Matlab matrix common operation method

The basic format of data in matlab is a matrix, and row vectors, column vectors, and scalars are all special cases of matrices. A matrix can be two-dimensional or multidimensional.

(1) Find the elements in the matrix

① find function

In matlab, you can call the find function to find elements that meet certain conditions in the matrix. The common calling format is as follows:

ind=find(X)
[m n]=find(X)

Where X is the matrix to be searched; ind is the linear index value in matrix X that satisfies the search condition. Because in matlab, the matrix is stored by column, the value of ind indicates the position of the element when it is stored by column in the matrix. m and n are column vectors, which store the row subscript and column subscript of the position of the element in the matrix, respectively.

For example

A=[1 2 3 4;5 6 7 8];
%查找A中大于3的元素，返回元素的索引
ind=find(A>3);

The results obtained are as follows:

How to understand it?

You can also write like this

A=[1 2 3 4;5 6 7 8];
[m n]=find(A>3);

How to understand this?

Well, this is the understanding and use of the find function

②ind2sub and sub2ind functions

These two functions convert between linear index values and row and column subscripts. The format of the call is as follows

[I,J]=ind2sub(size,IND)
IND=sub2ind(size,I,J)

A=[1 2 3 4;5 6 7 8];
ind=find(A>3);
[m n]=find(A>3);
[I J]=ind2sub(size(A),ind);
IND=sub2ind(size(A),I,J);

(2) Delete the specified element in the matrix

If you want to delete the specified elements in the matrix, you only need to assign these elements to be empty ("[ ]"). For example, assuming that A is an m*n-dimensional matrix, you can use the following command to delete the specified elements in the matrix:

A(sub2ind(size(A),i,j))=[] %删除A的第i行，第j列的元素
A(i,:)=[] %删除A的第i行的数据
A(i:j,:)=[] %删除A的第i行到第j行的数据
A(:,j)=[] %删除A的第j列的所有元素
A(:,i:j) %删除A的第i列到第j列的数据

For example

A=[1,2,3,4;5,6,7,8];
A(1,:)=[] %删除第1行的所有元素

A=[1,2,3,4;5,6,7,8];
A(:,1)=[] %删除A的第一列元素

For the deletion of a single element in a matrix, MATLAB only allows the use of a linear index value to specify that element.

(3) Get the specified element in the matrix

Users can use the following methods to get the elements of a certain (some) row or column in the matrix:

X=A(i,:) %取得A的第i行的数据，并赋值给变量X
X=A(i:j,:) %取得A的第i行到第j行数据，并赋值给变量X
Y=A(:,j) %取得A的第j列的数据
Y=A(:,i:j) %取得A的第i列到第j列的数据
Z=A(i:j,n:m) %取得矩阵第i行到第j行以及第n列到第m列之间的数据

(4) Query the size of the matrix

num=size(A) %返回矩阵的行数和列数，num是一个1*2的数组，第一个数值是矩阵的行数，第二个数值是矩阵的列数
num=length(A) %返回A的行数和列数的最大值，相当于max(size(A))
num=size(A,1) %返回矩阵A的行数
num=size(A,2) %返回矩阵A的列数

(5) Obtain the maximum and minimum values of the elements in the matrix: max and min functions

C=max(A) %取得矩阵A中每一列的最大值，组成行向量返回给C
C=max(A,B) %取得矩阵A和B对应元素的最大值
C=max(A,[],dim) %取得矩阵每行或每列的最大值，dim=1表示每列的最大值组成的行向量，dim=2表示每行的最大值组成的列向量
%同理,min也有上面的三种用法

For example

a=[2 3;3 6;4 9]
b=[1 4;4 5;5 8]
max(a)
min(a)
max(a,b)
max(a,[],2)
max(a,[],1)

The execution results are as follows

Matlab matrix common operation method

Guess you like