Pointers and Arrays Beyond Chapter 16

16-2 Pointers and Arrays What are the real differences? Pointer Holds the address of a variable Can be pointed anywhere in memory Array A contiguous chunk of allocated ROM The Array Name is the address of where the data starts The address in the Array Name may NOT be changed  E.g. It cannot point anywhere else Similarities Both passed as a reference to a function call

16-3 Passing arguments to functions int function(int *intArray, int size); intArray passed in stack May perform arithmetic on intArray (e.g. intArray++ )‏ int function(int intArray[], int size); Too slow to copy array into stack as argument intArray reference passed on stack (not array)‏ May perform arithmetic on intArray (e.g. intArray++) ‏ Since just a pointer is passed, the function does not know how big the array is, so the size is usually passed in separately.

16-4 Pointers vs Array – Compiler’s knowledge helps char charArray[6] = “world”; char *charPtr = “hello”; What do they look like? R0 = charArray[4] – translate into LC-3 R0 = charPtr[4] – translate into LC-3

Arrays, and Pointers Pointer and Array equivalence Array name is “pointer” to first element in array int x[] = {1,2,3,4,5,6,7,8,9,10}; int *y; y = x; *y = x[3]; *x = y[6]; y[4] = *x; y = &x[4]; *y = x[9]; 16-5

Pointer Arithmetic int x[10] = {1,2,3,4,5,6,7,8,9,10}; int *y; y = x + 1; ++y; ++x; /* can’t do */ *y = ++x[3]; *(y+1) = x[5]++; *x = y[6]; y[4] = *x; y = &*(x+4) *y = *(x + 9); 16-6

16-7 Arrays of Pointers char *names[] = {“hello”,”how”,”are”,”you?”}; What is it? What does this look like?

16-8 char *names[] = {“hello”,”how”,”are”,”you?”}; char *word = names[1]; char **all = names+1; What are the values of the following? names[3][1] **names *(names[0]+3)‏ *(++word)‏ *(*(++all)+1)‏

16-9 multidimensional array int myInts[3][2] = {{0,1},{5,7},{-1,-3}}; Allocated as one contiguous chunk of memory. Looks like: myInts[2][1] – compiler translates to:

16-10 On your own (or with a partner): Draw the memory for the following declarations int intArray[][3] = {{2,4,6},{1,4,9},{1,3,5},{5,7,9}}; int *squares = &intArray[3][1]; int *odd = intArray[2];

16-11 Returning arrays from functions Assume that str is never more than 128 characters What is wrong with this function? char *copyString(char *str)‏ { char buffer[128]; int index = 0; while ((str[index] != ‘\0’) && (index < 128))‏ { buffer[index] = str[index]; index++; } buffer[index] = ‘\0’; return buffer; }

Dynamic Memory Allocation void *malloc(size_t size) malloc allocates size number of bytes in memory and returns a pointer to it. The memory is not cleared. Use: char *line; int *x; line = (char *) malloc (sizof(char) * 80); x = (int *) malloc(sizeof(int) * 10); 16-12

Dynamic Memory Allocation void free(void *ptr) free releases the memory pointed to by ptr. The memory must have been created by malloc or one of its kind. Use: char *line; line = (char *) calloc (80, sizof(char));. free(line); 16-13

Memory Layout Dynamically allocated memory is placed on the heap Dynamically allocated memory is not automatically freed. Code (instructions) Global/Static Data Heap Run-time stack 16-14

16-15 Fixing the code: char *copyString(char *str)‏ { int len = strlen(str); char *buffer = ________________________; int index = 0; while (str[index] != ‘\0’)‏ { buffer[index] = str[index]; index++; } buffer[index] = ‘\0’; return buffer; }

How To Dynamically Allocate char *names[] = {“hello”,”how”,”are”,”you?”}; What does this look like? 16-16

How To Dynamically Allocate int myInts[][2] = {{0,1},{5,7},{-1,-3}}; What does this look like? 16-17