python - Numpy where function multiple conditions -

i have array of distances called dists. want select dists between 2 values. wrote following line of code that:

 dists[(np.where(dists >= r)) , (np.where(dists <= r + dr))] 

however selects condition

 (np.where(dists <= r + dr)) 

if commands sequentially using temporary variable works fine. why above code not work, , how work?

cheers

the best way in your particular case change 2 criteria 1 criterion:

dists[abs(dists - r - dr/2.) <= dr/2.] 

it creates 1 boolean array, , in opinion easier read because says, is dist within dr or r? (though i'd redefine r center of region of interest instead of beginning, r = r + dr/2.) doesn't answer question.

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the answer question:
don't need where if you're trying filter out elements of dists don't fit criteria:

dists[(dists >= r) & (dists <= r+dr)] 

because & give elementwise and (the parentheses necessary).

or, if want use where reason, can do:

 dists[(np.where((dists >= r) & (dists <= r + dr)))] 

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why:
reason doesn't work because np.where returns list of indices, not boolean array. you're trying and between 2 lists of numbers, of course doesn't have true/false values expect. if a , b both true values, a , b returns b. saying [0,1,2] , [2,3,4] give [2,3,4]. here in action:

in [230]: dists = np.arange(0,10,.5) in [231]: r = 5 in [232]: dr = 1  in [233]: np.where(dists >= r) out[233]: (array([10, 11, 12, 13, 14, 15, 16, 17, 18, 19]),)  in [234]: np.where(dists <= r+dr) out[234]: (array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12]),)  in [235]: np.where(dists >= r) , np.where(dists <= r+dr) out[235]: (array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12]),) 

what expecting compare boolean array, example

in [236]: dists >= r out[236]:  array([false, false, false, false, false, false, false, false, false,        false,  true,  true,  true,  true,  true,  true,  true,  true,         true,  true], dtype=bool)  in [237]: dists <= r + dr out[237]:  array([ true,  true,  true,  true,  true,  true,  true,  true,  true,         true,  true,  true,  true, false, false, false, false, false,        false, false], dtype=bool)  in [238]: (dists >= r) & (dists <= r + dr) out[238]:  array([false, false, false, false, false, false, false, false, false,        false,  true,  true,  true, false, false, false, false, false,        false, false], dtype=bool) 

now can call np.where on combined boolean array:

in [239]: np.where((dists >= r) & (dists <= r + dr)) out[239]: (array([10, 11, 12]),)  in [240]: dists[np.where((dists >= r) & (dists <= r + dr))] out[240]: array([ 5. ,  5.5,  6. ]) 

or index original array boolean array using fancy indexing

in [241]: dists[(dists >= r) & (dists <= r + dr)] out[241]: array([ 5. ,  5.5,  6. ])