Programming in C Pointer Basics Java Reference In

Programming in C Pointer Basics

Java Reference • In Java, the name of an object is a reference to that object. Here ford is a reference to a Truck object. It contains the memory address at which the Truck object is stored. Truck ford = new Truck( ); • The syntax for using the reference is pretty simple. Just use the “dot” notation. ford. start( ); ford. drive( 23 ); ford. turn (LEFT);

What is a pointer ? • In C, a pointer variable (or just “pointer”) is similar to a reference in Java except that – A pointer can contain the memory address of any variable type (Java references only refer to objects) – – – A primitive (int, char, float) An array A struct or union Dynamically allocated memory Another pointer A function – There’s a lot of syntax required to create and use pointers 1/14/10

Why Pointers? • They allow you to refer to large data structures in a compact way • They facilitate sharing between different parts of programs • They make it possible to get new memory dynamically as your program is running • They make it easy to represent relationships among data items. 1/14/10

Pointer Caution • They are a powerful low-level device. • Undisciplined use can be confusing and thus the source of subtle, hard-to-find bugs. – Program crashes – Memory leaks – Unpredictable results 1/14/10

C Pointer Variables To declare a pointer variable, we must do two things – Use the “*” (star) character to indicate that the variable being defined is a pointer type. – Indicate the type of variable to which the pointer will point (the pointee). This is necessary because C provides operations on pointers (e. g. , *, ++, etc) whose meaning depends on the type of the pointee. • General declaration of a pointer type *name. Of. Pointer; 1/14/10

Pointer Declaration The declaration int *int. Ptr; defines the variable int. Ptr to be a pointer to a variable of type int. Ptr will contain the memory address of some int variable or int array. Read this declaration as – “int. Ptr is a pointer to an int”, or equivalently – “*int. Ptr is an int” Caution -- Be careful when defining multiple variables on the same line. In this definition int *int. Ptr, int. Ptr 2; int. Ptr is a pointer to an int, but int. Ptr 2 is not! 1/14/10

Pointer Operators The two primary operators used with pointers are * (star) and & (ampersand) – The * operator is used to define pointer variables and to dereference a pointer. “Dereferencing” a pointer means to use the value of the pointee. – The & operator gives the address of a variable. Recall the use of & in scanf( ) 1/14/10

Pointer Examples int x = 1, y = 2, z[10]; int *ip; /* ip is a pointer to an int */ ip = &x; y = *ip; *ip = 0; *ip = *ip + 10; If ip points to x, then *ip can be used anywhere x can be used so in this example *ip = *ip + 10; and x = x + 10; are equivalent The * and & operators bind more tightly than arithmetic operators so y = *ip + 1; takes the value of the variable to which ip points, adds 1 and assigns it to y Similarly, the statements *ip += 1; and ++*ip; and (*ip)++; all increment the variable to which ip points. (Note that the parenthesis are necessary in the last statement; without them, the expression would increment ip rather than what it points to since operators like * and ++ associate from right to left. ) 1/14/10

Pointer and Variable types • The type of a pointer and its pointee must match int a = 42; int *ip; double d = 6. 34; double *dp; ip dp 1/14/10 = = &a; &d; &a; /* /* ok -- types match */ ok */ compiler error -- type mismatch */ compiler error */

More Pointer Code • • Use ampersand ( & ) to obtain the address of the pointee Use star ( * ) to get / change the value of the pointee Use %p to print the value of a pointer with printf( ) What is the output from this code? int a = 1, *ptr 1; /* show value and address of a ** and value of the pointer */ ptr 1 = &a ; printf("a = %d, &a = %p, ptr 1 = %p, *ptr 1 = %dn", a, &a, ptr 1, *ptr 1) ; /* change the value of a by dereferencing ptr 1 ** then print again */ *ptr 1 = 35 ; printf(“a = %d, &a = %p, ptr 1 = %p, *ptr 1 = %dn", a, &a, ptr 1, *ptr 1) ; 1/14/10

NULL • NULL is a special value which may be assigned to a pointer • NULL indicates that this pointer does not point to any variable (there is no pointee) • Often used when pointers are declared int *p. Int = NULL; • Often used as the return type of functions that return a pointer to indicate function failure int *my. Ptr; my. Ptr = my. Function( ); if (my. Ptr == NULL){ /* something bad happened */ } • Dereferencing a pointer whose value is NULL will result in program termination. 1/14/10

Pointers and Function Arguments • Since C passes all primitive function arguments “by value” there is no direct way for a function to alter a variable in the calling code. /* version 1 of swap */ void swap (int a, int b) { int temp; temp = a; a = b; b = temp; } /* calling swap from somewhere in main() */ int x = 42, y = 17; swap( x, y ); printf(“%d, %dn”, x, y); // what does this print? 1/14/10

A better swap( ) • The desired effect can be obtained by passing pointers to the values to be exchanged. • This is a very common use of pointers. /* pointer version of swap */ void swap (int *px, int *py) { int temp; temp = *px; *px = *py; *py = temp; } /* calling swap from somewhere in main( ) */ int x = 42, y = 17; swap( &x, &y ); printf(“%d, %dn”, x, y); // what does this print? 1/14/10

More Pointer Function Parameters • Passing the address of variable(s) to a function can be used to have a function “return” multiple values. • The pointer arguments point to variables in the calling code which are changed (“returned”) by the function. 1/14/10

Convert. Time. c void convert. Time (int time, int *p. Hours, int *p. Mins) { *p. Hours = time / 60; *p. Mins = time % 60; } int main( ) { int time, hours, minutes; printf("Enter a time duration in minutes: "); scanf ("%d", &time); convert. Time (time, &hours, &minutes); printf("HH: MM format: %d: %02 dn", hours, minutes); return 0; } 1/14/10

An Exercise • What is the output from this code? void my. Function (int a, int *b) { a = 7 ; *b = a ; b = &a ; *b = 4 ; printf("%d, %dn", a, *b) ; } int main() { int m = 3, n = 5; my. Function(m, &n) ; printf("%d, %dn", m, n) ; return 0; } 1/14/10 4, 4 3, 7

Pointers to struct /* define a struct for related student data */ typedef struct student { char name[50]; char major [20]; double gpa; } STUDENT; STUDENT bob = {"Bob Smith", "Math", 3. 77}; STUDENT sally = {"Sally", "CSEE", 4. 0}; STUDENT *p. Student; /* p. Student is a "pointer to struct student" */ /* make p. Student point to bob */ p. Student = &bob; /* use -> to access the members */ printf ("Bob's name: %sn", p. Student->name); printf ("Bob's gpa : %fn", p. Student->gpa); /* make p. Student point to sally */ p. Student = &sally; printf ("Sally's name: %sn", p. Student->name); printf ("Sally's gpa: %fn", p. Student->gpa); Note too that the following are equivalent. Why? ? p. Student->gpa and (*p. Student). gpa /* the parentheses are necessary */ 1/14/10

Pointer to struct for functions void print. Student(STUDENT *studentp) { printf(“Name : %sn”, studentp->name); printf(“Major: %sn”, studentp->major); printf(“GPA : %4. 2 f”, studentp->gpa); } Passing a pointer to a struct to a function is more efficient than passing the struct itself. Why is this true? 9/24/10