Lesson 17 · 17 of 20

Structs · C

Group related data into a C struct: a shop's products with a name, price and stock, the dot, the arrow and pointers to structs.

C · Structs in DMarlon Studio: the example's code and its output in the terminal

A structure (struct) puts several related pieces of data into one type, each with its own name and type. struct Product has three fields: name, price and stock. It is C's way of describing "a product".

To create one, give the values between braces in the same order as the fields, as with mug, or by name, as with lamp: .name = "Lamp". The dot reaches each field: mug.price = 6.95 changes the price. A text field would be changed with strcpy, like any string.

show takes the struct by value, that is, a copy: enough to read it. sell, on the other hand, has to change the stock, so it takes a pointer: sell(&mug, 5). With a pointer to a struct, fields are reached with the arrow: p->stock is the same as (*p).stock.

sell will not sell more than there is: with 4 lamps asked for and 3 in stock, it adjusts units and the lamp ends up at 0.

Code · main.c

#include <stdio.h>// A struct groups related data under one name.struct Product {    char name[20];    double price;    int stock;};void show(struct Product p) {  // gets a copy: enough to read it    printf("%-10s %6.2f  stock %2d\n", p.name, p.price, p.stock);}// A pointer lets the function change the struct; -> reaches its fields.void sell(struct Product *p, int units) {    if (units > p->stock) units = p->stock;    p->stock -= units;    printf("Sold %d x %s\n", units, p->name);}int main(void) {    struct Product mug = {"Mug", 7.50, 12};    struct Product lamp = {.name = "Lamp", .price = 24.90, .stock = 3};    mug.price = 6.95;  // the dot reaches each field    show(mug);    show(lamp);    sell(&mug, 5);    sell(&lamp, 4);  // only 3 left    show(mug);    show(lamp);    return 0;}

Output · cc main.c -o main && ./main

Mug          6.95  stock 12
Lamp        24.90  stock  3
Sold 5 x Mug
Sold 3 x Lamp
Mug          6.95  stock  7
Lamp        24.90  stock  0

Key points

A struct groups several fields into one type.
variable.field reads or changes a field.
Pass a pointer so the function can change the struct.
p->field is (*p).field.
You can initialise by order or with .field = value.

Exercise

Write void restock(struct Product *p, int units) that adds stock, and a double stock_value(struct Product p) function that returns price times stock; use them on both products.