colon, :

Create vectors, array subscripting, and for-loop iterators

Description

The colon is one of the most useful operators in MATLAB^®. It can create vectors, subscript arrays, and specify for iterations.

The colon operator uses the following rules to create regularly spaced vectors for scalar values i, j, and k:

`j:k`	is the same as `[j,j+1,j+2,...,j+m]`, where `m = fix(k-j)`. In the case where both `j` and `k` are integers, this is simply `[j,j+1,...,k]`. This syntax returns an empty matrix when `j > k`.
`j:i:k`	is the same as `[j,j+i,j+2i, ...,j+m*i]`, where `m = fix((k-j)/i)`. This syntax returns an empty matrix when `i == 0`, `i > 0` and `j > k`, or `i < 0` and `j < k`.

If i, j, or k is an empty input, then the colon operator returns an empty 1-by-0 matrix. If you specify nonscalar arrays, MATLAB interprets j:i:k as j(1):i(1):k(1).

You can use the colon to create a vector of indices to select rows, columns, or elements of arrays, where:

`A(:,j)`	is the `j`th column of `A`.
`A(i,:)`	is the `i`th 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 `k`th 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`.

When you create a vector to index into a cell array or structure array (such as cellName{:} or structName(:).fieldName), MATLAB returns multiple outputs in a comma-separated list. For more information, see How to Use the Comma-Separated Lists in the MATLAB Programming Fundamentals documentation.

Examples

Using the colon with integers,

D = 1:4

results in

D =
    1    2    3    4

Using two colons to create a vector with arbitrary real increments between the elements,

E = 0:.1:.5

results in

E =
    0    0.1000    0.2000    0.3000    0.4000    0.5000

The command

A(:,:,2) = pascal(3)

generates a three-dimensional array whose first page is all zeros.

A(:,:,1) =
     0     0     0
     0     0     0
     0     0     0

A(:,:,2) =
     1     1     1
     1     2     3
     1     3     6

Using a colon with characters to iterate a for-loop,

for x='a':'d',x,end

results in

x = 
    a
x = 
    b
x = 
    c
x = 
    d

Documentation

colon, :

Description

Examples

See Also

MATLAB Documentation

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