Creating a vector from recursive elements of another vector

Question

Elyse Lalonde 2020-10-6

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https://ww2.mathworks.cn/matlabcentral/answers/606636-creating-a-vector-from-recursive-elements-of-another-vector

回答： Peter O 2020-10-6

Say I create a matrix A (1, 100) and want to create a matrix B whose entries are b_i = a_i * a_(i+1), meaning it would be (1,99)

How would I go about this? Im new to MATLAB and this is puzzling me.

Thanks.

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Answer 1

Peter O 2020-10-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/606636-creating-a-vector-from-recursive-elements-of-another-vector#answer_506786

在 MATLAB Online 中打开

Welcome to MATLAB!

Matlab will allocate the memory for you on the fly, so this is a one line operation, no need for a loop :)

Use the colon to mark a continuous range of indices. For your question:

B = A(1,1:99).*A(1,2:100)

Note that the .* operator is used to mark element-wise multiplication, otherwise, you're trying to multiply [1x99] x [1x99], which isn't compatible and you'll receive an error. There are similar element-wise operations for exponentiation (.^) and division (./)

You can also go backwards, skip indices, and access a subset by using two colons for the range:

B = A(1, 1:99) .* A(1,100:-1:2) % Backwards
B = A(1, 1:3:99) .* A(1,2:3:100) % Every 3rd
B = A(1, [3,4,5]) .* A(1,[51,61,71]) % A subset of values having equal array dimensions
B = A(1, 1:end-1) .* A(1, 2:end); % Use the "end" keyword with add/subtraction to have flexible code without needing to query the matrix length