C Comments & Variables | C Programming for Beginners - Class 6 PDF Download

Comments in C

Comments can be used to explain code, and to make it more readable. It can also be used to prevent execution when testing alternative code.
Comments can be singled-lined or multi-lined.

Single-line Comments

• Single-line comments start with two forward slashes (//).
• Any text between // and the end of the line is ignored by the compiler (will not be executed).
• This example uses a single-line comment before a line of code:

Example

// This is a comment

printf("Hello World!");

This example uses a single-line comment at the end of a line of code:
Example

printf("Hello World!"); // This is a comment

C Multi-line Comments

• Multi-line comments start with /* and ends with */.
• Any text between /* and */ will be ignored by the compiler:

Example

/* The code below will print the words Hello World!

to the screen, and it is amazing */

printf("Hello World!");

C Variables

Variables are containers for storing data values.

In C, there are different types of variables (defined with different keywords), for example:

• int - stores integers (whole numbers), without decimals, such as 123 or -123
• float - stores floating point numbers, with decimals, such as 19.99 or -19.99
• char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes

Declaring (Creating) Variables

To create a variable, specify the type and assign it a value:

Syntax

type variableName = value;

Where type is one of C types (such as int), and variableName is the name of the variable (such as x or myName). The equal sign is used to assign a value to the variable.
So, to create a variable that should store a number, look at the following example:

Example

Create a variable called myNum of type int and assign the value 15 to it:

int myNum = 15;

You can also declare a variable without assigning the value, and assign the value later:

Example

int myNum;

myNum = 15;

Note: If you assign a new value to an existing variable, it will overwrite the previous value:

Example

int myNum = 15;  // myNum is 15

myNum = 10;  // Now myNum is 10

Output Variables

You learned from the output chapter that you can output values/print text with the printf() function:

Example

printf("Hello World!");

In many other programming languages (like Python, Java, and C++), you would normally use a print function to display the value of a variable. However, this is not possible in C:

Example

int myNum = 15;

printf(myNum);  // Nothing happens

To output variables in C, you must get familiar with something called "format specifiers".

Format Specifiers

• Format specifiers are used together with the printf() function to tell the compiler what type of data the variable is storing. It is basically a placeholder for the variable value.
• A format specifier starts with a percentage sign %, followed by a character.
• For example, to output the value of an int variable, you must use the format specifier %d or %i surrounded by double quotes, inside the printf() function:

Example

int myNum = 15;

printf("%d", myNum);  // Outputs 15

To print other types, use %c for char and %f for float:

Example

// Create variables

int myNum = 5;             // Integer (whole number)

float myFloatNum = 5.99;   // Floating point number

char myLetter = 'D';       // Character

// Print variables

printf("%d\n", myNum);

printf("%f\n", myFloatNum);

printf("%c\n", myLetter);

To combine both text and a variable, separate them with a comma inside the printf() function:

Example

int myNum = 5;

printf("My favorite number is: %d", myNum);

To print different types in a single printf() function, you can use the following:

Example

int myNum = 5;

char myLetter = 'D';

printf("My number is %d and my letter is %c", myNum, myLetter);

Add Variables Together

To add a variable to another variable, you can use the + operator:

Example

int x = 5;

int y = 6;

int sum = x + y;

printf("%d", sum);

Declare Multiple Variables

To declare more than one variable of the same type, use a comma-separated list:

Example

int x = 5, y = 6, z = 50;

printf("%d", x + y + z);

You can also assign the same value to multiple variables of the same type:

Example

int x, y, z;

x = y = z = 50;

printf("%d", x + y + z);

C Variable Names

• All C variables must be identified with unique names.
• These unique names are called identifiers.
• Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).
• Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

Example

// Good

int minutesPerHour = 60;

// OK, but not so easy to understand what m actually is

int m = 60;

The general rules for naming variables are:

• Names can contain letters, digits and underscores
• Names must begin with a letter or an underscore (_)
• Names are case sensitive (myVar and myvar are different variables)
• Names cannot contain whitespaces or special characters like !, #, %, etc.
• Reserved words (such as int) cannot be used as names