FAQ
Could anyone tell me the efficient way to do this? Extracting values
from an array for a single condition (say all values greater than 'x')
using 'where' and 'compress' is simple enough.
from Numeric import arange,where,compress
data= arange(10)
data= [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
mask= where((data > 3),1,0)
result= compress(mask,data)
result
array([4, 5, 6, 7, 8, 9])

But, 'where' doesn't appear to allow for multiple conditions in one
statement. For instance, I'd like to do something like:

mask= where((3 < data <= 7),1,0)

but, this won't work.

So, the best I could come up with was this more complicated process that
requires 2 separate masks and an extra temporary array.
mask1= where((data > 3),data,0)
mask1
array([0, 0, 0, 0, 4, 5, 6, 7, 8, 9])
mask2= where((mask1<= 7),mask1,0)
mask2
array([0, 0, 0, 0, 4, 5, 6, 7, 0, 0])
r= compress(mask2,data)
r
array([4, 5, 6, 7])

Is there a more concise way?

Thanks,

J.S.

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  • Robert Kern at Nov 29, 2004 at 11:17 pm

    Jorl Shefner wrote:
    Could anyone tell me the efficient way to do this? Extracting values
    from an array for a single condition (say all values greater than 'x')
    using 'where' and 'compress' is simple enough.

    from Numeric import arange,where,compress
    data= arange(10)
    data= [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    mask= where((data > 3),1,0)
    result= compress(mask,data)
    result
    array([4, 5, 6, 7, 8, 9])

    But, 'where' doesn't appear to allow for multiple conditions in one
    statement. For instance, I'd like to do something like:

    mask= where((3 < data <= 7),1,0)

    but, this won't work.
    Right. "3 < data" creates an array of 0s and 1s where the condition is
    false and true, respectively. You don't need where() at all.

    Try

    mask = logical_and(3 < data, data <= 7)

    --
    Robert Kern
    rkern at ucsd.edu

    "In the fields of hell where the grass grows high
    Are the graves of dreams allowed to die."
    -- Richard Harter
  • Jorl Shefner at Nov 30, 2004 at 2:48 pm

    In article <cogan6$kt0$1 at news1.ucsd.edu>, Robert Kern wrote:
    Right. "3 < data" creates an array of 0s and 1s where the condition is
    false and true, respectively. You don't need where() at all.

    Try

    mask = logical_and(3 < data, data <= 7)
    Great. That's exactly what I needed. Thanks.

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