As I mentioned in a footnote to my previous post, I just discovered that variable names in the `bc`

programming language cannot contain capital letters. I think I understand why: Capital letters are reserved for hexadecimal constants, though in a weird sort of way.

At first variable names in `bc`

could only be one letter long. (This is still the case in the POSIX version of `bc`

but not in Gnu `bc`

.) And since A through F were reserved, you might as well make things simple and just reserve all capital letters. Maybe that was the thinking.

If you enter A at the `bc`

prompt, you get back 10. Enter B you get 11, etc. So `bc`

assumes a number containing a hex character is a hex number, right? Actually no. It assumes that any *single* letter that could be a hex number is one. But in numbers with multiple digits, it interprets letters as 9’s. Yes, 9’s.

The full story is a little more complicated. `bc`

will work in multiple bases, and it lets you set the input and output bases with the variables `ibase`

and `obase`

respectively. Both are set to 10 by default. When a number contains multiple characters, letters less than `ibase`

are interpreted as you’d expect. But letters greater than or equal to `ibase`

are interpreted as `ibase`

– 1.

So in base 12 in a number represented by more than one character, A means 10 and B means 11. But C, D, E, and F also mean 11. For example, A0 is 120 and BB is 143. But CC is also 143.

If `ibase`

is set to 10, then the expression `E == F`

evaluates to false, because 14 does not equal 15. But the expression expression `EE == FF`

evaluates to true, because 99 equals 99.

If you set `ibase`

to 16, then you’re in hex mode and the letters A through F behave exactly as expected.

If you want to go back to base 10, you need to set `ibase`

to A, not 10. If you’re in hex mode, every number you enter is interpreted in hex, and so “10” is interpreted as the number we usually write as 16. In any base, setting `ibase`

to 10 does nothing because it sets the base equal to the base.