Lesson 12.1. Introduction to structures in C

Arrays allow to group a set of variables, but only of the same type. In a similar way, a structure allows to define a set of variables, but of different types.

Syntax

Let's suppose that we want to memorize for a student

• his first name;
• his last name;
• and his grade point average.

We then create the following structure:

struct student
{
  char firstname[50];
  char lastname[50];
  float gpa;
};

firstname, lastname and gpa are called the fields of the student structure.

The student structure is actually a new type that we can use in the same way as char, int, double ... It would be more coherent to use the following alternative syntax:

typedef struct
{
  char firstname[50];
  char lastname[50];
  float gpa;
} student;

With this syntax, and in particular the use of the keyword typedef, we understand better that we are creating a new type and not a new variable. In the rest of this course, we will prefer the first syntax which is mostly used.

Declaration of structures

When you have created a new structure, you can declare variables which will be of the type of the structure (here student):

struct student postGraduate;

postGraduate is a variable of the type of the student structure, which will have 3 fields: firstname, name and average.

Field access

Access to the fields of a structure is done, in reading and writing, by adding the name of the field separated by a dot :

// Assign 12.5 to the GPA field
postGraduate.gpa = 12.5;

// Display the last name of the student
printf ("Family name: %s", postGraduate.lastname);

Example

The following example creates a new structure and asks the user to enter some fields before displaying them:

// Declare the student structure
struct student
{
  char firstname[50];
  char lastname[50];
  float gpa;
};

int main(void) {
  // Declare a new student element
  struct student postGraduate;

  // Ask first and last name
  printf("Last name: ");
  scanf ("%s", postGraduate.lastname);
  printf("First name: ");
  scanf ("%s", postGraduate.firstname);

  // Display the name of the student
  printf ("Hello %s %s.\n", postGraduate.firstname, postGraduate.lastname);

  return 0;
}

Last name: Tucky
First name: Ken
Hello Ken Tucky.

Exercises

Exercise 1

Create a date structure that will contain the following fields:

• day, an integer between 0 and 31 ;
• month, an integer between 0 and 12 ;
• year, an integer between 0 and 3000.

Declare a structure of type date and assign to each field the date of birth of Linus Torvalds (founder of the Linux kernel) before displaying it:

Linus T. was born on 12/28/1969.

#include <stdio.h> // Create the structure date // COMPLETE HERE int main(void) { // Create a new variable of type date // COMPLETE HERE // Assign the birth date of Linus Torvalds (December 28, 1969.) // COMPLETE HERE // Display the birth date printf ("Linus T. was born on %d/%d/%d.\n", linus.month , linus.day , linus.year); return 0; }

Exercise 2

Create a structure named point with two fields:

• x, the ordinate of a point
• y, the abscissa of a point

Create a point array that contains the coordinates of the points of a unit circle with a step of 20 degrees. Recall that the coordinates of the unit circle can be calculated by the following formulas:

$$ \begin{bmatrix} x \\ y \end{bmatrix} = \begin{bmatrix} cos(\alpha) \\ sin(\alpha) \end{bmatrix} $$

Display the points according to this example:

P(0)    = (  1.00 ;  0.00 )
P(1)    = (  0.94 ;  0.34 )
P(2)    = (  0.77 ;  0.64 )
P(3)    = (  0.50 ;  0.87 )
P(4)    = (  0.17 ;  0.98 )
P(5)    = ( -0.17 ;  0.98 )
P(6)    = ( -0.50 ;  0.87 )
P(7)    = ( -0.77 ;  0.64 )
P(8)    = ( -0.94 ;  0.34 )
P(9)    = ( -1.00 ;  0.00 )
P(10)   = ( -0.94 ; -0.34 )
P(11)   = ( -0.77 ; -0.64 )
P(12)   = ( -0.50 ; -0.87 )
P(13)   = ( -0.17 ; -0.98 )
P(14)   = (  0.17 ; -0.98 )
P(15)   = (  0.50 ; -0.87 )
P(16)   = (  0.77 ; -0.64 )
P(17)   = (  0.94 ; -0.34 )
P(18)   = (  1.00 ; -0.00 )

#include <stdio.h> #include <math.h> // Create the structure point (x,y) // COMPLETE HERE int main(void) { // Declare an array of 19 points struct point P[19]; // Calculate the coordinates of each point for (int i=0 ; i<19 ; i++) { // COMPLETE HERE } // Display points // COMPLETE HERE return 0; }

Quiz

In C, what is the purpose of a structure?

---

In C, what do we call the elements composing a structure?

---

How do you access the fields of a structure?

---

Which syntaxes declare a variable of type article?

struct article {
  char name[50];
  float price[50];
  int quantity;
};

---

What does the following code display?

struct point {
  double x; double y;
};

int main(void) {
  struct point P = {12.3, 24.5};
  printf ("%.2lf %.2lf", P.y, P.x);
}

---

See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.1. Conditional branching (if...else)
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.3. For loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.2. Bit masking, how to clear a bit in C?
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.8. Random numbers in C
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.2. Syntax of C pointers
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.3. Structures and pointers
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

---