C array search with explicit length

Search only within the supplied array bounds.

#include <stdio.h>
#include <assert.h>
int find(const int values[], int n, int target) {
    for (int i = 0; i < n; i++) {
        if (values[i] == target) return i;
    }
    return -1;
}
int main(void) {
    int values[] = {5, 2, 7};
    printf("%d\n", find(values, 3, 7));
    assert(find(values, 0, 7) == -1);
    assert(find(values, 3, 9) == -1);
    return 0;
}

Output

Explanation

An array parameter is adjusted to a pointer in a C function. The function does not automatically know how many elements are available, so length must be passed separately.

This search returns the first matching index or -1. The caller promises that n is nonnegative and no larger than the accessible element count.

Explore the dry run

Guided walkthroughReady

1#include <stdio.h>

2#include <assert.h>

3int find(const int values[], int n, int target) {

4 for (int i = 0; i < n; i++) {

5 if (values[i] == target) return i;

6 }

7 return -1;

8}

9int main(void) {

10 int values[] = {5, 2, 7};

11 printf("%d\n", find(values, 3, 7));

12 assert(find(values, 0, 7) == -1);

13 assert(find(values, 3, 9) == -1);

14 return 0;

15}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Create three elements.	`values = [5, 2, 7]`	—
Index 0 does not match 7.	`i = 0, value = 5`	—
Index 1 does not match.	`i = 1, value = 2`	—
Index 2 matches; return it.	`return = 2`	—
Print the found index.	`return = 2`	2
Check zero-length and missing-target cases.	`return = 2`	2

Cost and common mistakes

O(n) worst-case time and O(1) auxiliary space.

Using sizeof on the pointer parameter does not recover the array length.
Passing a length larger than the actual array allows invalid reads.

Try it yourself

Search for 5 and then 4.

Reveal answer and reasoning

The results are 0 and -1 respectively.

C array search with explicit length

Explanation

Follow the example

Cost and common mistakes

Try it yourself

Further reading