Pointers Programming Applications

Pointer A pointer is a variable whose value is a memory address representing the location of the chunk of memory on either the run-time stack or on the heap. Pointers have names, values and types. Value of p versus value pointed to by p

Pointer  The definition of pointer is Int *p; float *p1; char *p2;  Which means that p is a pointer to integer data in memory and p1 is a pointer to floating point data in memory

Declaring pointer variables  it is not enough to say that a variable is a pointer. You also have to specify the type of variable to which the pointer points ! int * p1; // p1 points to an integer float * p2; // p2 points to a float  Exception: generic pointers (void *) indicate that the pointed data type is unknown may be used with explicit type cast to any type (type *) void * p; type * variable_name;

Pointer using  To use the pointer: int *p, k; K=10; P= &k;  Means that p has the address of k in memory  Then &k means the address of k and *p means the value in address p

Example main() { int v1=11, v2=22, *p; p=&v1; printf( “ %d ”, p); p=&v2; printf( “ %d ”, p); printf( “ %d ”, *p); }

Example main() { int v1=11, v2=22, *p; p=&v1; printf( “ %d ”, p); *p=33; printf( “ %d \n %d ”, p, *p); }

Example main() { int v1=11, v2, *p; p=&v1; printf( “ %d ”, p); v2=*p; printf( “ %d %d ”, p, v2); }

Pointer int u=3, v; int *pu, *pv; pu= &u; v= *pu; Pv=&v; Printf( “ %d %d %d %d ”, u, &u, pu, *pu); Printf( “ \n %d %d %d %d ”, v, &v, pv, *pv);

Pointer int u1, u2, v=3; int *pv; u1 = 2 * (v+5); pv = &v; u2 = 2 * (*pv +5); printf( “ %d %d ”, u1, u2);

Trace int x,y; int * px; x = 5; px = &x; printf("x = %d\n", x); printf("x = %d\n", *px); y = *px; printf("y = %d\n", y); *px = 10; printf("x = %d\n", x); printf("x = %d\n", *px); px = &y; *px = 15; printf("x = %d\n", x); printf("y = %d\n", y); printf("what = %d\n", *px);

Trace int x,z; float y; char ch, *chp; int *ip1, *ip2; float *fp; x = 100; y = 20.0; z = 50; ip1 = &x; ip2 = &z; fp = &y; chp = &ch; ip2 = ip1; ip1 = &z; *ip1 = *ip2; *ip1 = 200; *ip1 = *ip ; *fp = 1.2; printf("%f\n", *fp); printf("%d\n", ip1); printf("%d\n", *ip1);

Pointer and array  If we define the array int x[5]={2, 1, 5, 6, 8}  The variable x only means pointer to the first element of array x[]  If we define pointer int *p;  Then we can use p as p = &x[0] Which means that p is pointer to first element or p = x

Pointer and array  Both x and p can be used as printf( “ %d ”, *(x+2)); means the x[2] printf( “ %d ”, *(p+1)); means the x[1] printf( “ %d ”, (*x+2)); means the x[0]+2

Pointer Int x[8] = {10, 20, 30, 40, 50, 60, 70, 80,}; Printf( “ %d %d %d %d %d ”, x, x+2, *x, (*x+2), *(x+2));

Example float list[10], *p1; for(int i=0; i<10;i++, p1++) *p1=i; for( i=0; i<10;i++) printf( “ %f ”, *(pt-i));

Function and Pointer  We can call function by value f(s)  Or call function by reference f(&s)  Or define pointer p = &s and call function f(p)

Function and pointer  In the function call by value, any modification held in the function will not affect the original value  In call of reference, the modification in function affect the original value.

Example void set (int x); int main(void) {int x=10,y=11; set(x); set(y); printf( “ %d%d ”, x,y);} void set (int x){ int y=x+2; x*=2; }

Example void set (int *x); int main(void) {int x=10,y=11; set(&x); set(&y); printf( “ %d%d ”, x,y);} void set (int *x){ int y=*x+2; *x*=2; }

Example void set (int &); int main(void) {int x=10,y=11; set(&x); set(&y); printf( “ %d%d ”, x,y);} void set (int &x){ int y=x+2; x *=2; }

Passing array void cent (double *); void main(){ double varray[5]={10.0, 43.1, 95.9, 59.7, 87.3}; cent(varray); for(int j=0; j<5; j++) printf( “ %d ”, varray[j]);} void cent (double *pt){ for(int j=0; j<5; j++) *pt++ *= 2.54; }

Pointer & String void disp ( char *); void main() { char str[] = “ Idel people have the …” ; disp(str); } void disp (char *ps){ while (*ps != “ \0 ” ) printf( “ %c ”, *ps++); }

Array of string pointer We can define array of pointer to strings as: char * arrstr[7]={ “ Saturday ”, “ Sunday ”, “ Monday ”, “ Tuesday ”, “ Wednesday ”, “ Thursday ”, “ Friday ” } for (int j=0; j<7; j++) printf( “ %s ”, arrstr[j]);

Example float list[10] ={1, 5, 6, 9, 10, 0, 45, 4, 61, 7}; float *p, *d; P=list; d=&list[9]; for( i=0; i<10;i++) printf( “ %f ”, *(p+i),*(d-i) );

#include Void push(int *, int); Void pop (int *); Main() { int *p, value; For(int t=0; t<5;t++) {scanf( “ %d ”, &value); push(p, value);} Pop(p); pop(p); } Void push(int *p, int v){ p++; *p=v;} void pop (int *p){printf( “ %d ”, *(p--));}

Trace #include Int f (int); Void main() {Int y=f(0); Int z = f(4); Printf ( “ %d \t %d \n “, y, z); } int f(int x) {if (x==0) return 0; Else if (x>=1) return x+f(x-1); }

Example  Using pointer, write a program that read a list of numbers and sort them using swap function which accept two pointers to elements in this list and swap them.

Example  Write a program to read group of numbers from the user and then average them after stored them in an array and print the result. You should use pointer as possible.

Example  Write a function String which accept as argument the pointer to string and convert the string to all upper case. You can use the toupper() library function which takes a single character and return this character in upper case.

NULL pointers  Values of a pointer variable: Usually the value of a pointer variable is a pointer to some other variable Another value a pointer may have: it may be set to a null pointer  A null pointer is a special pointer value that is known not to point anywhere.  No other valid pointer, to any other variable, will ever compare equal to a null pointer !  Predefined constant NULL, defined in  Good practice: test for a null pointer before inspecting the value pointed ! #include int *ip = NULL; if(ip != NULL) printf("%d\n", *ip); if(ip ) printf("%d\n", *ip);

u Never use uninitialized pointers. u To increment a dereferenced pointer, use  (*p)++  rather than  *p++ /* correct but means … */

Generic Pointers and NULL Java: a reference to "any" kind of object  use a variable of type Object C: a reference to "any" kind of object  use a variable of type void*  void *p;  defines a generic, or typeless pointer p. Often casted to (T*)p NULL value can be assigned to any pointer, no matter what its type.

Generic Pointers and NULL  void *p = NULL;  int i = 2;  int *ip = &i;  p = ip;  printf("%d", *p);  printf("%d", *((int*)p ) );

Pointer Expressions and Pointer Arithmetic pv+n  pv + n*sizeof ( variable type that pointer point to )  Arithmetic operations can be performed on pointers Increment/decrement pointer ( ++ or -- ) Example: ++vPtr, vPtr++, --vPtr, vPtr-- Add an integer to a pointer( + or +=, - or -= ) Pointers may be subtracted from each other Operations meaningless unless performed on an array