Re: Computer programmers' habits in electronics

On Wed, 21 Dec 2005 17:47:36 -0500, Spehro Pefhany wrote:

> On Wed, 21 Dec 2005 12:28:17 -0800, the renowned Tim Wescott
> <tim@xxxxxxxxxxxxxxxx> wrote:
>
>>>>
>>Someone who likes to ask that question told me of an interviewee who got
>>as far as saying "I think it involves recursion...".
>>
>>So I wrote a version that did it using recursive function calls and sent
>>it to her. I don't know if I would have gotten the job -- they had
>>already made the mistake of hiring me.
>
> Something like this? (ugh)
>
> #include <string.h>
>
> rev2(char * istring, int start, int fin)
> {
> char t;
> t = istring[start];
> istring[start] = istring[fin];
> istring[fin] = t;
> start++; fin--;
> if (fin > start) rev2(istring, start, fin);
> }
>
>
> ...
>
> /* reverse the string in place */
> rev2(mystring, 0, strlen(mystring)-1);
>
> ...
>
>>Good thing the PC has a lot of stack space.
>>
>>For interviewing embedded SW engineers we finally settled on a fairly
>>basic scaling problem. We started with a little story problem that
>>required the interviewee to find the ratio of a couple of numbers and
>>multiply it to a third, then we said "oh, by the way, our floating point
>>library is too slow -- do it with integers". The question usually took
>>about 40 minutes to explore fully, with some folks never getting it and
>>some just glancing at the board and writing down the correct answer.
>>
>>It's amazing how you can separate the desktop programmers from the
>>embedded engineers with that one.
>
>
> Here's one I did to approximate e^(-x) over the range 0 .. 0.75
> (the end product would be written in assembler**, the C is just to
> explain).
>
>
>
> ---
>
> where xf, yf are 32-bit integers.
>
> xf = x * 128;
> yf = 32768 - xf + (xf* xf)/65536 - (((xf * xf)/32768
> )*xf)/(6*32768) + (((xf*xf)/32768)* ((xf*xf)/32768))/(32768 * 24)
> - (((((xf*xf)/32768)* ((xf*xf)/32768))/32768) * xf)/(32768 * 120)
> ;
>
> The result is ~= exp(-x/256) * 32768 over the range
> x = 1 to 192.
>
> yf/32768 agrees with exp(-x/256) within 0.001 over that range.
>
>
> You can easily extend this to additional terms, although it gets a bit
> tedious.
>
> It's based on the textbook Taylor/Maclaurin series
>
> infinity
> ---
> \
> exp(x) = > (x^n)/n!
> /
> ---
> n= 0
>
>
> ** actually assembler macros, but that's another story
>
> ---

Yabbut, none of youse guys will work as cheap as me.

Thanks,
Rich

.

Loading