An assignment of vectorization
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Say you have two column vectors vv and ww, each with 7 elements (i.e., they have dimensions 7x1). Consider the following code:
z = 0;
for i = 1:7
z = z + v(i) * w(i)
end
A) z = sum (v .* w);
B) z = w' * v;
C) z = v * w;
D) z = w * v;
According to the solutions, answers (A) AND (B) are the right answers, can someone please help me understand why?
6 个评论
madhan ravi
2019-2-2
Multiplication operator with dot represents element wise operation and the one without is matrix multiplication.https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html
John D'Errico
2019-2-2
Surely this question was given to you as a homework assignment, AFTER you have been taught about matrix multiplication. The point is, you have been given the clues in your school work. And Madhan gave you the perfect clue here. Remember that a vector IS a matrix. Just one with one row or column.
Luis URBINA
2019-8-1
I was struggling with this question, also, but finally I got the point.
The given code calculates z as the accumulated sum of the multiplied v(i) by w(i). So, z is accumulating scalar numbers in each (i) interaction in the for loop. At the end of the loop, the value of z=21.
The right answers are A and B. Thank you both.
John D'Errico
2019-8-1
All of them hinge on what a dot product means, and how that solves your problem.
In question they said that each column vector v and w as 7x1 dimension.
They gave v(i)*w(i). So, we cannot multiply 7*1 dimension and 7*1 dimension.
So, we should take transpose for anyone v(i) or w(i).
Then, only we get (1*7)(7*1) and (7*1)(1*7)dimension.
Finally, we get solution by multiply them.
Amr Soror
2021-10-14
for A) x .* y
Element-by-element multiplication. If both operands are matrices, the number of rows and columns must both agree, or they must be broadcastable to the same shape.
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