Multipling a multidimensinnal array with a vector
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Hi Everyone
I am trying to multiply a multidimensional 13*2560 (13 rows and 2560 coloumns) with 1*2560 (1 row and 2560 coloumns).
But matlab is giving error that matrix dimensions is nit match
For example as for simplest example
A=[1 2 3 4 ; 5 6 7 8]; 2*4 array
>> B=[11 22 33 44 ];1*4 vector
I want first row of A get multiplied with B and give its mean as a output then same for other row.
weighted_mean = mean(B.*A)
Please help
Thanks in advance
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Star Strider
2019-4-28
编辑：Star Strider
2019-4-28
If you want the mean of each row, specify the dimension argument as 2:
weighted_mean = mean(B.*A, 2);
That should do what you want.
EDIT —
To calculate a mean of the rows with respect to the elements of the weighting vector (as described in Mathematical definition (link) of the Weighted aritmentic mean), this works:
WeightedMean = sum(bsxfun(@times, B, A),2) / sum(B);
equivalently (R2016b and later releases):
WeightedMean = sum(B.*A,2) / sum(B);
This of course assumes that ‘B’ is the weighting vector.
Experiment to get the result you want.
6 个评论
John Hock
2019-4-28
@ Star Strider
Sir Thanks alot for ur time but it still giving the same error
I dont know what i am missing
I had selected the simplest example but still the same error.
Star Strider
2019-4-28
I gather you are using MATLAB version R2016a or earlier.
This should work for the true weighted mean, regardless of your MATLAB release:
WeightedMean = sum(bsxfun(@times, B, A),2) / sum(B);
If you only want the unweighted row mean of the product of ‘A’ and ‘B’, this will work, again regardless of your MATLAB version:
RowProductMean = mean(bsxfun(@times, B, A),2);
The R2016b version introduced ‘automatic implicit expansion’, so the bsxfun call would not have been necessary (although it remains highly desirable because the result will be as intended).
John Hock
2019-4-28
Star Strider
2019-4-28
As always, my pleasure!
John Hock
2019-5-1
@Star Strider
Sorry to disturb you again but i again facing some problem
My matrix whose which I want to calculate weighted average z3(22*2562) and weighted vector
abc(1*2562)
Using the above mentioned command
WeightedMean = sum(bsxfun(@times, abc, z3),2) / sum(abc)
But I am expecting the output 1*2562 but it is giving output of just 22*1 values
Pictures are attached
Please help me in this regard
Thanks in advance
Star Strider
2019-5-1
No worries.
If you want a (1 x 2562) result, sum down the columns rather than across the columns:
WeightedMean = sum(bsxfun(@times, abc, z3),1) / sum(abc);
or using the default column sum:
WeightedMean = sum(bsxfun(@times, abc, z3)) / sum(abc)
My only concern is that may not be the same thing as the true weighted mean, since you are applying the weighting vector across the columns, although summing down the columns.
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