In computer programming, a constant is an identifier with an associated value which cannot be altered by the program during normal execution – the value is constant. This is contrasted with a variable, which is an identifier with a value that can be changed during normal execution – the value is variable. Constants are useful for both programmers and compilers: for programmers they are a form of self-documenting code and allow reasoning about correctness; while for compilers they allow compile-time and run-time checks that constancy assumptions are not violated, and allow or simplify some compiler optimizations.