Constant

Question

A book says You can categorize data as variable or constant. A data item is constant when it cannot be changed after a program is compiled - in other words when it cannot vary. In my view after compilation you can go into the source code and change it if you want. I wish to know in the book what is meant by compilation.

Sam Hobbs · Answer

Programmers have a tendency to put data into their source code; in other words diameter * 3.14. Experienced programmers know that we often need to find where a value is used such as to change the value. And usually it is easier to remember a name instead of a value. So experienced developers use names for values and that is the most common use for a constant. It is to give a name to a value so it is easier to remember and so it is easier to find in the program; IDEs such as VS make it easy to find all uses of a name such as items that are declared as a const.

Maha · Answer

Thank you very much for your explanation.

Vulpes · Answer

Yes I think you've grasped it now Maha :)

Maha · Answer

Now I got the point that means if you declare a number as constant integer in same program your can't assigned different number to that constant integer . I wish to know whether my understanding is correct. For example: const int HIGH = 10; const int HIGH = 20; (in same program it is wrong) Let us assume int x = 2; (to initialize) int x = 4; (in the same program this is acceptable in a appropriate place)

Vulpes · Answer

It will do because the program hasn't been compiled at that stage. When you make a change to the source code and then recompile the compiler regards it as a totally new program and processes it from scratch. However if you try to do this then you will get the error I described: using System; public class IfElseDecision { public static void Main() { const int HIGH = 10; string numberString; int number; Console.Write(Enter an integer ); numberString = Console.ReadLine(); number = Convert.ToInt32(numberString); HIGH = 20; if (number > HIGH) Console.WriteLine({0} is greater than {1} number HIGH); else Console.WriteLine({0} is not greater than {1} number HIGH); Console.ReadKey(); } }

Maha · Answer

In this simple program 10 is declared as a constant integer even though you change the 10 to 20 program is compiling well. const int HIGH = 10; using System; public class IfElseDecision { public static void Main() { const int HIGH = 10; string numberString; int number; Console.Write(Enter an integer ); numberString = Console.ReadLine(); number = Convert.ToInt32(numberString); if (number > HIGH) Console.WriteLine({0} is greater than {1} number HIGH); else Console.WriteLine({0} is not greater than {1} number HIGH); Console.ReadKey(); } }

Vulpes · Answer

The point is that the compiler won't let you change it so you won't be able to execute the program. If you try to change a const value after it has been declared the compiler will produce an error - 'the left-hand side of an assignment statement must be a variable property or indexer.'

Maha · Answer

I couldn't grasp the point. Suppose we say in a program we declared const double PI = 3.14; after execution of the program we have changed the value to 10.5 therefore new value is const double PI = 10.5; if we execute the program what will happen.

Vulpes · Answer

Compilation is the process whereby the C# source code is translated to Intermediate language (IL) and metadata. Once this has been done constant data (i.e. data marked as 'const') cannot be changed and in fact if you have a line like this in the source code: const double PI = 3.14; then the compiler will simply replace PI wherever it occurs with the double value 3.14.