How can a constant be a variable?

Projects Programming
1

I don't quite see that
Mark S.

2

const int x = 5;
int y;

x is a variable like pi (3.14159), it does not change.
y is a variable that can change (y = voltage on A0)

.

3

markba633csi:
I don't quite see that
Mark S.

What confuses you about a named value that does not change? In math you have a name for the ratio of a circle's circumference to its diameter: Pi. In physics you have various constants like "C" (the speed of light).

4

Is it the fact it is called a "variable" but then it isn't being allowed to vary what is confusing?

5

As implied:
E = MC2

C = 299,792,458 m/s

'C' is a variable that does not change.

6

The term "variable" means a name associated with a memory location. It does NOT mean (though the name does seem to imply that it does) that the value in the memory location can be changed at will.

7

"symbolic reference" is a more generic term that covers both variables and constants.

8

I have 10 variables which are defined in the following array:

byte myArray[] = {0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F};

The array is initialized in RAM space of the MCU. I know very well about the nature of the elements of the array which are cc(common cathode codes) for the decimal digits: 0, 1, ..., 8, 9. I also know that the values of the array elements will remain as they are through out the whole life of the executing program. So, I wish to make the array elements 'Read-only' in order to give them some kind of 'Pseudo Protection' so that they can't be inadvertently written and corrupted. The compiler/programming language has told us to do it just by putting the attribute const for constant.

The mechanism of protecting data/code in RAM area has been elaborately explained in the Protected Mode Operation (PVAM) of Intel High Performance 80286/80386 architectures
//-------------------------------------------------------------------------------

const byte myArray[]=  {0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F};

myArray[0] = 0x23;   // attempt to change value; it is not allowed; there is a compilation error

//-------------------------------------------------------------------------------

byte myArray[]=  {0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F};

myArray[0] = 0x23;         // attempt to change value; it is allowed; there is no compilation error
9

The mechanism of protecting data/code in RAM area has been elaborately explained in the Protected Mode Operation (PVAM) of Intel High Performance 80286/80386 architectures

The above has nothing what so ever to do with C/C++ constants.

The idea of a constant dates back to at least the 1950's. In a typical programming language it is enforced by the compiler at compile time and totally forgotten by runtime.

Mark

10

evanmars:
Is it the fact it is called a "variable" but then it isn't being allowed to vary what is confusing?

That's language. It's messy and illogical, always has been and always will be.

11

Evanmars got my drift; the nomenclature was/is a little hard to grasp at first.
Mark S.
ps It took me a while to understand = vs == also

12

markba633csi:
Evanmars got my drift; the nomenclature was/is a little hard to grasp at first.
Mark S.
ps It took me a while to understand = vs == also

Also watch out for || vs |, && vs &, ++x and --x vs x++ and x--, etc.