Keywords, Datatypes and Variables

Keywords in C

Keywords are reserved words in C that have special meanings and cannot be used as identifiers. In C, all keywords are in lowercase letters.

auto break case char
const continue default do
double else enum extern
float for goto if
int long register return
short signed sizeof static
struct switch typedef union
unsigned void volatile while

Data Types in C

C provides various data types to store different kinds of data. In C, data types are categorized into three main groups:

  • Primary Data Types
  • Derived Data Types
  • User-Defined Data Types

Primary Data Types

Primary data types are the basic data types provided by C. They include:

Data Type Keyword Description Size (bytes) Range
int Integer type 4 -2,147,483,648 to 2,147,483,647
float Floating-point type 4 Approximately ±3.4E±38 (7 digits precision)
double Double-precision floating-point type 8 Approximately ±1.7E±308 (15 digits precision)
char Character type 1 -128 to 127 (or 0 to 255 if unsigned)
void No value type N/A N/A

Derived Data Types

Derived data types are formed by combining primary data types. They include:

  • Arrays
  • Pointers
  • Functions
  • Structures
  • Unions

User-Defined Data Types

User-defined data types are created by the programmer to suit specific needs. They include:

  • enum (Enumerations) - enum data type is used to define a set of named integer constants. For example:
    #include <stdio.h>
    int main() {
        enum Day { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday };
        enum Day today = Wednesday;
        printf("Day number: %d\n", today);
        return 0;
    }
    Output:
    Day number: 3
  • typedef (Type Definitions) - typedef is used to create a new name for an existing data type. For example:
    #include <stdio.h>
    typedef unsigned int uint;
    int main() {
        uint age = 25;
        printf("Age: %u\n", age);
        return 0;
    }
    Output:
    Age: 25

Type Conversion

Converting one data type to another is called type conversion or type casting.

Implicit Type Conversion:

The compiler automatically converts one data from type to another is called implicit type conversion. Compiler converts data from lower to higher data type.

For Example:
#include <stdio.h>
int main() {
    int a = 5;
    float b;
    b = a; // Implicit conversion from int to float
    printf("Value of b: %f", b);
    return 0;
}
Output:
Value of b: 5.000000
Explicit Type Conversion:

When a programmer manually converts one data from type to another, it is called explicit type conversion or type casting.

For Example:
#include <stdio.h>
int main() {
    int numaretor = 5, denominator = 2;
    float result;
    result = (float)numaretor / denominator; // Explicit conversion from int to float
    printf("Value of result: %f", result);
    return 0;
}
Output:
Value of result: 2.500000

Variables

A variable is a named location in memory used to store data values. In C, variables must be declared before they can be used. The declaration specifies the type and name of the variable.

Variable Declaration Syntax:
data_type variable_name;
Example:
int num
Variable Initialization Syntax:
data_type variable_name = value;
Example:
int num = 30;

Common Interview Questions

  1. What is keyword?

    A keyword is a reserved word that have special meaning in compiler.

  2. Difference between type conversion and type casting?
    • Type Conversion: The data is converted from one data type to another data type by the compiler at compile time.
    • Type Casting: The data is converted from one data type to another data type by the programmer using casting operator ().
  3. What is enum data type?

    An enumeration(enum) is an user-defined data type that contains set of named integer constants.

  4. What is an empty data type?

    void

  5. Difference between variable declaration, initialization and definition?
    • Declaration - Creation of variable.
    • Definition - Creation of variable and also provides implementation details.
    • Initialization - Assigning value at the time of creating variable.
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