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The colon(:) is one of the most useful operator in MATLAB. It is used to create vectors, subscript arrays, and specify for iterations.
If you want to create a row vector, containing integers from 1 to 10, you write −
1:10
MATLAB executes the statement and returns a row vector containing the integers from 1 to 10 −
ans =
1 2 3 4 5 6 7 8 9 10
If you want to specify an increment value other than one, for example −
100: -5: 50
MATLAB executes the statement and returns the following result −
ans = 100 95 90 85 80 75 70 65 60 55 50
Let us take another example −
0:pi/8:pi
MATLAB executes the statement and returns the following result −
ans =
Columns 1 through 7
0 0.3927 0.7854 1.1781 1.5708 1.9635 2.3562
Columns 8 through 9
2.7489 3.1416
You can use the colon operator to create a vector of indices to select rows, columns or elements of arrays.
The following table describes its use for this purpose (let us have a matrix A) −
| Format | Purpose |
|---|---|
| A(:,j) | is the jth column of A. |
| A(i,:) | is the ith row of A. |
| A(:,:) | is the equivalent two-dimensional array. For matrices this is the same as A. |
| A(j:k) | is A(j), A(j+1),…,A(k). |
| A(:,j:k) | is A(:,j), A(:,j+1),…,A(:,k). |
| A(:,:,k) | is the kth page of three-dimensional array A. |
| A(i,j,k,:) | is a vector in four-dimensional array A. The vector includes A(i,j,k,1), A(i,j,k,2), A(i,j,k,3), and so on. |
| A(:) | is all the elements of A, regarded as a single column. On the left side of an assignment statement, A(:) fills A, preserving its shape from before. In this case, the right side must contain the same number of elements as A. |
Example
Create a script file and type the following code in it −
A = [1 2 3 4; 4 5 6 7; 7 8 9 10] A(:,2) % second column of A A(:,2:3) % second and third column of A A(2:3,2:3) % second and third rows and second and third columns
When you run the file, it displays the following result −
A =
1 2 3 4
4 5 6 7
7 8 9 10
ans =
2
5
8
ans =
2 3
5 6
8 9
ans =
5 6
8 9
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