Pointers and Arrays

POINTERS AND ARRAYS

The concept of array is very much bound to the one of the pointers. An array occupies consecutive memory locations. Consider Figure 7.2. For example, if we have an array declared as

int arr[] = {1, 2, 3, 4, 5}; then in memory it would be stored as shown in Figure 7.2.3

Array notation is a form of pointer notation. The name of the array is the starting address of the array in memory. It is also known as the base address. In other words, base address is the address of the first element in the array or the address of arr [0] . Now let us use a pointer variable as given in the statement below.

int *ptr;

ptr = &arr [0];

Here, ptr is made to point to the first element of the array. Execute the code given below and observe the output which will make the concept clear to you.

Programming Tip: The name of the array is actually a pointer that points to the first element of the array.

main()

{

int arr[]={1,2,3,4,5);

printf("\n Address of array = %p %p %p", arr, &arr[0], &arr);

}

Similarly, writing ptr = &arr [2], makes ptr to point to the third element of the array that has index 2. Figure 7.3 shows ptr pointing to the third element of the array.

If pointer variable ptr holds the address of the first element in the array, then the address of successive elements can be calculated by writing ptr++.

int *ptr = &arr [0];

ptr++;

printf("\n The value of the second element of the array is %d", *ptr);

The printf() function will print the value 2 because after being incremented ptr points to the next location. One point to note here is that if x is an integer variable, then x++ adds 1 to the value of x. But ptr is a pointer variable, so when we write ptr +i, then adding i gives a pointer that points i elements further along an array than the original pointer.

Programming Tip: An error is generated if an attempt is made to change the address of the array.

Since ++ptr and ptr++ are both equivalent to ptr +1, incrementing a pointer using the unary ++ operator, increments the address it stores by the amount given by sizeof(type) where type is the data type of the variable it points to (i.e., 2 for an integer). For example, consider Figure 7.4.

If ptr originally points to arr [2], then ptr++ will point to the next element, i.e., arr [3]. This is shown in Figure 7.4.

Had this been a character array, every byte in the memory would have been used to store an individual character. ptr++ would then add only 1 byte to the address of ptr.

When using pointers, an expression like arr[i] is equivalent to writing * (arr+i). If arr is the array name, then the compiler implicitly takes

arr = &arr [0]

To print the value of the third element of the array, we can straightaway use the expression * (arr+2). Note that

arr[i] = * (arr + i)

Many beginners get confused by thinking of array name as a pointer. For example, while we can write

ptr = arr; // ptr = &arr [0]

we cannot write

arr = ptr;

This is because while ptr is a variable, arr is a constant. The location at which the first element of arr will be stored cannot be changed once has arr [] been declared. Therefore, an array name is often known to be a constant pointer.

Programming Tip: When an array is passed to a function, we are actually passing a pointer to the function. Therefore, in the function declaration you must declare a pointer to receive the array name.

To summarize, the name of an a pointer to the array is equivalent to the address of its first element, as a pointer is equivalent to the address of the element that it points to. Therefore, arrays and pointers use the same concept.

Note

arr[i], i[arr], *(arr+i), *(i+arr) gives the same value.

Look at the following code and understand the result of Look at the following code and understand executing them.

main()

{

int arr[]={1,2,3,4,5};

int *ptr, i;

ptr=&arr [2];

*ptr = -1;

* (ptr+1) = 0;

*(ptr-1) = 1;

printf("\n Array is: ");

for (i=0;i<5;i++)

printf("%d", * (arr+i);

}

Output

Array is: 1 1 -1 0 5

In C we can add or subtract an integer from a pointer 2010 23 to get a new pointer, pointing somewhere other than the original position. C also permits addition and subtraction of two pointer variables. For example, look at the code given below.

main()

{

int arr[]={1,2,3,4,5,6,7,8,9};

int *ptr1, *ptr2;

ptr1 = arr;

ptr2 = ptr+2;

printf("%d", ptr2 –ptr1);

}

Output

2

In the code, ptr1 and ptr2 are pointers pointing to the elements of the same array. We may subtract two pointers as long as they point to the same array. Here, the output is2 because there are two elements between ptr1 and ptr2. Both the pointers must point to the same array or one past the end of the array, otherwise this behaviour cannot be defined.

Moreover, C also allows pointer variables to be com- pared with each other. Obviously, if two pointers equal, then they point to the same location in the array. However, if one pointer is less than the other, it means that the pointer points to some element nearer to the beginning of the array.

17. Write a program to display an array of given numbers.

#include <stdio.h>

main()

{

int arr[]={1,2,3,4,5,6,7,8,9};

int *ptrl, *ptr2;

ptr1 = arr;

ptr2 = &arr [8];

while (ptr1<=ptr2)

{

printf("%d", *ptr1);

ptr1++;

}

}

Output

1 2 3 4 5 6 7 8 9

18. Write a program to read and display an array of n integers.

#include <stdio.h>

int main()

{

int i, n;

int arr [10], *parr = arr;

printf("\n_Enter the number of elements: ");

scanf("%d", &n);

printf("\n Enter the elements: ");

for(i = 0; i < n; i++)

scanf("%d", parr+i);

printf("\n The elements entered are: ");

for (i=0; i < n; i++)

printf("\t %d", * (parr+i));

return 0;

}

Output

Enter the number of elements: 5

Enter the elements: 1 2 3 4 5

The elements entered are: 1 2 3 4 5

Note

An object is a named region of storage; an Ivalue is an expression referring to an object.

19. Write a program to find mean of n numbers using arrays.

#include <stdio.h>

int main()

{

int i, n, arr [20], sum =0;

int *pn = &n, *parr = arr, *psum = &sum;

float mean = 0.0, *pmean = &mean;

printf("\n Enter the number of elements: ");

scanf("%d", pn);

for (i = 0; i < *pn; i++)

{

printf("\n Enter the number: ");

scanf("%d", (parr + i));

}

for (i=0; i < *pn; i++)

*psum += * (arr + i);

*pmean = (float) *psum / *pn;

printf("\n The numbers you entered are: ");

for (i=0; i < *pn; i++)

printf("%d ", * (arr + i));

printf("\n The sum is: %d", *psum);

printf("\n The mean is: %.2f", *pmean);

return 0;

}

Output

Enter the number of elements: 5

Enter the number: 1

Enter the number: 2

Enter the number: 3

Enter the number: 4

Enter the number: 5

20. Write a program to find the largest of n numbers using arrays. Also display its position.

#include <stdio.h>

int main()

{

int i, n, arr [20], large = -32768, pos = 0;

int *pn = &n, *parr = arr, *plarge = &large, *ppos = &pos;

clrscr();

printf("\n Enter the number of elements in the array: ");

scanf("%d", pn);

for (i = 0; i < *pn; i++)

{

printf("\n Enter the number: ");

scanf("%d", parr+i);

}

for (i = 0; i < *pn; i++)

{

if (* (parr+i) > *plarge)

{

*plarge = *(parr+i);

*ppos = i;

}

}

printf("\n The numbers you entered are:") ;

for (i = 0; i < *pn; i++)

printf("%d ", * (parr+i));

printf("\n The largest of these numbers is: %d", *plarge);

printf("\n The position of the largest number in the array is: %d", *ppos);

return 0;

}

Output

Enter the number of elements in the array: 5

Enter the number: 1

Enter the number: 2

Enter the number: 3

Enter the number: 4

Enter the number: 5

The numbers you entered are:

1 2 3 4 5

The largest of these numbers is: 5

The position of the largest number in the array is: 4