pointers pointer is a variable that contains the address of a variable here p is sahd to point to...

Pointers

Pointer is a variable that contains the address of a variable

Here P is sahd to point to the variable C

C

7 3 4… …

173172 174 175 176 177 178 179 180 181

174 3 4… …

P

833832 834 835 836 837 838 839 840 841

Referencing

The unary operator & gives the address of a variable

The statement P=&C assigns the address of C to the

variable P, and now P points to C To print a pointer, use %p format.

Referencing

int C;int *P; /* Declare P as a pointer to int

*/C = 7;P = &C;

C

7 3 4… …

173172 174 175 176 177 178 179 180 181

174 3 4… …

P

833832 834 835 836 837 838 839 840 841

Dereferencing

The unary operator * is the dereferencing operator

Applied on pointers Access the object the pointer

points to The statement *P=5; Puts in C (the variable pointed by

P) the value 5

Dereferencing

printf(“%d”, *P); /* Prints out ‘7’ */*P = 177;printf(“%d”, C); /* Prints out ‘177’ */P = 177; /* This is unadvisable! */

C

7 3 4… …

173172 174 175 176 177 178 179 180 181

174 3 4… …

P

833832 834 835 836 837 838 839 840 841

177

177

Example

pointers.c

pointers.c – step by step

int x=1, y=2, z[10]={5,6,7};int *ip; /* ip is a pointer to int */

ip = &x; /* ip now points to x */printf("ip now points to x that contains the value %d\n",*ip);y = *ip; /* y is now 1 */printf("y is now %d\n",y);*ip = 0; /* x is now 0 */printf("x is now %d\n",x);ip = &z[2]; /* ip now points to z[2] */printf("ip now points to z[2] that contains the value %d\n",*ip);*ip = 1; /* z[2] is now 1 */printf("z[2] is now %d\n", z[2]);printf("ip is %p\n", ip);

x y

z ip

1 2

364

5 6 7

Z[0] Z[1] Z[2]

120 248

364 368 372

…

564 772




x y

z ip

1 2

364

5 6 7

Z[0] Z[1] Z[2]

120 248

364 368 372

120

564 772




x y

z ip

1 1

364

5 6 7

Z[0] Z[1] Z[2]

120 248

364 368 372

120

564 772




x y

z ip

0 1

364

5 6 7

Z[0] Z[1] Z[2]

120 248

364 368 372

120

564 772




x y

z ip

0 1

364

5 6 7

Z[0] Z[1] Z[2]

120 248

364 368 372

372

564 772




x y

z ip

0 1

364

5 6 1

Z[0] Z[1] Z[2]

120 248

364 368 372

372

564 772

Common errors It is impossible to define pointers to constants

or expressions. It is also impossible to change a variable’s

address (because it is not for us to determine!).

Therefore, the following are errors: i = &3; j = &(k+5); k = &(a==b); &a = &b; &a = 150;

Pass arguments by value

The functions we saw till now accepted their arguments “by value”

They could manipulate the passed values

They couldn’t change values in the calling function

Wrong Swap

val_swap.c

How can we fix it?

We can define swap so it gets pointers to integers instead of integers

void swap(int *x, int *y) {

…swap *x and *y… } We then call swap by swap(&x,&y); This is passing values by address

Right Swap

add_swap.c

Back to scanf

We can now understand the & in scanf(“%d”,&a);

The argument list in scanf is simply passed by address, so scanf can change its content

Exercise

Write a function that accepts a double parameter and returns its integer and fraction parts.

Write a program that accepts a number from the user and prints out its integer and fraction parts, using this function

Solution

dbl_split.c

Exercise

The relation between rectangular and polar coordinates is given by –

r = sqrt(x2+y2)θ = tan-1(y/x)

Implement a function that accepts two rectangular coordinates and returns the corresponding polar coordinates Use the function atan defined in math.h

Solution

rec_to_polar.c

Pointers and Arrays

Recall that an array S holds the address of its first element S[0]

S is actually a pointer to S[0] int S[10]; int *P; P=S; /* From now P is equivalent to S */

Both P and S are now pointing to S[0]

Pointer-array equivalence Arrays are actually a kind of pointers! When an array is defined, a fixed

amount of memory the size of the array is allocated. The array variable is set to point to the

beginning of that memory segment When a pointer is declared, it is

uninitialized (like a regular variable) Unlike pointers, the value of an array

variable cannot be changed

Passing arrays to functions Arrays can be passed to functions and

have their values changed. This is possible because an array

variable is actually an address. The two following function argument

declarations are equivalent – int arr[] int *arr

Example – vec_mul.c

Arrays as function arguments Functions can accept arrays as

arguments Usually the array’s size also needs

to be passed (why?) For example -

int CalcSum(int arr[], int size); Within the function, arr is accessed

in the usual way Example – calc_sum.c

Exercise

Implement a function that accepts two integer arrays and returns 1 if they are equal, 0 otherwise

Write a program that accepts two arrays of integers from the user and checks for equality

Solution

compare_arrays.c

Exercise

Implement the functionint Subsequence(int arr1[], int

size1, int arr2[], int

size2); The function returns 1 iff arr2 is a

subsequence of arr1 For example, if arr1 = {1, 4, 6, 8} and

arr2 = {4, 6}.

Solution

subsequence.c

Exercise Write a program that accepts

positive integers from the user until a negative one is entered

The program then displays the 5 largest integers entered Not necessarily sorted by size!

Note – there’s no need to define functions other than main

Solution

top_numbers.c

pointers pointer is a variable that contains the address of a variable here p is sahd to point to...

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