question about vector multiplication
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The Matlab vector v1 has a dimension of n-by-1 and the vector v2 has a dimension of 1-by-n, which of the following is true?
A) The+operation v1*v2 does+not+return+an+error
B) The+operation v2*v1 does+not+return+an+error
C) The+operation v1.*v2 does+not+return+an+error
D) None+of+the+above
6 Comments
Walter Roberson
on 7 Dec 2019
v1.*v2 is an example of implicit expansion as of R2016b.
The question said A and B are vectors
Yes, but the question talks about returning an error, not about whether the result is an array, a vector, or a scalar.
>> v1 = rand(5,1); v2 = rand(1,5);
>> v1*v2
ans =
0.772181449374705 0.136904501442802 0.406952626197035 0.883582709529619 0.764391769792849
0.126134670907917 0.0223631430789796 0.0664750954870026 0.144331898126657 0.124862238539422
0.77674644648747 0.137713856102018 0.409358456543161 0.888806290750275 0.768910715728357
0.766002014144726 0.135808913741177 0.403695959778422 0.87651177804287 0.758274672010212
0.388436651747121 0.0688681736560934 0.204712656160906 0.444475724597195 0.384518146507958
>> v2*v1
ans =
2.46493297354767
>> v1.*v2
ans =
0.772181449374705 0.136904501442802 0.406952626197035 0.883582709529619 0.764391769792849
0.126134670907917 0.0223631430789796 0.0664750954870026 0.144331898126657 0.124862238539422
0.77674644648747 0.137713856102018 0.409358456543161 0.888806290750275 0.768910715728357
0.766002014144726 0.135808913741177 0.403695959778422 0.87651177804287 0.758274672010212
0.388436651747121 0.0688681736560934 0.204712656160906 0.444475724597195 0.384518146507958
No errors.
Answers (1)
Sourav Bairagya
on 10 Dec 2019
The first two options of your question are performing matrix multiplication of the vectors A and B by considering them nX1 and 1Xn matrices respectively. In first case it will produce a nXn matrix and in 2nd case it will give 1X1 matrix i.e. a scalar.
In 3rd option, it is the case of element-wise multiplication.
C = A.*B performs the element-wise multiplication of vectors A and B, where either A and B are of same size or having compatible sizes. If A and B have compatible sizes, then the two vector implicitly expand to match each other.
Like, if one of them is a scalar, then the scalar is combined with each element of the other vector. Also, vectors with different orientations (one row vector and one column vector) implicitly expand to form a matrix. Hence, in these cases, this elememt-wise multiplications will sucessfully performed without throwing any error.
To know more about the compatible sizes of two vectors, you can leverage the following link:
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