Functions and Pointers

Functions and Pointers Dr. SajibDatta CSE@UTA Oct 6, 2014

Passing multi-dimensional array #include <stdio.h> void print(int [][3], int , int); int main(void) { int a[2][3] = {{1,2,3}, {4,5,6}}; printf("%d\n", sizeof(a)); print(a,2,3); return 0; } void print(int b[][3], int r, int c) { inti,j; for(i=0;i<r;i++) { for(j=0; j<c;j++) { printf("%d", b[i][j]); } printf("\n"); } }

Pointers

Addresses in Memory • Everything in memory has an address. C allows us to obtain the address that a variable is stored at. • scanf() is an example using the address of a variable • scanf("%d", &year);

Addresses in Memory • Preceding a variable name by an ampersand, known as the address operator, will return its address: • #include <stdio.h> • int main(void) • { • int x; • /* notice the format specifier in printf() for an • address is %p */ • printf("The address for the memory allocated to x is %p\n", &x); • return 0; • }

hexadecimal (or called base-16) 0123456789ABCDEF • The address for the memory allocated to x is 0012FF60 in the previous example. • For 32 bits system the 0012FF60 hexadecimal will be converted to 32 bits binary (one hex character represented by 4 bits).

Pointer • A pointer is a variable whose value is a memory address. • Note the type of a pointer indicates the type of variable which it points to. • E.g., int * (called pointer-to-int type) should be initialized to point to a variable of type int. • Similarly, we have char*, float *, …… • Given a pointer variable, assignment can be done by using: • int * ptr = &pooh; /*assigns pooh’s address to ptr, we say ptr points to pooh*/ • ptr = &bah; /*make ptr point to some other variables*/

#include<stdio.h> • int main(void) • { • int num = 3, num1 = 5; • int* numptr; /* numptr is a pointer */ • printf("content of num is %d\n", num); • printf("address of num is %p\n", &num); • printf("address of num1 is %p\n", &num1); • numptr = &num; /* initialize numptr with the address of num */ • printf("content of numptr is %p\n", numptr); • numptr = &num1; • printf("content of numptr is %p\n", numptr); • return 0; • }

Output: content of num is 3 address of num is 0012FF60 address of num1 is 0012FF54 content of numptr is 0012FF60 content of numptr is 0012FF54 Press any key to continue . . . • #include<stdio.h> • int main(void) • { • int num = 3, num1 = 5; • int* numptr; /* numptr is a pointer */ • printf("content of num is %d\n", num); • printf("address of num is %p\n", &num); • printf("address of num1 is %p\n", &num1); • numptr = &num; /* initialize numptr with the address of num */ • printf("content of numptr is %p\n", numptr); • numptr = &num1; • printf("content of numptr is %p\n", numptr); • return 0; • }

Why pointers • C was developed when computers were much less powerful • The raw ability to work with particular memory locations was obviously a useful option to have. • Programming microcontrollers still need this. • Optimize a program to run faster or use less memory that it would otherwise.

#include<stdio.h> • int main(void) • { • intarr[3] = {2, 4, 6}, i; • for(i = 0; i<3; i++) • printf("The address of %d element is %p.\n", i, &arr[i]); • return 0; • } • The address of 0 element is 0032FE08. • The address of 1 element is 0032FE0C. • The address of 2 element is 0032FE10.

Indirection operator • Pointers allow us to modify content in memory by using indirection operator (or called dereference operator). • Putting an asterisk before your pointer variable • See the example in the next slide

What’s going here? • #include <stdio.h> • int main(void) • { • int bah = 10, val; • int* ptr = &bah; • val = *ptr; • printf("The value of val is %d.\n", val); • return 0; • }

Pointers We can use pointers in much the same way we do the variables that they point to. • #include <stdio.h> • int main(void) • { • int a = 3, b = 3; /* a and b start with equal values */ • int* bptr = &b; /* we’ll modify b using a pointer */ • a *= 4; • *bptr *= 4; • printf("a is %d, b is %d\n", a, b); • a--; • (*bptr)--; /* parentheses are necessary here to override the order of precedence */ • printf("a is %d, b is %d\n", a, b); • return 0; • }

Output: • a is 12, b is 12 • a is 11, b is 11

Functions and Pointers