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Constants in C language

Constant value is understandable as nonchangeable value like PI=3.141592… value in math. Usually, you use constants in your programs, but don’t realize that they are constants. For instance:

x=x+3; The number 3 is a constant which will be compiled directly in addition operation. Constants can be character or string. Like in function printf(“Hello World\n”); “Hello World” is a string constant that is placed in program memory and will never change.

It is usually recommended to declare constants by using identifier with reserved word const:

const int No=44;

Identifying the variable as constant will cause the compiler to store this variable in program memory rather than in RAM, thus saving space in RAM. If special functions are used, then constants can also be stored in EEPROM memory.

Few words for the numeric constants. Numeric constants can be declared in many ways indicating their base.

  • Decimal integer constants (base 10) consist of one or more digits, 0 through 9, where 0 cannot be used as the first digit.
  • Binary constants (base 2) begin with a 0b or 0B prefix, followed by one or more binary digits (0, 1).
  • Octal constants (base 8) consist of one or more digits 0 through 7, where the first digit is 0.
  • Hexadecimal constants (base 16) begin with a 0x or 0X prefix, followed by a hexadecimal number represented by a combination of digits 0 through 9 and characters A through F.
  • Floating-point constants consist of:
    • an optional sign – or +;
    • an integer part a combination of digits 0 through 9;
    • a period;
    • a fractional part a sequence of digits 0 through 9;
    • an optional exponent e or E, followed by an optionally signed sequence of one or more digits

For example, the following floating-point constant contains both the optional and required parts: +1.15e-12.

Character constant can be represented as a character in quotation marks like ‘t’ or as character code like ‘\x74’. Backslash may confuse the compiler. Let us say you want to assign backslash as a character, then you should write like this ‘\\.’

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