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C array search with explicit length

Search only within the supplied array bounds.

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Before you start

Arrays, functions, and loops.

Search only within the supplied array bounds.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

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.

Code example

#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

Example details

Follow the code

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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

Behind the scenes

How C processes this code

The C compiler produces a native program. Variables occupy memory, and execution enters main.

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.