matlab find函数详解

Find 这个函数用处也挺大的，这几天看很多程序都见到这一函数，今天要好好给阐述，了解下

这个函数是为了找到矩阵或者是数组，向量中的非零元素。下面一大段英文没耐心看。看看例子就行了。

第一个用法是

ndices = find(X)

X = [1 0 4 -3 0 0 0 8 6];

indices = find(X)

返回x中非零元素的序号从1开始

indices =

    1     3     4    8     9

若x是个矩阵呢？

x= [1  0 ;1  5 ;4 0 ]

find(x)得到的是

    1

    2

    3

    5

  是一列矢量，非零元素的序号是按一列一列的算

第二个用法

ind = find(X, k)
ind = find(X, k, 'first')
ind = find(X, k, 'last')

这三个是找前K个非零元素，前两式功能一致，第三式指从最后一个元素算起。

重点说一下

第三个用法

[row,col] = find(X, ...)
[row,col,v] = find(X, ...)

例如

X = [3 2 0; -5 0 7; 0 0 1];

[r,c,v] = find(X>2)

r =1

  2

c =1

  3

v =1

  1

这样看看不出什么？看下式就比较明了。

 

1  2  1    就是说矩阵中第一行第二列的元素满足X>2为真用1来表示。余下的类似。

1  3  1   

1 1  1  

[r,c,v]

要注意一下这一用法

X = [3 2 0; -5 0 7; 0 0 1];

[r,c,v]= find(X)                           这个得到的v是X中非零的元素，要注意这个区别

r =

    1                                     

    2                                  

    1            

    2

    3

c =

    1

    1

    2

    3

    3

v =

    3

   -5

    2

    7

    1

Ps：如果你想求出矩阵中满足某种特定要求的元素，你会想到用find，但这并不是一个好方法

有更加简便的方法如要找f1中小于等于 t的元素。用下式就能够做到

r1=f1(f1<=t);   

求矩阵所有元素的均值也不要像C语言那样把所有一个个都加起来利用下式temp1=mean(r1(:));  

 

find

Find indices and values of nonzero elements

Syntax

ind = find(X)
ind = find(X, k)
ind = find(X, k, 'first')
ind = find(X, k, 'last')
[row,col] = find(X, ...)
[row,col,v] = find(X, ...)

Description

ind = find(X)locates all nonzero elements of array X, and returns the linear indices ofthose elements in vector ind. If X is a row vector, then ind is a row vector;otherwise, ind is a column vector. If X contains no nonzero elements or is anempty array, then ind is an empty array.

ind = find(X, k) or ind = find(X, k, 'first') returns at most the first k indicescorresponding to the nonzero entries of X. k must be a positive integer, but itcan be of any numeric data type.

ind = find(X, k,'last') returns at most the last k indices corresponding to the nonzero entriesof X.

[row,col] = find(X, ...) returns the rowand column indices of the nonzero entries in the matrix X. This syntax isespecially useful when working with sparse matrices. If X is an N-dimensionalarray with N > 2, col contains linear indices for the columns. For example,for a 5-by-7-by-3 array X with a nonzero element at X(4,2,3), find returns 4 inrow and 16 in col. That is, (7 columns in page 1) + (7 columns in page 2) + (2columns in page 3) = 16.

[row,col,v] =find(X, ...) returns a column or row vector v of the nonzero entries in X, aswell as row and column indices. If X is a logical expression, then v is alogical array. Output v contains the non-zero elements of the logical arrayobtained by evaluating the expression X. For example,

A= magic(4)

A =

   16     2     3   13

    5    11    10    8

    9     7     6   12

    4    14    15    1

 

[r,c,v]= find(A>10);

 

r', c', v'

ans =

    1     2     4    4     1     3

ans =

    1     2     2    3     4     4

ans =

    1     1     1    1     1     1

Here the returned vector v is a logicalarray that contains the nonzero elements of N where

N=(A>10)

Examples

Example 1

X = [1 0 4 -3 0 0 0 8 6];

indices = find(X)

returns linear indices for the nonzeroentries of X.

indices =

    1     3    4     8     9

Example 2

You can use a logical expression to defineX. For example,

find(X > 2)

returns linear indices corresponding to theentries of X that are greater than 2.

ans =

    3     8     9

Example 3

The following find command

X = [3 2 0; -5 0 7; 0 0 1];

[r,c,v] = find(X)

returns a vector of row indices of thenonzero entries of X

r =

    1

    2

    1

    2

    3

a vector of column indices of the nonzeroentries of X

c =

    1

    1

    2

    3

    3

and a vector containing the nonzero entriesof X.

v =

    3

   -5

    2

    7

    1

Example 4

The expression

X = [3 2 0; -5 0 7; 0 0 1];

[r,c,v] = find(X>2)

returns a vector of row indices of thenonzero entries of N where N=(X>2)

r =

    1

    2

a vector of column indices of the nonzeroentries of N where N=(X>2)

c =

    1

    3

and a logical array that contains thenonzero elements of N where N=(X>2).

v =

    1

    1

Recall that when you use find on a logicalexpression, the output vector v does not contain the nonzero entries of theinput array. Instead, it contains the nonzero values returned after evaluatingthe logical expression.

Example 5

Some operations on a vector

x = [11 0  33  0 55]';

 

find(x)

ans =

    1

    3

    5

 

find(x == 0)

ans =

    2

    4

 

find(0 < x & x < 10*pi)

ans =

    1

Example 6

For the matrix

M = magic(3)

M =

    8     1     6

    3     5     7

    4     9     2

 

find(M > 3, 4)

returns the indices of the first fourentries of M that are greater than 3.

ans =

    1

    3

    5

    6

Example 7

If X is a vector of all zeros, find(X)returns an empty matrix. For example,

indices = find([0;0;0])

indices =

  Empty matrix: 0-by-1

 

最后

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