Lecture 18: struct

The three file cabinets store things of the same type, just like arrays. The left one could be "bytes", the middle one "integers" and the right one "reals".

The file cabinet in the center is used for storing things of mixed type. In the same way, a struct is used for storing variables of different type, integers, real, or whatever, all together in the same box.

A visualization of a struct, namely a motley set of variables. Each variable inside a struct is called a field. Here we have 5 fields: an int (b), a float (f), a char (c), a single array of floats (r) and a double array of doubles (m).

To declare a struct we do the following
 

As an example, the definition of a struct containing the information of a student might have fields for name, year, and tuition fees paid:

struct {    char name[20];    int year;    int propinas;   } student;

This struct can only contain the information of a single student. Later we will see how to make an array of structs.

For example, to assign values to the struct student we can do the following

  strcpy(student.name, "Peter Stallinga");
  student.year = 2002;
  student.propinas = 1;

for the explanation of the function strcpy, see the lecture on strings.
Another example:

  struct {
      float x;
      float y;
  } coordinate;

  coordinate.x = 1.0;
  coordinate.y = 0.0;

This is not exactly a set of variables of mixed type, so in principle we could also do this with an array:

  float coordinate[2];

  coordinate[0] = 1.0;
  coordinate[1] = 0.0;

but, the first version, with the struct is more logical.
Another example:

  struct {
    char rua[20];
    int numero;
    int andar;
    char porta;
  } address;

  strcpy(address.rua, "Rua Santo Antonio");
  address.numero = 34;
  address.andar = 3;
  address.porta = 'E';

  printf("%s %d", address.rua, address.numero);
  printf("%d %c", address.andar, address.porta);

Rua Santo Antonio 34
3 E

struct {    char name[20];    int year;    int propinas;   } students[2000];

Note the structure of arrays of structs. students is an array of structs, therefore, students[i] is one of these structs and if we want to assign something to a field we use the period and the fieldname, so students[i].year is an int containing the year of student number i.
Wrong syntax would be students.year[i] (we could use this if we had a single struct students containing a field year that is an array)
Also wrong: students.[i]year, which doesn't make sense at all. Be careful where you put the dots.

Later we will see that we can much more easily declare variables of type array of structs (see lecture 19).