ADVANCED POINTERS

Overview Review on pointers and arrays Common troubles with pointers Multidimensional arrays Pointers as function arguments Functions returning pointers Pointers to functions

Review: Pointers and Arrays flizny == &flizny[0]; // name of array is the address of the first element Both flizny and &flizny[0] represent the memory address of that first element. Both are constants because they remain fixed for the duration of the program. They can be assigned as values to a pointer variable, and the variable can be changed.

Pointers and Arrays short double pointers + 0: 0x0064fd20 0x0064fd28 pointers + 1: 0x0064fd22 0x0064fd30 pointers + 2: 0x0064fd24 0x0064fd38 pointers + 3: 0x0064fd26 0x0064fd40

Pointers and Arrays The value of a pointer is the address of the object to which it points. How the address is represented internally is hardware dependent. The address of a large object, such as type double variable, typically is the address of the first byte of the object. Applying the * operator to a pointer yields the value stored in the pointed-to object. Adding 1 to the pointer increases its value by the size, in bytes, of the pointed-to type. dates +2 == &date[2] /* same address */ *(dates + 2) == dates[2] /* same value */

Troubles with pointers List of some pointer errors Assigning values to uninitialized pointers int *p, m = 100; *p = m;// Error Assigning values to a pointer variable int *p, m = 100; p = m;// Error Not dereferencing a pointer when required int *p, x = 100; *p = &x; printf(“%d”, p);// Error Assigning the address of an uninitialized variable int m, *p; p = &m; Comparing pointers that point to different objects char name1 [20], name2 [30]; char *p1 = name1; char *p2 = name; If (p1 > p2)… //Error

Multidimensional Arrays Two dimensional array definition: float rain[5][12]; // array of 5 arrays of 12 floats This tells us that each element is of type float[12]; that is, each of the five elements of rain is, in itself, an array of 12 float values. rain  Pointer to first row rain + 1  pointer to ith row *(p+i)  pointer to first element in the ith row *(p+i) + j  pointer to jth element in the ith row *(*(p+i) + j )  value stored in the cell (i,j) *(rain + 2) + 3

Initializing a Two-Dimensional Array More Dimensions int box[10][20][30]; You can visualize a one-dimensional array as a row of data, a two- dimensional array as a table of data, and a three-dimensional array as a stack of data tables. Typically, you would use three nested loops to process a three- dimensional array, four nested loops to process a four-dimensional array, and so on. char *name[3] = {“New Zealand”, “Australia”, “India”};

The program goal is to find a)the total rainfall for each year, b)the average yearly rainfall, and c)the average rainfall for each month. To find the total rainfall for a year, you have to add all the data in a given row. To find the average rainfall for a given month, you have to add all the data in a given column. The two-dimensional array makes it easy to visualize and execute these activities.

Pointers as functions arguments The function parameters are declared as pointers; The dereferenced pointers are used in the function body; When the function is called, the addresses are passed as actual arguments. array passed as a pointer Example 1: Sorting function Example 2: copying strings

Functions returning pointers The function larger receives the addresses of the variables a and b, decides which one is larger using the pointers x and y and the returns the address of its location. The returned value is then assigned to the pointer.

Pointers to functions A pointer to a function is declared as follows: type (*fptr) (); This tells the compiler that fptr is a pointer to a function, which returns type value. Assigning the name of the function to the pointer double mul(int, int); double (*p1)(); p1 = mul; Now we can call the function by the pointer p1 with the list of parameters. (*p1)(x,y)  mul(x,y)

Example: A program that uses a function pointer as a function argument A program prints the function values over a given range of values. The printing is done by the function table by evaluating the function passed to it by the main.