Arrays And Pointers (1d Arrays)

C++ Notes: Arrays as Pointers Using an array name as a pointer An array name is really a pointer to the first element of the array. For example, the following is legal. int b[100];

// b is an array of 100 ints.

int* p;

// p is a pointer to an int.

p = b;

// Assigns the address of first element of b to p.

p = &b[0];

// Exactly the same assignment as above.

Array name is a const pointer When you declare an array, the name is a pointer, which cannot be altered. In the previous example, you could never make this assignment. p = b; b = p; type.

// Legal -- p is not a constant. // ILLEGAL because b is a constant, although the correct

Pointer arithmetic "Meaningful" arithmetic operations are allowed on pointers. • Add or subtract integers to/from a pointer. The result is a pointer. • Subtract two pointers to the same type. The result is an int. • Multiplying, adding two pointers, etc. don't make sense.

Pointer addition and element size When you add an integer to a pointer, the integer is multiplied by the element size of the type that the pointer points to. // Assume sizeof(int) is 4. int b[100];

// b is an array of 100 ints.

int* p;

// p is a a pointer to an int.

p = b;

// Assigns address of first element of b. Ie, &b[0]

p = p + 1;

// Adds 4 to p (4 == 1 * sizeof(int)). Ie, &b[1]

Equivalence of subscription and dereference Because of the way C/C++ uses pointers and arrays, you can reference an array element either by subscription or * (the unary dereference operator). int b[100];

// b is an array of 100 ints.

int* p;

// p is a a pointer to an int.

p = b;

// Assigns address of first element of b. Ie, &b[0]

*p = 14;

// Same as b[0] = 14

p = p + 1;

// Adds 4 to p (4 == 1 * sizeof(int)). Ie, &b[1]

*p = 22;

// Same as b[1] = 22;

Example - Two ways to add numbers in an array The first uses subscripts, the second pointers. They are equivalent. int a[100];

int a[100];

. . .

. . .

int sum = 0;

int sum = 0;

for (int i=0; i<100; i++) {

for (int* p=a; p
sum += a[i]; }

sum += *p; }