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What does the colon operator (:) in MATLAB do?

Asked by Andrew on 21 Mar 2011

The code below is used to draw a simple octagon on the screen with the word stop in the middle.

My question is, can anyone explain what is happening in the top statement. (variable t). What does the (1:2:15)' do?

Also, for example, if I wanted to draw a circle on the screen, how would I modify this code to do so.

t = (1:2:15)'*pi/8;
  x = sin(t);
y = cos(t);
fill(x,y,'r')
axis square off
text(0,0,'STOP','Color',[0 0 0],'FontSize',80,'FontWeight','bold','HorizontalAlignment','center')
title('Figure STOP')

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Andrew

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3 Answers

Answer by Paulo Silva on 21 Mar 2011
Accepted answer

1:2:15 creates a vector like this:

[1,3,5,7,9,11,13,15]

doc colon

The ' puts the values in lines (rows) instead of columns.

[1;3;5;7;9;11;13;15]

doc transpose

first vale is 1, step is 2 and last value is 15

If you want to create the circle you can do this

t=0:0.1:2*pi;

and keep the rest of the code without any changes

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Paulo Silva
Answer by the cyclist on 21 Mar 2011

x and y are being created via a "parameterization" through t. The variable t is running from 0 to 2pi (i.e. around the circumference of the circle), but it does so in eight discrete intervals. (That's why you get eight sides.)

If you breakpoint your code after the first line, you will see that t takes equally spaced values around the circle. (The "linspace" command would have been another way to do this.) The part "1:2:15" is the vector [1 3 5 7 9 11 13 15].

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the cyclist
Answer by Alexander on 14 Aug 2013

Yes but it's been seen as := as well, what does that do?

2 Comments

Jan Simon on 15 Aug 2013

At 1st this is a new question and not an answer of the original question. At 2nd ":=" might be seen anywhere, but not in valid Matlab code. In a mathematical context a ":=" means, that the left side is defined as the right side.

Image Analyst on 15 Aug 2013

I'm not even sure that Alexander "Answered" the question he thought he was. His response is a total non sequiter.

Alexander

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