0x80Lesson 9 of 15

Arrays

Store many values of the same type side by side in memory, and loop over them safely.

14 min 5-question quiz 2 code exercises

By the end of this lesson you can

Declare, initialize and index arrays
Loop over an array without going out of bounds
Pass arrays to functions along with their length

An array is a fixed number of values of the same type, stored one after another in memory. Instead of score1, score2, score3... you write int scores[3] and reach each one by its index.

scores.c

1#include <stdio.h>
2
3int main(void) {
4    int scores[5] = {90, 72, 85, 64, 99};
5
6    printf("First: %d\n", scores[0]);
7    printf("Last: %d\n", scores[4]);
8
9    scores[1] = 75;   // change an element
10    printf("Second is now %d\n", scores[1]);
11    return 0;
12}

Output

First: 90
Last: 99
Second is now 75

Indexes start at zero

An array of 5 elements has indexes 0 to 4. The index is really an offset: "start of the array, plus this many elements". So scores[0] is the first element and scores[4] the last. There is no scores[5].

Looping over an array

The standard loop visits indexes 0 up to, but not including, the length. You can compute the length of an array (in the same function where it is declared) with sizeof:

average.c

1#include <stdio.h>
2
3int main(void) {
4    int scores[] = {90, 72, 85, 64, 99};
5    size_t count = sizeof scores / sizeof scores[0];
6
7    int total = 0;
8    for (size_t i = 0; i < count; i++) {
9        total += scores[i];
10    }
11    printf("%zu scores, average %.1f\n", count, (double)total / count);
12    return 0;
13}

Output

5 scores, average 82.0

With an initializer you can leave the size empty (int scores[] = {...}) and the compiler counts for you. If you give a size and fewer values, the rest are set to zero: int counts[26] = {0}; is the idiomatic way to zero an array.

Arrays and functions

When you pass an array to a function, C does not copy it. The function receives the address of the first element, which has an important consequence: inside the function, sizeof can no longer tell you the length. So you always pass the length as a separate parameter. It also means a function can modify the caller's array.

max.c

1#include <stdio.h>
2
3int find_max(const int values[], size_t length) {
4    int best = values[0];
5    for (size_t i = 1; i < length; i++) {
6        if (values[i] > best) {
7            best = values[i];
8        }
9    }
10    return best;
11}
12
13int main(void) {
14    int data[] = {3, 41, 7, 26, 9};
15    printf("Max: %d\n", find_max(data, 5));
16    return 0;
17}

Output

Max: 41

Quick check

Predict the output

What does this print?

1#include <stdio.h>
2
3void zero_first(int values[]) {
4    values[0] = 0;
5}
6
7int main(void) {
8    int numbers[3] = {5, 6, 7};
9    zero_first(numbers);
10    printf("%d %d %d\n", numbers[0], numbers[1], numbers[2]);
11    return 0;
12}

Output

Two-dimensional arrays

int grid[3][4]; is 3 rows of 4 columns. Access elements with two indexes, grid[row][col], and loop with nested for loops. In memory it is still one flat run of 12 ints, row after row.

Key takeaways

An array of N elements has indexes 0 to N-1. C never checks bounds for you.
Loop with for (size_t i = 0; i < length; i++). Get the length with sizeof array / sizeof array[0] where the array is declared.
Arrays passed to functions are not copied: pass the length separately, and use const if the function should not modify it.

Lesson quiz

5 questions · pass with 4 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write real C

These run for real. Write the program, press Run tests, and read what the compiler tells you - learning to read compiler messages is half of learning C.

Exercise 1

Print in reverse

+25 XP

Read 5 integers into an array, then print them in reverse order separated by spaces, followed by a newline.

For input 1 2 3 4 5 print 5 4 3 2 1.

1 2 3 4 5
Mixed numbers

main.c

Compiler Explorer

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Exercise 2

Above average

+25 XP

The first number in the input is n (at most 100), followed by n integers. Print how many of them are strictly greater than their average.

For 5 then 1 2 3 4 10 the average is 4, so the answer is 1.

1 2 3 4 10
All equal
Half above

main.c

Compiler Explorer

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Your code is compiled with gcc 14 (-Wall -Wextra) and run on Compiler Explorer (godbolt.org), a free public service.

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