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:
Syntax
int myNumbers[] = {25, 50, 75, 100};
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:
Example
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:
Syntax
myNumbers[0] = 33;
Example
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:
Example
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:
Example
// 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.
Get Array Size or Length
To get the size of an array, you can use the sizeof operator:
Example
int myNumbers[] = {25, 50, 75, 100};
printf("%lu" sizeof(myNumbers)); // Prints 20
Why did the result show 20 instead of 5, when the array contains 5 elements?
- It is because the sizeof operator returns the sizeof 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 one array element):
Example
int myNumbers[] = {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:
Example
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]);
}