MATLAB find()函数

MATLAB find()函数（还有一些问题不明白）(2009-11-05 00:00:00) 标签：教育
find
Find indices索引and values值of nonzero elements非零元素Syntax句法
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非零元素of array X,
and returns the linear indices线性下标of those elements in vector ind在向量ind中.
If X is a row vector行向量, then ind is a row vector行向量; otherwise, ind is a column vector列向量.
If X contains no nonzero elements or is
an empty array, then ind is an empty array空向量.
ind = find(X, k) or ind = find(X, k, 'first')
returns at most至多，不超过the first k indices
corresponding to与...一致the nonzero entries个数of X. % 也就是至多只能输出X中符合条件的前k项
k must be a positive integer正整数, but it can be of any numeric data type数字数据类型.
ind = find(X, k, 'last') returns at
most the last k indices corresponding to the nonzero entries of X. [row,col] = find(X, ...) returns
the row and column indices of the nonzero entries in the matrix X.
This syntax句法is especially useful when working with sparse matrices 稀疏矩阵. If X is
an N-dimensional arrayN维数组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 in row and 16 in col.
That is, (7 columns in page 1) + (7 columns in page 2) + (2 columns in page 3) = 16.
[row,col,v] = find(X, ...) returns
a column or row vector v of the nonzero entries in X,
as well as row and column indices. If X is a logical expression_r_r_r逻辑
表达式,
then v is a logical array逻辑数组.
Output v contains the non-zero elements of the logical array obtained by evaluating the expression_r_r_r表达式X.
For example,
A= magic(4)
A =
16 2 3 13
5 11 10 8
9 7 6 12
4 14 1
5 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 logical array逻辑数组that
contains the nonzero elements非零项of N where N=(A>10) % N是X
中>10的元素组成的向量
Examples
Example 1
X = [1 0 4 -3 0 0 0 8 6];
indices = find(X) returns linear indices线性下标for the nonzero entries 非零项of X.
indices =
1 3 4 8 9
Example 2
You can use a logical expression_r_r_r逻辑表达式to define X.
For example,
find(X > 2) returns linear indices线性下标corresponding to the entries 项of X that
are greater than 2.
ans =
3 8 9
Example 3
The following find command下面的find命令
X = [3 2 0; -5 0 7; 0 0 1];
[r,c,v] = find(X)
returns a vector of row行indices of the nonzero entries个数of X
r =
1
2
1
2
3
a vector of column列indices of the nonzero entries个数of X c =
1
1
2
3
3
and a vector containing包含the nonzero entries非零项of X. v =
3
-5
2
7
1
Example 4
The expression_r_r_r表达式
[r,c,v] = find(X>2)
returns a vector of row indices of the nonzero entries个数of X(>2)
r =
1
2
a vector of column indices of the nonzero entries个数of X(>2)
c =
1
3
and a logical array逻辑数组that contains the non zero elements非零元素of N（矩阵）where N=(X>2). % N是X中>2的元素组成的向量，且v=find(N)
v =
1
1
Recall that when you use find on a logical expression_r_r_r逻辑表达式, the output vector v does not contain the nonzero entries非零项
of the input array输入的数组. Instead, it contains the nonzero values非零值returned after
evaluating the logical expression_r_r_r逻辑表达式.
Example 5
Some operations操作on a vector
x = [11 0 33 0 55]';
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下标of the first four entries前四项（也就是只输出前四项）of M that are greater than 3.
1
3
5
6
Example 7
If X is a vector of all zeros, find(X) returns an empty matrix一个空矩阵. For example,indices = find([0;0;0])
indices =
Empty matrix空矩阵: 0-by-1。