Conditional calculation in python

Question

I'm attempting to use my own modified version of the logsumexp() from here: https://github.com/scipy/scipy/blob/v0.14.0/scipy/misc/common.py#L18

On line 85, is this calculation:

out = log(sum(exp(a - a_max), axis=0))

But I have a threshold and I don't want a - a_max to exceed that threshold. Is there a way to do a conditional calculation, which would allow the subtraction to take place only if the difference isn't less than the threshold. So something like:

out = log(sum(exp( (a - a_max < threshold) ? threshold : a - a_max), axis = 0))

You could check `a` against `threshold + a_max`, i.e. a modified threshold. — fuesika, Sep 04 '14 at 08:36

score 1 · Answer 1 · answered Sep 04 '14 at 08:38

1

How about

out = log(sum(exp( threshold if a - a_max < threshold else a - a_max), axis = 0))

answered Sep 04 '14 at 08:38

qingbo

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score 1 · Accepted Answer · answered Sep 04 '14 at 08:38

1

There is a conditional inline statement in Python:

Value1 if Condition else Value2

Your formula transforms to:

out = log(sum(exp(threshold if a - a_max < threshold else a - a_max), axis = 0))

answered Sep 04 '14 at 08:38

bogs

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I'm getting this error: ` out = log(sum(exp(threshold if a - a_max < threshold else a - a_max), axis=0)) ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()` – user961627 Sep 04 '14 at 09:00
Can you give me some sample data? What data types are each variable? – bogs Sep 04 '14 at 12:20
The thread is old, but for completeness sake: the problem is that the condition tries to evaluate the whole condition array, not per index. Meaning, for each index it does “if condition_array“, instead of “if condition_array[0]“, then “if condition_array[1]„ ..., see also: https://stackoverflow.com/questions/18397869/placing-a-condition-while-calculating-using-numpy-array – Michael S. Feb 16 '18 at 07:09

Conditional calculation in python

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