Lecture 10 - Introduction to Pointers

Question 1

what is a pointer?

Answer 1

a variable that stores the address of a variable

Question 2

normal variables: int x; vs. x = 5;

Answer 2

• with int x; the x variable occupies some space in memory
• with x = 5;. the compiler “knows” how to compute the address of the x variable and can arrange to have a value put there

Question 3

the operator &

Answer 3

• used to determine the location of any storage element
• NOT equivalent to the value that is stored at a given address

Question 4

p = &x explained

Answer 4

• p now holds the address that x resides at
• is it NOT equivalent to x
• instead we say that p POINTS to x

Question 5

the operator *

Answer 5

• used to access the value stored at the memory address
• if *p = x is equivalent to x
• put asterisk on variable you are assigning value to

Question 6

how to define a pointer

Answer 6

• int *p;
• this means that p is a pointer to an integer

Question 7

how to manipulate a pointer?

Answer 7

• the c language has an operator * called the indirection operator or contents-of operator
• *p = 5 places the value 5 into the memory location pointed to by p

Question 8

p = &x vs. *p = 5

Answer 8

• p = &x now “points” to x (its memory location, not its value)
• *p is equivalent to x

Question 9

why does x not increment?
#include <stdio.h>
void increment(int n) {
n = n + 1;
}
int main() {
int x = 0;
increment(x);
printf(“The value of x = %d\n”, x);
return 0;
}</stdio.h>

Answer 9

• x’s address was not passed to the increment function, just its value
• the new value computed inside increment() was not stored back into x
• there is NO way to increment() about the memory location of x

Question 10

proper way to increment a value using pointers/a method

Answer 10

include <stdio.h></stdio.h>

void increment(int *p) {
*p = *p + 1;
}
int main() {
int x = 0;
increment(&x);
printf(“value of x= %d\n”, x);
}

Question 11

passing by reference/pointer/address

Answer 11

• all same thing
• when you don’t pass the variable, but what the address of the variable is
• ex. scan(“%d\n”, &x);

Question 12

why don’t we need ‘&’ when we pass an array?

Answer 12

• pointers can be used just as arrays
• arrays are EQUIVALENT to pointers

Question 13

more pointer basics with &

Answer 13

• “Address-of” (&) can be used on array elements
• “Address-of” can be used for structs

Question 14

using a pointer as an array

Answer 14

• when you obtain the address of a variable ptr = &x,
• you can “dereference” it two ways
• y = *ptr or z = ptr[0]
• they mean the same thing

Question 15

differences between arrays and pointers

Answer 15

• you can reassign values to a pointer, but an array always points to the same thing
• a pointer definition allocates space only for the pointer value, array allocate space for all elements
• a function parameter defined as an array is really just a pointer

Question 16

examples of how pointers and arrays are equivalent

Answer 16

• when you assign an array (ptr = array), you’re assigning a pointer
• when you pass an array to something, you’re passing a pointer (strcpy(array1, array2))
• when you return array, you return a pointer