MATLAB Answers

C Zeng

General questions about function and if command.

Asked by C Zeng
on 23 Oct 2012

1. If I have a variable x, and I use a nested function f and in f.m, there is also a variable x but not equal to the first x. Will it bother the program? The x's are defined in different functions, will matlab recognize these?

2. I have an error message: --- Operands to the and && operators must be convertible to logical scalar values.

Error in V_F (line 16) if P(n)>0 && x>n-1 ---

However, P(n) is scalar and x is also scalar. What is wrong here?




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2 Answers

Answer by Matt Fig
on 23 Oct 2012
Edited by Matt Fig
on 23 Oct 2012

For the first question, it makes a difference how and where you are accessing the variables. Post a simple example for anlaysis. For the second question, are you sure P is not a cell array and P(n) is not nan? Are you sure n is a scalar and not empty or nan? Are you sure x is a scalar and not empty or nan?

  1 Comment

C Zeng
on 23 Oct 2012

Hi, Matt, thanks in advance!

(1) For example, in my current program, I define x as a vector, like x=zeros(1,N). I also define a function f(a,b,c) note f has nothing with x. In f.m file, I write a code to find the f_value, including define auxiliary variable x which is a scalar. By doing so, it seems to me that it will not affect any confusion, and by convention, matlab will not make any problem, right?

(2)Let me double check it. I think they are all well defined and will not be empty or infinity.

Answer by Walter Roberson
on 23 Oct 2012

There might perhaps be a code path in which your local "x" does not get initialized in the subfunction on and so inherits the scalar value.


C Zeng
on 23 Oct 2012

Can you be clear, Walter? I am curious how does it happen.

Suppose you have

if a > 3
  x = 15;
elseif a < 3
  x = 22;

then it might at first look like x is always being initialized, but in fact in the case that a is exactly 3, there would be a path in which x was not set, and so in that case would inherit its value from the nesting function.

C Zeng
on 26 Oct 2012

Thanks, Walter, so in this case x is not well defined? Will it cause any problem to nested function?

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