Code to select random conditions is incorrect #3256
Description
From PsychoPy's documentation:
The parameter Select rows allows this. You can specify which rows you want to use by inserting values here:
- 0,2,5 gives the 1st, 3rd and 5th entry of a list - Python starts with index zero)
- random(4)*10 gives 4 indices from 0 to 10 (so selects 4 out of 11 conditions)
The first item is incorrect: 0,2,5 gives the 1st, 3rd, and 6th entries.
The second item is just plain wrong as far as I understand. numpy's random function generates random numbers in the interval [0, 1), thus random(4)*10 (which by the way should be $random(4)*10) generates 4 floating-point numbers in that interval. Multiplying by 10 thus generates 4 floating-point numbers in the [0, 10) interval. So these numbers are floats, not integers. But as far as I understand PsychoPy converts them to integers like this:
# the selection might now be a slice or a series of indices
if isinstance(selection, slice):
trialList = trialList[selection]
elif len(selection) > 0:
allConds = trialList
trialList = []
for ii in selection:
trialList.append(allConds[int(round(ii))])
Thus the numbers are rounded before being converted to integers. This means that numbers from 0 to 0.5 are converted to 0, numbers from 0.5 to 1.5 are converted to 1 and so on. Thus, there's a lower probability that 0 or 10 will be selected.
The probability for every number should be 1/11, but it's not.
In my opinion, int(round(ii)) should be changed to int(np.floor(ii)) and the documentation to "random(4)*10 gives 4 indices from 0 to 9 (so selects 4 out of 10 conditions)." This will make sure every number has the same probability of being selected.
