Week 7: Pointers and Arrays. BJ Furman 02OCT2009

Transcription

1 Week 7: Pointers and Arrays BJ Furman 02OCT2009

2 The Plan for Today Closer look at pointers What is a pointer? Why use pointers? How can we use pointers? Pointer Examples Pointer Practice

3 Learning Objectives Explain what a pointer is Explain how it is used Write a function that uses pointers

4 What is a Pointer? - 1 Variables in general Variable declaration informs compiler of two things: 1. Name of the variable 2. Data type of the variable Actions caused: Bytes allocated in memory Symbol table with name, address, and value Can think of a variable as having two "values" Value of what is stored at the memory location (rvalue) Value of the memory location (its address) (lvalue) int var1 = 0; Address Memory (8-bit) 0x10FE x10FF x x Bit Symbol Table Var name Var type Var address Var value var1 int 10FE 0

5 What is a Pointer? - 2 Variables in general, cont. Assigning a value to a variable (i = 3;) Value is copied to the address listed in the symbol table Using a variable in an expression (j = i;) Value of what is stored at the memory location is accessed Address 0x10FE 0x10FF Name i j short int i, j; i = 3; j = i; Memory (8-bit) Bit Symbol Table Type short int short int Address 10FE 1100 Value

6 What is a Pointer? - 3 Pointer variable A variable that contains an address Declaring a pointer variable type * varname int* ptr1; float *prt2; char * prt3; Read as: #include <stdio.h> "prt1 is a pointer to an integer" "prt2 is a pointer to a float" "prt3 is a pointer to a char" &num1 means: the address for the variable num1 int main() { int num1; int *ptr1 = &num1, *ptr2; num1 = 7; printf("size of num1: %d\n",sizeof(num1)); printf("value of num1: %d\n",num1); printf("address of num1: %p\n",&num1); printf("address in ptr1: %p\n",ptr1); return 0; } What is the size of num1? Is &num1 == ptr1? What address is stored in ptr2?

7 Accessing What a Pointer Points To The indirection operator * gets the value at the address stored in the pointer #include <stdio.h> int main() { int num1; int *ptr1 = &num1; num1 = 7; printf("value of num1: %d\n", num1); printf("value of num1: %d\n", *ptr1); } return 0;

8 Pointer - Practice 1 Declare: x_ptr, a pointer to an integer y_ptr, a pointer to a double What do the following statements do? char *my_ptr, my_char1 = 'c', my_char2; my_ptr = &my_char1; my_char2 = *m_ptr; What is in: my_ptr (in declaration? in second line?) my_char1 my_char2 (in declaration? in last line?)

9 ASCII Table

10 Why Use Pointers? Allows a function to modify variables in the calling function (without using global variables) Return more than one value from a function call Pass a pointer to a large data structure rather than copying the whole structure Arrays (probably next lecture)

11 How Can We Use Pointers? Recall last week's lab exercise Write a function that returns the square and cube of a number. The function must: Prompt the user for the number Print the number, its square, and its cube The prompting, calculation of square and cube, and printing must all be done by calling separate functions: num_prompt() prompts and gets the number from the user sq_cube() calculates the square and cube of the number print_sq_cube() prints out the square and cube of the number sq_cube() is a good candidate for using pointers needs to return two values: square and cube of a number

12 Program Development 1. See previous page 2. Inputs number Outputs number square of number cube of number Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code

13 Solution Algorithm - 1 Pseudocode Start at a high level Get number from the user Calculate square and cube Print number, square, and cube Refine Get number from the user Prompt user to enter number Get number Echo number to monitor

14 Solution Algorithm - 2 Pseudocode, cont. Refine, cont. Calculate square and cube Calculate square Calculate cube Print number, square, and cube

15 Solution Algorithm - 3 'Sketch the functions get_num() return_type get_num( arg_list ) What should the return type be? Does the function need to return a value by a return statement? If yes, then what type? If no, then what should the return type be? What arguments should the function have?

16 Solution Algorithm - 4 'Sketch the functions, cont. sq_cube() return_type sq_cube( arg_list ) What should the return type be? Does the function need to return a value by a return statement? If yes, then what type? If no, then what should the return type be? What arguments should the function have? Does it need to take a value in? Does it need to pass values out?

17 Solution Algorithm - 5 'Sketch the functions, cont. print_sq_cube() return_type print_sq_cube( arg_list ) What should the return type be? Does the function need to return a value by a return statement? If yes, then what type? If no, then what should the return type be? What arguments should the function have? Does it need to take a value in? Does it need to pass values out?

18 Arrays

19 What is an Array? So far we've dealt with scalar variables contain just one value Arrays are collections of data of the same type that occupy contiguous memory locations Individual values in the collection are called elements of the array Need to declare before use Format type array_name [num_elements]; Ex. Array of 10 integers named, 'nums' int nums[10];

20 Accessing Array Elements Individual elements of an array are accessed by their index number ****** Important Note****** Array indexing starts at zero char my_array[10]; the first element is my_array[0] the tenth element is my_array[9] Be careful! C will happily give you the value in or allow you to write to the next location in memory!

21 Review

22 References Jensen, T. (2003). A Tutorial on Pointers and Arrays in C, available from: ers.htm.visited 02OCT Visited 03OCT2009.