| Indexing arrays with arrays [message #55835] |
Fri, 21 September 2007 08:18  |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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A couple times I've asked about this. I sometimes would like to be
able to index arrays with arrays without using for loops. For intance
imagine I have:
data = findgen(20)
st = [5,2,10]
ed = [6,4,15]
I would like to do:
res = data[st:ed]
obviously, IDL doesn't allow this. The last time I posted about this,
I got this response:
Well, if end_ind is always a constant offset from start_ind, it's easy
just to contruct the index vector yourself:
t=[end_ind[0]-start_ind[0]+1,n_elements(start_ind)]
extract=res[reform(rebin(1#start_ind,t)
+rebin(indgen(t[0]),t),t[0]*t[1])]
If the start-end difference can be any variable amount, this is a
problem for HISTOGRAM related to chunk indexing. See the HISTOGRAM
tutorial
This was very helpful and got me started. After much pondering, I
came up with this for the general case. Assume that st and ed are row
arrays (as per the above example)
function array_index,st,ed
nst = n_elements(st)
ned = n_elements(ed)
diff = ed - st + 1
h = histogram(total(diff,/cumulative)-1,/
binsize,min=0,reverse_indices=ri)
i = ri[0:n_elements(h)-1]-ri[0]
arr = [fltarr(1,nst),transpose(diff)-1]
maxdiff = max(diff)
x = rebin(findgen(maxdiff),maxdiff,nst)/(maxdiff-1)
y = rebin(findgen(1,nst),maxdiff,nst)
int = interpolate(arr,x,y)
ref = reform(ceil(int),maxdiff*nst)
adds = ref[uniq(ref)]
return,st[i]+adds
Given the above example:
data = indgen(20)
st = [5,2,10]
ed = [6,4,15]
print,data[array_index(st,ed)]
; prints 5 6 2 3 4 10 11
12 13 14 15
I'm rather happy about the result. There's been a couple times when I
really wish I had this available. Anyway, a couple questions:
1) Anyone see a better way to do this?
2) Anyone want to generalize this to n dimensions?
i.e. inds = array_index([[st1,ed1],[st2,ed2],[st3,ed3]])
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