MATLAB Colon

Introduction to MATLAB Colon

‘Colon’ is used as an operator in MATLAB programming and is one of the frequently used operators. This operator comes into the picture in order to create vectors defining with simple expressions, specifying‘for’ iterations or subscribing arrays, or getting access to a set of elements of an existing vector in a sequence. When a vector is created to index into a cell or structure type array using the colon operator in the form of ‘cellName{:}’ or ‘structName(:).fieldName)’, MATLAB results in multiple outputs putting in a comma-separated list.

Syntax

Various forms of syntax are supported to use the operator ‘:’. The functionalities of different syntaxes are described as below:

Examples to Implement MATLAB Colon

Below are some examples mentioned:

Example #1

Code:

list_name = -3:3

Output:

Explanation: The command has generated a list of values from -3 to 3 having different between 2 consecutive elements as ‘1’.

Example #2

Code:

list_name = -3:3:30

Output:

Explanation: The command has generated list of values from -3 to 30 having different between 2 consecutive elements as ‘3’.

Example #3

Code:

Mat_A = magic(3)Mat_A(2,:)

Output:

Explanation: The command has displayed the elements from the second row of the matrix ‘Mat_A’

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Example #4

Code:

Mat_A = magic(3)Mat_A(:,2)

Output:

Explanation: The command has displayed the elements from the second column of the matrix ‘Mat_A’

Example #5

Code:

Mat_A = magic(3)Mat_A(:)

Output:

Explanation: The command has displayed the elements of the matrix ‘Mat_A’ in a single column.

Example #6

Code:

Mat_A = magic(4)Mat_A(2:4)

Output:

Explanation: The command has displayed the elements of the matrix ‘Mat_A’ indexed between 2 to 4.

Application of MATLAB Colon

Below are the applications:

1. Using colon to create a list

A vector with evenly-spaced numbers can be generated using a colon operator.

Code:

Mat_A =1:2:10

Output:

2. Creating a vector with only column format

The colon operator can be used to transform the input of the row vector type to that of the column vector type. The colon operator can also be used with reshaping or permute functions to create a column vector.

Code:

M = rand(3,2,6,4);
M32sum = sum(M(3,2,:))
M32sumAll = sum(M(3,2,:,:))
%Creating column vector using reshape method
M32vecR = reshape(M32sumAll,[],1)
%Creating column vector using %permute method
M32vecP = permute(M32sumAll, [4 1:3])
%Creating column vector using only colon operator
M32vec = M32sumAll(:)
allthesame = isequal(M32vec, M32vecP, M32vecR)

Output:

3. Maintaining the shape of an array during assignment operation

The application of the assignment operator in the assignment operation on the input matrix, assures the shape of the array remains unchanged.

Code:

M32sumAll(:) = [1 2; 3 5]

Output:

4. Working with all the entries in specified dimensions

The colon operator can also be used to manipulate specific dimensions of the input array.

Code:

M = zeros(3,3,3)M(:) = 1:numel(M)M1 = M(:,[2:size(M,2) 1],:)

Output:

Explanation: The indices of the elements in the input matrix ‘M’ is redefined and created new input matrix M1 with the application of the colon operator.

5. Create Unit-Spaced Vector

Using a colon operator, a vector having a list of consecutive numbers within a specified range, can be generated using the syntax li=j: k.

Code:

list_vector = 1:15

Output:

Explanation: A list of having a number from 1 to 15 is generated from the MATLAB command using a colon operator.

6. Create a Vector with Specified Increment

Using colon operator, a vector having a list of numbers having an equal difference between two consecutive numbers within a specified range can be generated using the syntax li=j:i: k.

Code:

list_vector = 1:3:15

Output:

Explanation: A list of having numbers from 1 to 15 with a common difference of 3, are generated from the MATLAB command using a colon operator.

7. Specify for-loop Iterations

Colon operator is also used in designing the looping operation in MATLAB programming.

Code:

for m = 1:4
m^3+3
end

Output:

Additional point:

In case I being a non-integer and k is not equals to j+m*i, in the command form of li=j:i:k, floating-point arithmetic determines about colon including the endpoint k or not.

If no scalar array is specified, then MATLAB assumes i:k as j(1): i(1):k(1).

li = colon(j,k) and li = colon(j,i,k) are rarely used alternate ways for the commands li=j:k and li=j:i:k respectively.While implementing class, these syntaxes can be used to apply operator overloading.

Linspaceexhibit similar behavior to that of colon operator. The difference lies aslinespacegives direct control over the number of points and endpoints are always gets included.

Logspace, the sibling function is used to generate values being logarithmically spaced.

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