Arrays of Structures

Imagine a list of mountains, each with a name and a height. One mountain is one structure. To keep more of them, you already know what to use: an array. A structure is a type like any other, so you can make an array of structures:

#include <stdio.h>

typedef struct {
    char name[20];
    int height;
} Mountain;

int main(void) {
    Mountain mountains[3] = {
        {"Everest", 8849},
        {"K2", 8611},
        {"Lhotse", 8516}
    };

    for (int i = 0; i < 3; i++) {
        printf("%-8s %5d m\n", mountains[i].name, mountains[i].height);
    }
    return 0;
}

This prints:

Everest   8849 m
K2        8611 m
Lhotse    8516 m

Each element of the array is written as a structure of its own in curly braces, and the elements are separated by commas, as in any array. mountains[0] is the whole first mountain, Everest. mountains[i].name is read from left to right: first take element i of the array, then its member name.

A typical mistake is to swap the order and write mountains.name[i]. That means "the member name of the array mountains", but an array has no members, only elements:

main.c:16:41: error: member reference base type 'Mountain[3]' is not a structure or union
   16 |         printf("%-8s %5d m\n", mountains.name[i], mountains[i].height);
      |                                ~~~~~~~~~^~~~~

The message says that what stands before the dot, of type Mountain[3] (an array of three mountains), is not a structure. (A union is something similar to a structure that this course doesn't use.)

As in the lesson on strings, the printf in the loop sets the width of each column, so that the columns line up. %-8s prints the text in a column 8 characters wide, aligned to the left (that's what the - does), and %5d prints the number in a column 5 characters wide, aligned to the right. With decimal numbers, %6.2f means 2 decimal places and a width of 6 characters in total.

Instead of writing the count 3 in the loop, you can let the program work it out, as with any array: const int COUNT = sizeof(mountains) / sizeof(mountains[0]); The size of the whole array divided by the size of one element is the number of elements, whatever the elements are.