C Array

Arrays

Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.

To create an array, define the data type (like int) and specify the name of the array followed by square brackets [].

To insert values to it, use a comma-separated list inside curly braces, and make sure all values are of the same data type:

We have now created a variable that holds an array of four integers.

Access the Elements of an Array

To access an array element, refer to its index number.

Array indexes start with 0: [0] is the first element. [1] is the second element, etc.

This statement accesses the value of the first element [0] in myNumbers:

int myNumbers[] = {25, 50, 75, 100};
printf("%d", myNumbers[0]);

// Outputs 25

Change an Array Element

To change the value of a specific element, refer to the index number:

int myNumbers[] = {25, 50, 75, 100};
myNumbers[0] = 33;

printf("%d", myNumbers[0]);

// Now outputs 33 instead of 25

Loop Through an Array

You can loop through the array elements with the for loop.

The following example outputs all elements in the myNumbers array:

int myNumbers[] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
  printf("%d\n", myNumbers[i]);
}

Set Array Size

Another common way to create arrays, is to specify the size of the array, and add elements later:

// Declare an array of four integers:
int myNumbers[4];

// Add elements
myNumbers[0] = 25;
myNumbers[1] = 50;
myNumbers[2] = 75;
myNumbers[3] = 100;

Using this method, you should know the number of array elements in advance, in order for the program to store enough memory.

You are not able to change the size of the array after creation.

Avoid Mixing Data Types

It is important to note that all elements in an array must be of the same data type.

This means you cannot mix different types of values, like integers and floating point numbers, in the same array:

int myArray[] = {25, 50, 75, 3.15, 5.99};

In the example above, the values 3.15 and 5.99 will be truncated to 3 and 5. In some cases it might also result in an error, so it is important to always make sure that the elements in the array are of the same type.

Array Size

Get Array Size or Length

To get the size of an array, you can use the sizeof operator:

int myNumbers[] = {10, 25, 50, 75, 100};
printf("%lu", sizeof(myNumbers)); // Prints 20

- It is because the sizeof operator returns the size of a type in bytes.

You learned from the Data Types chapter that an int type is usually 4 bytes, so from the example above, 4 x 5 (4 bytes x 5 elements) = 20 bytes.

Knowing the memory size of an array is great when you are working with larger programs that require good memory management.

But when you just want to find out how many elements an array has, you can use the following formula (which divides the size of the array by the size of the first element in the array):

int myNumbers[] = {10, 25, 50, 75, 100};
int length = sizeof(myNumbers) / sizeof(myNumbers[0]);

printf("%d", length);  // Prints 5

Making Better Loops

In the array loops section in the previous chapter, we wrote the size of the array in the loop condition (i < 4). This is not ideal, since it will only work for arrays of a specified size.

However, by using the sizeof formula from the example above, we can now make loops that work for arrays of any size, which is more sustainable.

Instead of writing:

int myNumbers[] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
  printf("%d\n", myNumbers[i]);
}

It is better to write:

Example

int myNumbers[] = {25, 50, 75, 100};
int length = sizeof(myNumbers) / sizeof(myNumbers[0]);
int i;

for (i = 0; i < length; i++) {
  printf("%d\n", myNumbers[i]);
}

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Multidimensional Arrays

In the previous chapter, you learned about arrays, which is also known as single dimension arrays. These are great, and something you will use a lot while programming in C. However, if you want to store data as a tabular form, like a table with rows and columns, you need to get familiar with multidimensional arrays.

A multidimensional array is basically an array of arrays.

Arrays can have any number of dimensions. In this chapter, we will introduce the most common; two-dimensional arrays (2D).

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Two-Dimensional Arrays

A 2D array is also known as a matrix (a table of rows and columns).

To create a 2D array of integers, take a look at the following example:

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

The first dimension represents the number of rows [2], while the second dimension represents the number of columns [3]. The values are placed in row-order, and can be visualized like this:

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Access the Elements of a 2D Array

To access an element of a two-dimensional array, you must specify the index number of both the row and column.

This statement accesses the value of the element in the first row (0) and third column (2) of the matrix array.

Example

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

printf("%d", matrix[0][2]);  // Outputs 2

Remember that: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.

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Change Elements in a 2D Array

To change the value of an element, refer to the index number of the element in each of the dimensions:

The following example will change the value of the element in the first row (0) and first column (0):

Example

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };
matrix[0][0] = 9;

printf("%d", matrix[0][0]);  // Now outputs 9 instead of 1

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Loop Through a 2D Array

To loop through a multi-dimensional array, you need one loop for each of the array's dimensions.

The following example outputs all elements in the matrix array:

Example

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

int i, j;
for (i = 0; i < 2; i++) {
  for (j = 0; j < 3; j++) {
    printf("%d\n", matrix[i][j]);
  }
}

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Real-Life Example

To demonstrate a practical example of using arrays, let's create a program that calculates the average of different ages:

Example

// An array storing different ages
int ages[] = {20, 22, 18, 35, 48, 26, 87, 70};

float avg, sum = 0;
int i;

// Get the length of the array
int length = sizeof(ages) / sizeof(ages[0]);

// Loop through the elements of the array
for (i = 0; i < length; i++) {
  sum += ages[i];
}

// Calculate the average by dividing the sum by the length
avg = sum / length;

// Print the average
printf("The average age is: %.2f", avg);

And in this example, we create a program that finds the lowest age among different ages:

Example

// An array storing different ages
int ages[] = {20, 22, 18, 35, 48, 26, 87, 70};

int i;

// Get the length of the array
int length = sizeof(ages) / sizeof(ages[0]);

// Create a variable and assign the first array element of ages to it
int lowestAge = ages[0];

// Loop through the elements of the ages array to find the lowest age
for (i = 0; i < length; i++) {
  if (lowestAge > ages[i]) {
    lowestAge = ages[i];
  }
}