Lauri Tamminen

I submitted the new version of the function with much more efficient algorithm and more neater code.

I think that the new version (after review process) is the most optimal way string to double conversion can be done in any circumstances. Performance gains have risen from about 20x to about 80x-100x!

Also for Jan: New version is much faster than your (cryptic) one liner!

The idea is very good, but the results are not reliable. I cannot suggest to use this for productive work. A fair rating is not easy in this case, therefore I've hesitated for some years now.

@Lauri, good work...
Some fine tuning is still required in your function:
str2doubleq('') % NaN instead of 0

@Lauri: An excellent speedup! Thanks for this update.
The main time of my suggested one-liner in pure M (which looks less cryptic when it is expanded 3 lines) is wasted by SPRINTF. Using CStr2String (see my FEX page) a smarter pre-allocation allows a faster creation of the long string. This is 20% slower than your str2doubleq.

Isn't it surprising, that your parsing is so much faster than ATOF or STRTOD? What could the compiler manufacturers hide in their implementations? The excellent and fast parsing of Google's V8 engine is worth to be inspected: http://code.google.com/p/double-conversion . Rounding problems are handled smart and reliable, what is a very hard and complicated job.

Some fine tuning is required in your function:
str2doubleq('Inf') % NaN instead of Inf
str2doubleq('.i5') % 5 instead of NaN
str2doubleq('i') % 0 instead of 0 + 1i
str2doubleq('1e1.4') % 0.4 instead of NaN
str2doubleq('--1') % -1 instead of NaN
s = '12345678901234567890';
str2doubleq(s) - str2double(s) % 2048
s = '123.123e40';
str2doubleq(s) - str2double(s) % 1.547e26

Mal-formed input is an evil test, I know. But it would be very fine, if your very efficient implementation would be as reliable as Matlab's STR2DOUBLE.