Lesson 2.1. Types and variables

Variables

As in all programming languages, C uses variables. The variables can contain integers, reals, text .... Unlike constants, the content of variables can change during the execution of a program:

• the content of a constant cannot change;
• the content of a variable may change during the execution of the program.

C, a typed language

C is a typed language, that is to say that each variable is defined for a given type (integer, real, text...):

• Variables must be declared, it is not possible to use an undeclared variable.
• At the time of the declaration, the type of the variable must be specified (integer, float ...).
• The declaration is used to allocate the memory needed to store the variable.

Here is an example of Python code that creates a variable named A before displaying its value. No type is declared where creating the variable:

# A is not declared before its first use
A = 5

# Display the value of A
print (A)

Let's now consider the same example in C. We notice here that the type of the variable (int) is specified at the time of its declaration:

// Declaration of the variable A and its type (int = integer)
int A = 5;

// Display the value of A
printf("%d\n", A);

Specifying the type of variables may seem restrictive for the developer. But it has the has the advantage of minimizing memory allocation and maximizing performance. The vast majority of critical industrial systems, embedded or real-time systems are programmed in C.

Variables types

In C, there are 4 classes of variable types:

• Integers (int) which are used to store integer numbers : 0, 1, 25, -1024 ...
• Floats (float) which are used to store real numbers : 0.2 , 15.5, -3.1415 ...
• Characters (char) which are used to store alpha-numeric single characters : A, B, C ... Z, 0, 1, 2 ...9, ?, /, %, # ...
• The strings (char*) which are used to store text: This is an informative message.

Vocabulary

In C, there is a precise vocabulary for the use of variables. To be sure to be understood, make sure you are familiar with :

The declaration of a variable corresponds to the line where the type is associated with the variable:

// Declare a variable with its type
int A;

The assignment of a variable is the line where the content of the variable is specified:

Assignment of variable A
A = 5;

It is common to perform the declaration and assignment in a single line:

int A = 5;

Variable names

There are some restrictions on the naming of variables:

• Variable names can be composed of several characters.
• Variable names can contain upper or lower case letters (a to z and A to Z).
• Variable names can contain numbers (0 to 9).
• Variable names can contain an underscore (_).
• Variable names cannot begin with a number.
• It is preferable to avoid the use of reserved language keywords (main, return, int ...).
• The name of a variable must represent its usefulness (pressure_sensor, screen_width, nb_players... ).

Here are some examples of variables names that are syntactically correct:

int a;
int Z;
int Stuff;
int _duMP;
int Stuff_49;

Exercises

Exercise 1

Complete the following C code with:

• the declaration of the variable Var with type (int) ;
• its assignment with the value 128.

#include <stdio.h>

int main(void) {
  // Declaration of an integer variable named var

  // COMPLETE HERE

  // Assignment of 128 to var

  // COMPLETE HERE

  // Display the content of var
  printf ("The value of the variable is %d\n", Var);
  return 0;
}

#include <stdio.h> int main(void) { // Declaration of an integer variable named var // COMPLETE HERE // Assignment of 128 to var // COMPLETE HERE // Display the content of var printf("The value of the variable is %d\n", Var); return 0; }

Exercise 2

Modify the previous program so that the declaration and the assignment are done on the same line. on the same line.

#include <stdio.h> int main(void) { // Declaration and assignment of variable var // COMPLETE HERE // Display the content of var printf("The value of the variable is %d\n", Var); return 0; }

Quiz

What are the variable types in C?

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**Which type is used to define integers?

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Which type is used to define numbers with a decimal point?

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What is the declaration of a variable?

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Which variable names are correct?

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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 2.2. 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 12.1. Introduction to structures 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

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