

int i,n,hours;

long id;

float rate, wage;

char name[50], n2[50];

FILE *inputFile, *outputFile;

inputFile = fopen("employeeData.txt","r");

outputFile = fopen("employeeWage.txt","w");

if (inputFile) printf("File succesfully opened for reading!\n");

else {

printf(“File not opened\n”);

exit(1);

}

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Example 6: File I/O

printf("How many employees do you have>");

scanf("%d", &n);

for(i=0;i<n;i++){

fscanf(inputFile, "%[0-9]%[^0-9]%d%f",

&id,name,&hours,&rate);

wage = hours*rate;

fprintf(outputFile, "%ld\t%d\tSR%5.2f\t\tSR%5.2f\n",

id,hours,rate,wage);

}

fclose(inputFile);fclose(outputFile);

return 0;

}

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Strings in C

• Java strings are objects of class java.lang.String or

java.lang.StringBuffer, and represent sequences of characters

• Strings in C are just arrays of, or pointers to, char

• Functions that handle strings typically assume that strings are terminated with a null character ‘\0’, rather than being passed a length parameter

• Utilities for handling character strings are declared in <string.h>. For example:• strcpy, strncpy, strcmp, strncmp, strcat, strncat, strstr,strchr

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Example 7: Strings in C

#include <stdio.h>

int main(){

char name[] = {'i', 'c', 's', ‘4', '1', ‘0', '\0'};

char *dept = "ICS, KFUPM";

int i = 0;

printf("Course name = %s\n", name);

for(; i<6; i++)

printf("%c", *(name+i));

printf("\n");

for(i=0; i<10; i++)

printf("%c", *(dept+i));

printf("\n");

return 0;

}

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Example 8: Strings Processing

#include <stdio.h>

#include <string.h>

int main() {

char first[80], second[80];

printf("Enter a string: ");

gets(first);

strcpy(second, first);

printf("first: %s, and second: %s\n", first, second);

if (strcmp(first, second))

puts("The two strings are equal");

else

puts("The two strings are not equal");

return 0;

}

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Pointers and Functions

• What is the parameter passing method in Java?

• Passing arguments to functions

– By value

– By address

• Returning values from functions

– By value

– By address

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Example 9:Pass By Value

#include <stdio.h>

int sum(int a, int b);

/* function prototype at start of file */

int main(){

int a=4,b=5;

int total = sum(a,b);

printf("The sum of 4 and 5 is %d\n", total);

return 0;

}

int sum(int a, int b){ /* function definition */

return (a+b);

}

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Example 10: Pass By Address

#include <stdio.h>

int sum(int *a, int *b);

int main(void){

int a=4, b=5;

int *ptr = &b;

int total = sum(&a,ptr);

printf("The sum of 4 and 5 is %d\n", total);

return 0;

}

int sum(int *pa, int *pb){

return (*pa+*pb);

}

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Example 11: Pass By Value

#include <stdio.h>

void swap(int, int);

int main() {

int num1 = 5, num2 = 10;

swap(num1, num2);

printf("num1 = %d and num2 = %d\n", num1, num2);

return 0;

}

void swap(int n1, int n2) { /* passed by value */

int temp;

temp = n1;

n1 = n2;

n2 = temp;

printf("num1 = %d and num2 = %d\n", n1, n2);

}

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Example 12: Pass By Address

#include <stdio.h>void swap(int *, int *);

int main() { int num1 = 5, num2 = 10; swap(&num1, &num2); printf("num1 = %d and num2 = %d\n", num1, num2); return 0;}

void swap(int *n1, int *n2) { /* passed and returned by reference */ int temp; temp = *n1; *n1 = *n2; *n2 = temp; printf("num1 = %d and num2 = %d\n", *n1, *n2);}

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One-Dimensional Arrays

#include <stdio.h>

int main(){

int number[12];

int index, sum = 0;

/* Always initialize array before use */

for (index = 0; index < 12; index++) {

number[index] = index;

}

for (index = 0; index < 12; index = index + 1) {

printf("number[%d] = %d\n", index,number[index]);

sum += number[index];

}

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

printf("The sum is: %d\n", number[12]);

return 0;

}

Part - III

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Overview

• Storage classes

• Structures– Type aliasing via typedef

– enum types

– struct types

– union types

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Storage Classes: An Introduction

• For any variable declared in a program (e.g., int x = 11;), the compiler is told the following information about the variable: its

– type (preceded by an optional qualifier, e.g., long int x;)

– name

– cell size (with an implicit address)

– storage class

• A storage class indicates the degree of the life span of a variable.

• A variable can either have an extern, an auto, a register or a static storage class.

• A register, auto and a local static variable has a temporary, local scope.

• A global extern and a global static variable has a permanent, global scope.

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Storage Classes: An Introduction

• In functions other than main(), make a variable static if its value must be retained between calls to the function.

• If a variable is used by most or all of the program's functions, define it with the extern storage class.

• auto is the default storage class for local variables. auto can only be used within functions, i.e. local variables.

• register is used to define local variables that should be stored in a register instead of RAM.

• This means that the variable has a maximum size equal to the register size (usually one word) and can’t have the unary `&' operator applied to it (as it does not have a memory location).

• register should only be used for variables that require quick access - such as counters.

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Storage Classes: An Introduction

• static is the default storage class for global variables. static can also be defined within a function. If this is done, the variable is initialized at compilation time and retains its value between calls.

• Because it is initialized at compilation time, the initialization value must be a constant.

• extern defines a global variable that is visible to ALL object modules.

• An extern variable cannot be initialized because all extern does is point the variable name at a storage location that has been previously defined.

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Example 1: Storage Classes

#include <stdio.h>

int x = 71; /* Equivalently, static int x = 71; */

void func1(void) {

extern int x; /* Illegal to initialize! */

printf("In func1(), x = %d\n", x);

}

int main() {

auto x = 23; /* Equivalently, auto int x = 23; */

printf("In main(), x = %d\n", x);

func1();

return 0;

}

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Example 2: Storage Classes

#include <stdio.h>

void func1(void); /* function declaration */

static count=10; /* Global variable - static is the default */

main() {

while (count--) func1();

return 0;

}

void func1(void) {

/* 'x' is local to 'func1' - it is

* only initialized at run time. Its value

* is NOT reset on every invocation of

* 'func1' */

static x=5;

x++;

printf(" x is %d and count is %d\n", x, count);

}

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Example 3: Storage Classes

#include <stdio.h>

char *f(void);

int main() {

char *result;

result = f();

printf("%s",result);

return 0;

}

char *f(void) {

char name[13]="Amr Ali";

return name;

}

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Structures: Motivations• Our programs so far used only the basic data types provided by C, i.e.,

char, int, float, void and pointers.

• Programmers can define their own data types using structures

• Structures can be used to conveniently model things that would otherwise be difficult to model using arrays

• Arrays versus structures: • arrays elements and structure components are contiguously laid out in memory

• arrays are homogeneous, structures can be heterogeneous

• arrays are passed to functions by reference while structures have to be explicitly passed by reference

• arrays (of the same type, of course!) cannot be assigned to each other (why?) while structures can

• array identifiers can be compared while structure identifiers cannot

• Different structures (in the same program) can have the same component name. A structure component name can also be the same as an ordinary variable name

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Creating Structures: the Tools

1. the typedef specifier

2. the enum specifier

3. the struct specifier

4. the union specifier

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Creating Structures: Examples

typedef struct date { int day; int month; int year;} DATE;

enum Days {SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY};

struct date { int day; int month; int year;};

union mixed { char c; float num; int age; };

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Creating Structures: Examples

typedef is a mechanism, which allows a type to be explicitly associated with an identifier.

Some examples are

typedef char * string; typedef int INCHES, FEET, YARDS; typedef float vector [10];

In each of these type definitions the named identifiers can be used later to declare variables or functions in the same way ordinary types can be used. Thus

INCHES length, width; String s1 = "abc", s2 = "xyz", vector x;

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Example 4: Structures

#include <stdio.h>

struct date {

int day;

int month;

int year;

};

main() {

struct date today;

today.day = 17;

today.month = 11;

today.year = 1998;

printf("Todays date is %d/%d/%d.\n",

today.month, today.day, today.year );

return 0

}

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Example 5: Structures

#include <stdio.h>

struct date {int day, month, year;};

void modify_date(struct date *yaum) {

yaum->month = 3;

yaum->year = 1999;

printf("The date is %d/%d/%d.\n", yaum->day, yaum->month, yaum->year );

}

main() {

struct date *today;

today->day = 16;

today->month = 7;

today->year = 1998;

printf("The date is %d/%d/%d.\n", today->day, today->month, today->year );

modify_date(today);

printf("The date is %d/%d/%d.\n", today->day, today->month, today->year );

}

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Creating Union

A union, like a structure, is a derived type. Unions follow the same syntax as structures but have members that share storage. A union type defines a set of alternative values that may be stored in a shared portion of memory. The programmer is responsible for interpreting the stored values correctly. Consider the declaration

union int_or_float { int i; float f;};

In this declaration union is a keyword, int_or_float is the union tag name, and the variables i and f are members of the union. This declaration creates the derived data type union int_or_float. The declaration can be thought of as a template; it creates the type, but no storage is allocated.

The tag name, along with the keyword union, can now be used to declare variables of this type.

union int_or_float a, b, c;This declaration allocates storage for the identifiers a, b, and c. For each variable the

compiler allocates a piece of storage that can accommodate the largest of the specified members.

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Creating Union

Example:

tpedef union int_or_float {int i; float f;} number; main ( ) {

number n;n.i = 4444;printf("i: %10d f: %16.10e\n", n.i, n.f);n.i = 4444.0;printf("i: %10d f: %16.10e\n", n.i, n.f);

}

This little experiment demonstrates how your system overlays an int and a float. The output of this program is system dependent. Here is what is printed on our system:

i: 444 f: 0.6227370375e-41

i: 1166729216 f: 4.4440000000e+03