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> Hi,
>> The idea of "Pascal-like languages" is: Don't do at runtime, what
>> you can already do at compile-time. Type-safety-checks can ALWAYS
>>be done at runtime.
>
>you are quite right - I was too loose with my language. What I really
>mean is that the specification of CAP should not lead to an undefined
>result, because it forces the programmer to include run-time checks of
>the input. So the efficiency of the CAP implementation is buried under
>the inefficiency of the range checks.
But that's the fun part about it! There is no "undefined result". It is very much defined. It is not necessarily a USEFULL result (at least when you want to print it out on screen or printer), but is is defined.
If you are in a situation where only printable characters are allowed, then you have to do the type checking or use Strings.Upper. If that's none of your concern (and there are applications where this doesn't matter), then CAP will do the trick.
>Every possible CHAR maps to an upper case equivalent, even if it is a
>map back to itself. So, a->A, b->B, space->space, {([at]})nowhere.xy>reason why this should not be so, and no reason why it should be >inefficient to implement.
So maybe you should do some studies in Assembler then, the explanation why CAPS is that incredible fast (compared to Strings.Upper) has already been given by Chris Burrows, IIRC.
But since it seems you missed his excellent explanation, here it is, in short words, again (it's been a long time since I dabbled in ASM, so forgive me, if there's something inexact in the following):
In the Latin-1 character set, "a" differs from "A" by 1 bit that is set in the case of "a", and not set in the case of "A".
a = 1100001
A = 1000001
b = 1100010
B = 1000010
As you see, the upper case letter differs from the lower case letter only in the "second bit from the left", where the UC has the bit set, whereas the LC has not.
So, if you want to shift from LC to UC, all you have to do is clear the "second bit from the left". No translation table is involved! It's just ONE machine instruction: A single bitwise AND operation, where the "clear" (0) is a constant!
And ADD is a very simple instruction, machine internally.
Now, this was easy, I think.
So what is done in case of the translation table you propose?
Somewhere in the memory, you need a table. That table has lower case letters in column 1 and upper case letters in column 2. You have to read (1st instruction) the table, and if you have indexed it cleverly, this is just one instruction, and not a LOOP UNTIL lower case letter is found END. So you move the contents of the indexed row of your table to a register (1st instruction), then you move the contents of the register to the variable that should go uppercase (2nd instruction). And - tata - you need TWO instructions instead of one.
Further, MOV is not a simple instruction, but the encoding for the mov instruction is probably the most complex in the instruction set.
More complex means longer execution time!
So you could say that 1 single instruction of the "table version" (of which there are two) is slower than the 1 instruction the ADD takes! So it is actually MORE than double time that is needed by the table version.
Does that help?
Rene
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Received on Thu Sep 16 2004 - 16:10:58 UTC