1-1 An Introduction to C Muhammed Al-Mulhem Jan. 2008

1-2 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

1-3 Overview Basic types in C Some simple programs Format specifiers More example programs

1-4 C Programs: General Form preprocessor directives int main(){ variable declarations; statements; } Example: #include int main(){ int x = 1; printf(“Salaam Shabab\n”); printf(“This is program %d\n”, x); return 0; }

1-5 Basic Data Types (some) int char double short Basic pointer types (associated with each basic type)

1-6 Format Specifiers (some) Variable typeConversion Specifiers char %c Int%d Float%f Double%lf String%s

1-7 Format Specifiers (some) #include 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: A string: ICS 313 Mixed: s

1-8 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;

1-9 Example #include #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

1-11 Overview Introduction to pointers Arrays and pointers Strings in C Pointers as function parameters

1-12 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 int main(){ int x = 1; int *xp; xp = &x; printf(“x = %d\n”, x); printf(“*xp = %d\n”, *xp); return 0; }

1-13 Pointers: A Pictorial View float f; float *fp; ?? f fp ? any float any address ?4300 f fp fp = &f;

1-14 Pointers: A Pictorial View (cont’d) *fp = 3.2; /* indirection operator */ f fp f fp float g=*fp; /* indirection: g is now 3.2 */ f = 1.3;

1-15 Example 2:Dereferencing Pointers #include 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; }

1-16 Example 3: Pointer Compatibility #include 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; }

1-17 Arrays and Pointers Arrays in C are not objects: no methods, no ‘length’ field The name of an array is a constant pointer

1-18 Example 4: Arrays and pointers #include 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; }

1-19 Example 5: Dynamic Arrays #include 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;

1-20 Example 6: File I/O #include 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); }

1-21 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; }

1-22 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. For example: strcpy, strncpy, strcmp, strncmp, strcat, strncat, strstr,strchr

1-23 Example 7: Strings in C #include 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; }

1-24 Example 8: Strings Processing #include 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; }

1-25 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

1-26 Example 9:Pass By Value #include 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); }

1-27 Example 10: Pass By Address #include 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); }

1-28 Example 11: Pass By Value #include 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); }

1-29 Example 12: Pass By Address #include 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); }

1-30 One-Dimensional Arrays #include 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

1-32 Overview Storage classes Structures –Type aliasing via typedef –enum types –struct types –union types

1-33 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.

1-34 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.

1-35 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.

1-36 Example 1: Storage Classes #include 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; }

1-37 Example 2: Storage Classes #include 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); }

1-38 Example 3: Storage Classes #include char *f(void); int main() { char *result; result = f(); printf("%s",result); return 0; } char *f(void) { char name[13]="Amr Ali"; return name; }

1-39 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

1-40 Creating Structures: the Tools 1.the typedef specifier 2.the enum specifier 3.the struct specifier 4.the union specifier

1-41 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; };

1-42 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 INCHESlength, width; Strings1 = "abc", s2 = "xyz", vector x;

1-43 Example 4: Structures #include 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 }

1-44 Example 5: Structures #include 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 ); }

1-45 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; floatf;}; 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.

1-46 Creating Union Example: tpedef union int_or_float {int i; float f;} number; main ( ) { numbern; n.i = 4444; printf("i: %10df: %16.10e\n", n.i, n.f); n.i = ; printf("i: %10df: %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: 444f: e-41 i: f: e+03