C Arrays Tutorial | 1D and 2D Array Examples

📖 What to learn on this page

✅ Must-know essentials

int a[5]; declares an array of 5
Index from a[0] to a[4]
Out-of-bounds access is a bug

⭐ Read if you have time

Initializer: int a[] = {1,2,3};
2D arrays: int m[3][4]
sizeof(a)/sizeof(a[0]) for element count

Array — group values of the same type

Indices (subscripts) start at 0.

int a[5] = {10, 20, 30, 40, 50};
printf("%d\n", a[0]); // -> 10
printf("%d\n", a[4]); // -> 50

⚠️ The 5 in int a[5] is the count. a[5] itself does not exist!

This is the top beginner mistake in C.
● In the declaration, [5] means "how many" → it reserves space for 5 elements.
● In an access, [ ] means "which one" → only 0 through 4 are valid.

int a[5];  // reserves 5 slots: a[0], a[1], a[2], a[3], a[4]

a[0] = 10; // ✅ first element

a[4] = 50; // ✅ last element

a[5] = 99; // ❌ out of range — writes into memory that isn't yours (undefined behavior)

You get a[0] through a[4] — exactly 5 slots. Remember: the declaration size 5 and the last valid index 4 are always off by one.

Why? An index is "how far from the start." The first element is "0 away," and with 5 slots the last is "4 away."

💡 How bad is out-of-range access? C won't flag a[5] or a[100] as an error (often not even a warning). At runtime it may clobber another variable, crash with a Segmentation fault, or — worst of all — appear to work while quietly corrupting data.
That's why loops use i < 5 (strict less-than) for "0 through 4." Writing i <= 5 would touch a[5] — the classic off-by-one bug.

Array memory layout

Array elements sit contiguously in memory. Click a cell to inspect it.

Click a cell to see its details.

Step execution — array with for loop

array_demo.c

Variable state

Name	Type	Value

Array memory

Standard output

2D arrays

For tabular data with rows and columns, use a 2D array.
Declare it as type name[rows][cols];.

Declare and initialize

// 3 rows x 4 columns
int matrix[3][4] = {
    {1,  2,  3,  4},   // row 0
    {5,  6,  7,  8},   // row 1
    {9, 10, 11, 12}    // row 2
};

Memory layout

In memory, a 2D array sits in one contiguous line (row-major order).
matrix[1][2] is the third element of row 1, which is 7.

Blue = row 0, green = row 1 (orange = matrix[1][2]), red = row 2

Traverse with nested loops

for (int i = 0; i < 3; i++) {        // row
    for (int j = 0; j < 4; j++) {    // column
        printf("%3d", matrix[i][j]);
    }
    printf("\n");
}

Multiplication tables and matrix math are classic use cases for 2D arrays.
When you pass one to a function, you must specify the column count: void func(int m[][4], int rows).

Try it yourself — array

my_array.c

Output

Press "Run" to execute...

⚠️ Is "Average = 80" really right? — the integer-division trap

The sum is 80+65+92+78+88 = 403, so the true average is 403 ÷ 5 = 80.6.
But the program prints 80. That's because in C, int divided by int truncates toward zero (int / int → int).

To get 80.6 properly:
Cast one operand to double: double avg = (double)sum / 5;
Then print with printf("Average = %.1f\n", avg); (%.1f = one decimal place).

💡 "Integer division truncates" is the top gotcha for C beginners. See Lesson 8 (Arithmetic operators) for more.

💡 Try these ideas too

Use double and a (double)sum cast to show the true average 80.6
Grow the array from 5 to 10 elements and compute the mean
Find the min and max using a for loop
Print the array in reverse order
Double every element

Review Quiz

Check your understanding of this lesson!

Q1. How many elements does `int a[5];` allocate?

a[5] gives you 5 elements, from a[0] through a[4].

Q2. What index does a C array start from?

Any

C array indices always start at 0. In a[5], a[0] is the first element.

Q3. For `int a[3] = {1, 2};`, what is the value of `a[2]`?

Undefined

When an array is partially initialized, the leftover elements are zero-initialized. So a[2] is 0.

Lesson 20: Arrays

Array — group values of the same type

Array memory layout

Step execution — array with for loop

Variable state

Array memory

Standard output

2D arrays

Declare and initialize

Memory layout

Traverse with nested loops

Try it yourself — array

Related Lessons

Review Quiz

Q1. How many elements does `int a[5];` allocate?

Q2. What index does a C array start from?

Q3. For `int a[3] = {1, 2};`, what is the value of `a[2]`?

Array — group values of the same type

Array memory layout

Step execution — array with for loop

Variable state

Array memory

Standard output

2D arrays

Declare and initialize

Memory layout

Traverse with nested loops

Try it yourself — array

Related Lessons

Review Quiz

Q1. How many elements does int a[5]; allocate?

Q2. What index does a C array start from?

Q3. For int a[3] = {1, 2};, what is the value of a[2]?

Q1. How many elements does `int a[5];` allocate?

Q3. For `int a[3] = {1, 2};`, what is the value of `a[2]`?