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C Data Types

To understand C code you must grasp idea of data type.  You can not declare anything without using a data type. However you can define your own types using keyword: typedef. C has very few basic data types named "primitive":

Primitive types:

C has very few primitive types that are predefined. All other types are derived from these basic types.

To detect the real size of the data type you can use function: sizeof()

Example:

#include <stdio.h>
int main() {
    short a;
    long b;
    double d;
    printf("size of short = %d bytes\n", sizeof(a));
    printf("size of long = %d bytes\n", sizeof(b));
    printf("size of long double= %d bytes\n", sizeof(d));
    return 0;
}

Note: You can use following modifiers for integer type:

• short
• long 
• signed
• unsigned

Derived types:

You can create new data types from basic types using a derived data types. There are different methods of derivation. One method is to group multiple elements into data collection or structure. These are called improperly user defined data types.

• enumerations
• arrays
• pointers
• function types
• structures 

I will present derived types in future articles.

Identifiers

To make a C program you must declare names or identifiers for things that can be constants, variables, functions or data types. C has no "define" keyword: instead, a statement beginning with the name of a type. There is no "function" keyword: instead, a function is indicated by the parentheses following the function name. In parentheses you can enumerate formal parameters.

Variables

Next, you will learn about variables and rules for naming a variable.  A variable is like a labeled container or storage area in computer memory used to hold data for a short period of time. Variable names are symbolic representation of a memory location. Content of this location can be changed.

Example:

//declare variable ch with initial value 'a'
char ch = 'a';

//alter variable ch content using operator '='
ch = 'l';

Notes:

1. A variable name can have only Latin letters, digits and underscore,
2. The first letter of a variable should be either a letter or an underscore,
3. Identifier name in C should not be longer than 31 characters.

Constants

You can define variables that are immutable. These area actually called constants. 

Example:

const double PI = 3.14; //protected
PI = 2.9; //Error: PI is a constant

Literals

Now is time to understand something more. C source code can contain numbers and characters after the symbol "=", for example: 3.14.  These elements of the language are also constants, but are different kind of constants named Literals. The literals can be changed only if you change the source code.

Literal examples:

• Character: ‘A’, ‘b’, ‘1’
• Decimal: 0, -9, 22
• Octal: 021, 077, 033
• Hexadecimal: 0x7f, 0x2a, 0x521
• Floating point: -2.0
• Scientific notation -0.25E-5 
• String: "abc"

Escape sequence:

Some characters are not printable. For representing these characters in string literals you can use escape notation. Use symbol "\" follow by a letter. This will be replaced by the compiler with the proper code point.

Example:

"One line \nSecond line."

Boolean Type

Boolean algebra is a mathematic theory and is an essential part of any programming language. In C there is no Boolean type per say. {0 = False, 1 = True}. Any value = 0 is considered False. Any value ≠ 0 is considered True.

• True = 1
• False = 0
• True > False

Boolean Operators:

Boolean operators can be used in C with any numeric value. There is no Boolean reserved type. So any numeric literal = 0, = 1 or numeric value >=0 will do. There is no other type checking so you may do it all wrong but C will not worn you about type mismatch.

There are 3 Boolean Operators:

• Logical NOT is called bang: "!"
• Logical AND is two ampersand: "&&"
• Logical OR is two vertical bars: "||"

Table of truth:

Relation Operators

In practice common logical expressions are comparison between two variables, constants or literals using comparison operators. In the next table you can analyze these operators and two examples: positive and negative.

Logical Expressions

Read next: C Pointers