Array size
- C course
- Beginner
- Lesson 55/116
- 3 min
Learn to compute the array size with sizeof.
Array size with sizeof
C has no operator that directly gives array length. But sizeof tells you both the total bytes of the array and the bytes of one element; the division gives the length.
int scores[5] = {85, 92, 78, 90, 88};
int size = sizeof(scores) / sizeof(scores[0]);
printf("%d\n", size); // 5Here sizeof(scores) is 20 bytes (5 × 4), sizeof(scores[0]) is 4 bytes. The division gives 5. This idiom is useful for loop bounds.
sizeof as loop bound
Using the sizeof idiom for loop bounds is safer than hard-coding the size. If you change the size later, the loop adapts automatically.
int scores[5] = {85, 92, 78, 90, 88};
int n = sizeof(scores) / sizeof(scores[0]);
for (int i = 0; i < n; i++) {
printf("%d\n", scores[i]);
}This style makes maintenance easier. If the array grows to 10, the loop runs 10 times. Hard-coding 5 would risk forgetting the update.
Practice Print the array length
Write a C program. Declare int scores[5] = {85, 92, 78, 90, 88};. Compute the length with int n = sizeof(scores) / sizeof(scores[0]);. Print with printf("%d\n", n);. Put #include <stdio.h> at the top.
Starter code
#include <stdio.h>
int main(void) {
// Declare scores, compute length with sizeof and print it
return 0;
}Sample solution
#include <stdio.h>
int main(void) {
int scores[5] = {85, 92, 78, 90, 88};
int n = sizeof(scores) / sizeof(scores[0]);
printf("%d\n", n);
return 0;
}Expected output
5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the sizeof pieces
| Term | Meaning |
|---|---|
sizeof(scores) |
Total bytes |
sizeof(scores[0]) |
Single element bytes |
sizeof(int) |
Type bytes |
/ sizeof |
Division |
2 How many bytes is sizeof(scores) for int scores[5]?
int scores[5];
sizeof(scores);- 4
- 5
- 10
- 20 Correct answer
Explanation: 5 int × 4 bytes = 20.
3 Complete the length idiom
int n = sizeof(scores) / sizeof(____);int n = sizeof(scores) / sizeof(scores[0]);Explanation: scores[0] gives the single element bytes.