[FIXED] numpy argmax not working when value exceeds maximum in array

Issue

list_val = [ 30, 120, 240, 510]
num = 900

np.argmax(np.asarray(list_val) > num)

I want to find the position of first element in array which is greater than num

The output from code above is 0. However, since 900 is greater than 510, the result should be 3. How can I fix it?

--EDIT

I want a solution that works if num is 20, 200 or 900.

Solution

In [248]: x = np.array([ 30, 120, 240, 510])
In [249]: x>200
Out[249]: array([False, False,  True,  True], dtype=bool)
In [250]: np.argmax(_)
Out[250]: 2
In [251]: x>400
Out[251]: array([False, False, False,  True], dtype=bool)
In [252]: np.argmax(_)
Out[252]: 3
In [253]: x>600
Out[253]: array([False, False, False, False], dtype=bool)
In [254]: np.argmax(_)
In [255]: np.max(__)
Out[255]: False

With the large threshhold, the comparison produces all False. The maximum value is then False, and the 0th item is that.

You may have to test the x>n for all False and return a different value in that case. This is not a universally defined behavior.

Lists have a find

In [261]: (x>200).tolist().index(True)
Out[261]: 2
In [262]: (x>400).tolist().index(True)
Out[262]: 3
In [263]: (x>600).tolist().index(True)
...
ValueError: True is not in list

The string find returns a -1 if the value is not found.

---

In [266]: def foo(test):
     ...:     if not test.any():
     ...:         return -1
     ...:     return np.argmax(test)
     ...: 
In [267]: foo(x>200)
Out[267]: 2
In [268]: foo(x>400)
Out[268]: 3
In [269]: foo(x>600)
Out[269]: -1

Answered By - hpaulj