Re: Concatenate elements of string array

[kluegel(at)lanl.gov]

In article <37B1730A.41C6@icg.tu-graz.ac.SIG>,
  Ruhaltinger Norbert <norbertr@icg.tu-graz.ac.SIG> wrote:
> string can be used like sprintf in C
> You have to create the Format specification on the fly
>
> IDL> string_array=['one ','string ','split ',$
> 		'into ','many ','array elements']
> IDL> help,string_array & print,string_array
> STRING_ARRAY    STRING    = Array[6]
> one  string  split  into  many  array elements
> IDL> one_string = string(string_array, $
> 		format = '(' + string(n_elements(string_array)) + 'A)')
>
> IDL> help,one_string & print,one_string
> ONE_STRING      STRING    = 'one string split into many array
elements'
> one string split into many array elements

Thank you, Norbert!

You gave me the idea that I can use a format that isn't variable at all,
but just larger than I'll ever need:

result = STRING( string_array, FORMAT='(16(16384(16384A)))' )

I ran some benchmarks which showed that the above approach is more than
20 times faster (Pentium II 400) than the more obvious explicit looping
algorithm:

result = ''
FOR i=0, N_ELEMENTS(string_array)-1 DO result = result + string_array[i]

As usual, when we avoid explicit looping, etc. in interpreted code we
get a decent speedup.  For greater generality, I made it capable of
handling very long arrays.  The new approach allows an array of up to
2^32 elements.  It was necessary to nest the repeat counts in order to
get around IDL's maximum of 32767.  Note that the only way such a large
array could be used would be if many elements were empty strings.
Otherwise the result would be a string longer than 32767 characters,
IDL's limit for string length.

I think this is a bit kludgey, but its so much faster than looping I'll
live with it!

Thank you everyone who took the time to brainstorm this little puzzle
with me.

-- Tom Kluegel


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