Custom Data Types (Structs)

Custom Data Types (Structs)

All forms of digital data (strings, integers, chars, floats, etc…) are ultimately all just 1s and 0s in the system.

Their data type is what allows their binary information to be interpreted in different meanings.

It would be very useful if we were able to define and use our own type definitions

Custom Data Types (Structs)

Let’s consider some things to define a Person.

We would need a name, age, and perhaps an address.

char name[] = "John Citizen";
int age = 30;
char address[] = "1234 Street St";

What if we had to define two people?

char name_1[] = "John Citizen";
int age_1 = 30;
char address_1[] = "1234 Street St";

char name_2[] = "Jane Smith";
int age_2 = 25;
char address_2[] = "555 Main St";

This looks like it can grow out of hand…

Custom Data Types (Structs)

We can create a data structure in C using a struct.

struct structure_name {
    char your;
    int data;
    char here[];
};

For example

struct Person {        // Create a type 'struct Person'
    char name[30];     // Create a string variable
    int age;           // Create an age variable
    char address[100]; // Create an address variable
};

Aside: Structs vs Class

struct != class

In languages that implement the Object Oriented Programming (OOP) paradigm, a struct may look similar to a class or an object.

A struct is purely a data structure whereas an object is ‘alive’ and can perform functions. With structs, it is up to external code to access and modify the data.

Using structs

struct Person {
    char name[30];
    int age;
    char address[100];
};

To define a variable with our Person data type

struct Person p;

// We can then initialise values

p.name = "John Citizen";
p.age = 30;
p.address= "1234 Street St";

We can also define and assign our values together

struct Person p = {
    .age = 30,
    .name = "John Citzen",
    .address = "1234 Street St"
};

Note: We prepend a . before each variable name

If our data values are declared in the same order as they are defined, then we can do this

struct Person p = {
    "John Citzen", 30, "1234 Street St"
};

Accessing structs

struct Person p = {
    .name = "John Citzen",
    .age = 30,
    .address = "1234 Street St"
};

We can access a values from the struct

printf("Name: %s\n", p.name); // Output => Name: John Citizen
printf("Age: %d\n", p.age);   // Output => Age: 30
printf("Name: %s\n", p.name); // Output => Address: 1234 Str

Struct Size

The size of a struct is the sum of its component sizes.

struct Person {
    char name[30];
    int age;
    char address[100];
};

What is the size of struct Person?

sizeof(struct Person) == 134

struct Person {
    char* name;
    int age;
    char* address;
};

What is the size of struct Person?

sizeof(struct Person) == 12

Struct Operations

When creating functions that modify structs, we must remember to pass it by reference.
Otherwise changes will not be saved.

// Pass by reference
void set_age(struct Person *person, int new_age) {
    person->age = new_age;   // This is fine
    (*person).age = new_age; // This is also fine
}

// Pass by value
void set_age(struct Person person, int new_age) {
    //                     ^ This is NOT fine!
    person.age = new_age;
}
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