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extract first row from matrix

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Hello Matlab
Hello Matlab 2015 年 2 月 5 日
コメント済み: Stephen Porter 2020 年 11 月 28 日
I'm trying to extract the first row from my matrix and put in a vector like [m,n]
my matrix is 2x5 size.
but I'm getting errors, Indexing cannot yield multiple results.
Please any solution to that?

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Zabir Al Nazi Nabil
Zabir Al Nazi Nabil 2017 年 4 月 4 日
To extract first row from matrix X, you can do this - XX = X(1:1,:); Then, to copy it in a vector, you just do this - Y = XX;

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回答 (6 件)

Hikaru
Hikaru 2015 年 2 月 5 日
You have the wrong syntax.
Lets say A is your 2x5 matrix.
A = rand(2,5)
Then to extract the first row, you simply use:
V = A(1,:);

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Hello Matlab
Hello Matlab 2015 年 2 月 5 日
yes but I want to get the first row to a vector of 1x2, and to by [m,n] where the first element in the new vector is m and the second is n... how I can assign those two value to [m,n]??
Hikaru
Hikaru 2015 年 2 月 5 日
I'm not exactly sure if I understood your question correctly. How are you supposed to get 1x2 vector when the first row contains 5 elements? Did you mean that your original matrix is 5x2 matrix (i.e. 5 rows and 2 columns)?
You got the error 'Indexing cannot yield multiple results' because you tried to assign the values to m and n in one command line.
Assuming the above, why don't you try:
A = rand(5,2)
V = A(1,:); % extract the first row and assign it to V
m = V(1)
n = V(2)
[m, n] % this is essentially V, but if this is what you wanted
Stephen Cobeldick
Stephen Cobeldick 2015 年 2 月 5 日
@Hello Matlab: Your explanation is inconsistent: "...to get the first row to a vector of 1x2..." and "assign those two value to [m,n]" means two quite different things. Do you want to obtain the vector of the first row, OR to get two variables m,n corresponding to the elements of the first row of the matrix? So far Hikaru and I have addressed both of these, so if there is something that is not clear, you need to explain it to us again.

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MathWorks Support Team
MathWorks Support Team 2018 年 11 月 27 日
To extract any row from a matrix, use the colon operator in the second index position of your matrix. For example, consider the following:
A = [1 2 3; 4 5 6];
row1 = A(1,:)
row2 = A(2,:)
“row1” is the first row of “A”, and “row2” is the second row.
For more on basic indexing, see:

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Stephen Porter
Stephen Porter 2020 年 11 月 28 日
If you use a for loop to iterate so many times, how do you acess the first elemeent after the loop is complete?

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Stephen Cobeldick
Stephen Cobeldick 2015 年 2 月 5 日
編集済み: Stephen Cobeldick 2015 年 2 月 5 日
What you are trying to do is create a comma-separated list from a numeric vector. As far as I know, there is no inbuilt way to achieve this. This question has also been asked before , you might find other discussions on this online.
Usually in MATLAB it makes a lot more sense to keep your data together in arrays, so it may not be necessary to do this multiple-assignment. You should consider this. However if it really is necessary to do this multiple assignment, then you could:
  1. Assign individually: n = A(1,1); m = A(1,2);
  2. Assign via a cell array:
B = num2cell(A(1,1:2));
[n,m] = B{:};
Although the first option is much clearer!
EDIT: on re-reading your question, it seems that perhaps all you are after is just the vector of the first row of A, in which case you do not need to assign to two separate values first, as you can just extract the vector directly:
V = A(1,:);
If you just need the row (vector), then there is no need in MATLAB to create intermediate variables m,n.

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naazneen vhora
naazneen vhora 2018 年 5 月 11 日
if i wanted to extract a certain column without the first element how would i do that.
for example, if my column has the values: 2,3,4,5,6... and i want to access all the values in the column after 2 how would i do that? Yes i mean column for typing reasons i wrote it horizontally.

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Stephen Cobeldick
Stephen Cobeldick 2018 年 5 月 11 日
Using basic MATLAB indexing:
M(2:end,C)
for column C of matrix M. Basic MATLAB indexing is explained in the introductory tutorials:

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SANTOSH MOHAPATRA
SANTOSH MOHAPATRA 2019 年 4 月 17 日
I have a matrix of dimension(256, 32), means rows=256 and column=32. I want to select 128 rows of the particular matrix and make another Vector of dimension (1, 256*32) means to represent all the elements in a single row. How can I do that?
Please help...can I apply a loop program for it??

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Stephen Cobeldick
Stephen Cobeldick 2019 年 4 月 17 日
No loop required, just use indexing and reshape (and possibly transpose, depending on the order you want).

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Abhishek Mishra
Abhishek Mishra 2020 年 10 月 21 日
You can combine indexing with assignment to change array values to equal other elements. For example, this code would change the value of x(1) to x(2):
x(1) = x(2)
Try changing the first column of data to the second column of data.
Answer for this please

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