Arrays

Arrays

Master arrays with examples and explanations

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1D Arrays:

Declaration and initialization of 1D arrays

1D arrays store a collection of similar type data elements using a single name.

Declaration:

type array_name[size];

Where type is the data type of the elements and size is the total number of elements.

Initialization:

  int marks[5] = {34, 45, 78, 90, 98};
  float prices[4] = {10.5, 20.2, 30.1, 40.0};  
  char names[6] = {'J','o','h','n',' ','D'};

Accessing array elements

Accessing Elements:

Elements are accessed using a subscript (index number) within square brackets [ ]. The index starts from 0.

array_name[subscript]

  marks[0]  // 34
  marks[1]  // 45
  marks[4]  // 98

Examples:

  int roll_nos[20];  

  char names[5][20]; 

  float temp[10] = {98.6, 99.2, 100.4, 101.0};
  • Arrays are stored in contiguous memory locations.

  • The variable name itself represents the base address of the array.

  • Arrays have a static size that needs to be specified during declaration.

  • The size of an array must be an integer constant.

Pointers to Arrays:

Declaring a pointer to an array

A pointer to an array is a variable that stores the base address of an array.

Declaring:

type *ptr_name = array_name;

Where type is the element type of the array.

Example:

  int marks[5] = {10, 20, 30, 40, 50};

  int *ptr = marks; // ptr points to the first element of marks array

Here ptr is a pointer to an array of integers that stores the base address of marks array.

Accessing array elements

We can access array elements using:

  1. Subscript [ ]
    marks[i]

  2. Pointer arithmetic * and + *(ptr + i)

Both are equivalent and yield the i-th element.

  marks[0] == 10
  *(ptr + 0) == 10

  marks[1] == 20  
  *(ptr + 1) == 20

  marks[2] == 30
  *(ptr + 2) == 30

Example

  #include <stdio.h>

  int main() {
     int marks[5] = {10, 20, 30, 40, 50};

     int *ptr = marks;  // ptr points to the first element of marks array

     // Accessing elements using subscript    
     printf("Element at index 0: %d\n", marks[0]);
     printf("Element at index 1: %d\n", marks[1]);

     // Accessing elements using pointer arithmetic
     printf("Element at index 0: %d\n", *ptr);
     printf("Element at index 1: %d\n", *(ptr + 1));

     // Traversing the array using pointers
     for (int i = 0; i < 5; i++) {
         printf("%d ", *(ptr + i));
     }   

     return 0;
  }

Output:

Element at index 0: 10
Element at index 1: 20 Element at index 0: 10
Element at index 1: 20
10 20 30 40 50

2D Arrays:

Declaration

A 2D array is declared as:

type array_name[row_size][col_size];

Where type is the data type of elements and row_size and col_size are the number of rows and columns respectively.

Initialization

2D arrays can be initialized in two ways:

  1. Using Initializer List:

     int marks[2][3] = {
       {34, 45, 78},
       {90, 98, 67}  
     };
    
  2. Using a For Loop:

     int numbers[3][4];
    
     for (int i = 0; i < 3; i++) {
       for (int j = 0; j <4; j++) {
           numbers[i][j] = i*j;
       } 
     }
    

Accessing elements

Elements are accessed using two subscripts - one for the row and one for the column.

array_name[row][column]

Example:

  marks[0][1]  // 45
  marks[1][2]  // 67

Example

Adding Two Matrices:

#include <stdio.h>

int main() 
{
   int r, c, i, j;

   printf("Enter rows and columns: ");
   scanf("%d %d", &r, &c);

   int a[r][c], b[r][c], sum[r][c];

   printf("Enter elements of first matrix: \n"); 
   for (i = 0; i < r; i++)
       for (j = 0; j < c; j++)
           scanf("%d", &a[i][j]);

   printf("Enter elements of second matrix: \n");    
   for (i = 0; i < r; i++)    
       for (j = 0; j < c; j++)      
           scanf("%d", &b[i][j]);

   for (i = 0; i < r; i++) {
       for (j = 0; j < c; j++) {
           sum[i][j] = a[i][j] + b[i][j];
       }
   }

   printf("Sum of the matrices: \n");
   for (i = 0; i < r; i++) {
       for (j = 0; j < c; j++) {
           printf("%d ", sum[i][j]);    
       }
       printf("\n");
   }   
}

OUTPUT:

Enter rows and columns: 2 2  
Enter elements of first matrix:
1 2  
3 4
Enter elements of second matrix:    
4 3
2 1

Sum of the matrices:

5 5     
5 5

3D Arrays:

Declaration

A 3D array is declared as:

type array_name[x][y][z];

Where type is the data type of elements and x,y and z are the number of elements in the first, second and third dimensions respectively.

Initialization

3D arrays can be initialized using initializer lists:

  int marks[2][3][4] = {
      {   
          {1, 2, 3, 4},
          {5, 6, 7, 8},  
          {9, 10, 11, 12}  
      },
      {
          {13, 14, 15, 16}, 
          {17, 18, 19, 20},
          {21, 22, 23, 24}   
      }
  };

Accessing elements

Elements are accessed using three subscripts - one for each dimension.

array_name[x][y][z]

Example:

  marks[0][1][2] // 11  
  marks[1][2][3] // 24

Example

  #include <stdio.h>

  int main() {
      int marks[2][3][4] = { ... };

      printf("Element at 0 1 2 is %d", marks[0][1][2]);
  }

Passing Arrays to Functions:

Passing 1D Arrays:

A 1D array can be passed to a function like this:

  void func(int array[]) {
      // array used here
  }

  int main() {
      int array[5] = {1, 2, 3, 4, 5};

      func(array); 
  }

The array name acts as a pointer to its first element. So the function receives a pointer to the first element of the array, not a copy of the whole array.

Passing 2D Arrays:

A 2D array is passed as:

  void func(int array[][]) {
      // array used here
  }

  int main() {
      int array[3][4] = { ... };

      func(array);
  }

Again, the array name acts as a pointer to the first element. The second dimension size can be omitted.

Passing 3D Arrays:

A 3D array is passed similarly:

  void func(int array[][][]) {
      // array used here
  }

  int main() {
      int array[2][3][4] = { ... };

      func(array); 
  }

Example:

  void sum(int array[][3], int rows) {
      int sum = 0;

      for (int i = 0; i < rows; i++) {
          for (int j = 0; j < 3; j++) {
              sum += array[i][j];   
          }    
      }

      printf("Sum is %d", sum);
  }

  int main() {
      int array[2][3] = { ... };

      sum(array, 2); 
  }

Function Pointers:

Declaring function pointers

A function pointer is declared as:

return_type (*function_pointer_name)(arguments);

For example:

  int (*sum)(int, int); // pointer to a function that returns an int and has 2 int parameters

Defining and initializing function pointers

A function pointer can be initialized to point to an actual function:

  int sum(int a, int b){
      return a + b;
  }

  int main(){
      int (*sum_ptr)(int, int) = &sum;  // initialize pointer to point to sum()
  }

Calling functions using pointers

The function is called using the pointer:

  int result = sum_ptr(10, 20);

This calls the sum() function.

Example program

  int sum(int a, int b) { ... }
  int multiply(int a, int b) { ... }

  int main() {
      int (*func_ptr)(int, int);

      func_ptr = sum; 
      func_ptr(10, 20);  // Calls sum()

      func_ptr = multiply;
      func_ptr(10, 20);  // Calls multiply()
  }

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