| A function to locate values... without using histogram [message #84301] |
Thu, 16 May 2013 14:11 |
penteado
Messages: 866
Registered: February 2018
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Senior Member
Administrator |
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I am writing this just to share on the solution I wrote to a problem I
encounter frequently: Locating all occurrences of each different value
contained in an array. Often this happens when I am working with
database-like operations, when I have tables (arrays of structures)
and need to do what would be the equivalent of several call to
where():
foreach value,values do begin
w=where(array eq value)
(do stuff with w)
endforeach
Which is inneficient, due to the repeated searches over the entire
array. So I would usually do this with a single call to histogram()
and use its reverse indices. But when the array values are not
sequential integers (I often need to do this with strings and
doubles), they have to be converted to sequential integers, using
uniq(). What I realized then is that uniq(), alone, does all the work:
s=sort(array)
sarray=array[s]
sr=(ul64indgen(n_elements(array)))[s]
;sr maps sarray back into array: sarray=array[s] and array=sarray[sr]
uinds=uniq(sarray)
uarray=sarray[uinds]
ret=hash()
last=0ULL
foreach el,uinds,i do begin
nels=el-last+1
els=sr[last+ul64indgen(nels)]
ret.set,uarray[i],els
last=el+1
endforeach
So I wrote a function to do this, pp_locate(), which I can use as
array=['a','j','kk','a','a','b','zrdc','29','b','29','-19',' 0']
loc=pp_locate(array,sorted_values=sarray,unique_values=uarra y,histogram=h)
help,loc
;LOC HASH <ID=140 NELEMENTS=8>
print,loc
;zrdc: 6
;a: 0 3 4
;j: 1
;0: 11
;-19: 10
;b: 5 8
;kk: 2
;29: 7 9
It can be found at
http://www.ppenteado.net/idl/pp_lib/doc/pp_locate.html
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