What is constant in C?

• Integer constants
• Real or Floating point constants
• Octal & Hexadecimal constants
• Character constants
• String constants
• Backslash character constants

• C variable is a named location in a memory where a program can manipulate the data. This location is used to hold the value of the variable.
• The value of the C variable may get change in the program.
• C variable might be belonging to any of the data type like int, float, char etc.

Constant values can’t be modified by the program once it is declared. But, variable values can be modified by the program.

No. Variable names can’t start with numbers as per the variable naming rule.

• Variable declaration tells the compiler about data type and size of the variable. Whereas, variable definition allocates memory to the variable
• Variable can be declared many times in a program. But, definition can happen only one time for a variable in a program.
• Variable declaration is for assignment of properties and identification to a variable. Whereas, variable definition is for assignments of storage space to a variable

• Local variable
• Global variable
• Environment variable

• The variables which are having scope/life only within the function are called local variables.
• These variables are declared within the function and can’t be accessed outside the function.

• The variables which are having scope/life throughout the program are called global variables.
• Global variable is defined outside the main function. So, this variable is visible to main function and all other sub-functions.

• Environment variable is a variable that will be available for all C applications and C programs.
• Once environment variables are exported, we can access them from anywhere in a C program without declaring and initializing in an application or C program.

• The symbols which are used to perform logical and mathematical operations in a C program are called C operators.
• These C operators join individual constants and variables to form expressions. Operators, functions, constants and variables are combined together to form expressions.