if statement in MatLab environment

Question

I used the following simple code to check the properties of elseif command in MATLAB:

x = 10;
if x < 0
   y = x;
elseif 0 <= x < 2
   y = 3*x;
elseif x >= 2
   y = 8*x;
end
y

I would have expected the result of this to be 80 since x >= 2. But something amazing happened! The result is 30, not 80!

But why? Any ideas?

Update: when I change the code (as recommended in the answers) to

x = 10;
if x < 0
   y = x;
elseif (0 <= x) && ( x < 2 )
   y = 3*x;
elseif x >= 2
   y = 8*x;
end
y

I get 80 as expected. It's that double condition that threw me off.

@user2667048 - glad to hear it. I am deleting my "old" comments now since the question is now clear, and there is no need to preserve the chatter for future visitors... — Floris, May 19 '14 at 05:18

score 4 · Accepted Answer · edited May 23 '17 at 12:20

When you write

if 0 <= x < 2

you are really writing (without realizing it)

if (0 <= x) < 2

Which is evaluated as

if (true) < 2

which is true, since true evaluates to 1.

So here is what is happening, line by line:

x = 10;            % x is set to 10
if x < 0           % false
   y = x;          % not executed
elseif 0 <= x < 2  % condition is (true) < 2, thus true
   y = 3*x;        % y is set to 3*x, i.e. 30
elseif x >= 2      % since we already passed a condition, this is not evaluated
   y = 8*x;        % skipped
end                % end of composite if 
y                  % display the value of y - it should be 30

As an aside, when you use scalars, you should really use the && operator, not the & operator. See for example What's the difference between & and && in MATLAB?

You're right. I made a mistake in copying the code. Thanks for your description. — user2667048, May 18 '14 at 20:29
+1 for the effort to make a detailed explanation for such a simple problem — Luis Mendo, May 18 '14 at 20:49

Luis Mendo · Answer 2 · 2014-05-18T20:50:17.110

2

0 <= x < 2 doesn't behave as you may expect. Use (0 <= x) & (x < 2)

How does 0 <= x < 2 behave? It is evaluated from left to right. First 0 <= x gives 1 (true), and then 1 < 2 gives 1 (true). So the condition gives true, not false as you would expect.

Since the second condition in your code (0 <= x < 2) is true, you get a value 30 for y. Changing the condition to (0 <= x) & (x < 2) you get a value 80 for y.

edited May 18 '14 at 20:50

answered May 18 '14 at 20:24

Luis Mendo

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if statement in MatLab environment

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