I'm getting an error in my function I don't understand

My function is as follows

function [I,V] = MeasureResistance(R0,N,Imin,Imax,sigmaI,typeV)

      if nargin < 2
         N = 1;
         I0 = -1 + 2*rand(N,1);
         Ve = randn(N,1);
         Ie = randn(N,1);
      elseif nargin < 3
         I0 = -1 + 2*rand(N,1);
         Ve = randn(N,1);
         Ie = randn(N,1);
      elseif nargin < 7
         I0 = Imin + (Imax - Imin)*rand(N,100000);
         Ie = sigmaI;
         if typeV == 'N'
             Ve = randn(N,1);
         elseif typeV == 'U'
             Ve = rand(N,1);
         end
      end

V0 = R0*I0;

I = I0 + Ie;
V = V0 + Ve;

However when I run the function I get this error

Error: File: MeasureResistance.m Line: 18 Column: 17 The expression to the left of the equals sign is not a valid target for an assignment.

Any insight as to what may be causing this?

[Image Analyst]

Like almost all other people, you didn't copy the full error message - all the red text. You just snipped out a part and left out the crucial part of what line 18 is. What is line 18? The error message will tell you. Is it

Ve = rand(N,1);

because that's what I get when I count down your code. If so, there's nothing wrong with that line.

[Obsidian]

Sorry about that I'm still new to the forums.

Either way I'm not getting the error message anymore for that particular issue. I am now receiving another error.

The code is now

function [I,V] = MeasureResistance(R0,N,Imin,Imax,sigmaI,typeV)

      if nargin < 2
         N = 1;
         I0 = -1 + 2*rand(N,1);
         Ve = randn(N,1);
         Ie = randn(N,1);
      elseif nargin < 3
         I0 = -1 + 2*rand(N,1);
         Ve = randn(N,1);
         Ie = randn(N,1);
      elseif nargin < 7
         I0 = Imin + (Imax - Imin)*rand(N,100000);
         if typeV == 'N';
             Ve = randn(N,100000);
         elseif typeV == 'U';
             Ve = rand(N,100000);
         end
         Ie = sigmaI;
      end

V0 = R0*I0;

I = I0 + Ie;
V = V0 + Ve;

When I run the function with N = 1 I have no issues, however once I bumped it to N = 2 I get the error

"Error using + Matrix dimensions must agree. Error in MeasureResistance (line 25) V = V0 + Ve;"

I have to take Measurements with N = 1,2,4,8. I don't understand why it suddenly doesn't work. I did the math outside of the function step by step and it works, I'm almost sure the matrix dimensions are correct.

Answer 1

[per isakson]

To add two arrays they must have the same size. Obviously, with N=2 V0 and Ve don't have the same size. That's what the error message says.

Set

    dbstop if error

and run the function with N=2. Execution will stop at line 25. Inspect V0 and Ve and you will see which size differs from your expectation.

&nbsp

The debugging tools of Matlab are good! Here are some links on debugging in Matlab

• Debugging in MATLAB
• Debugging session
• Debugging Approaches

Answer 2

[Guillaume]

The best way for you to find out what is going wrong is to type:

dbstop if error

at the command line before calling your function. When the error occurs, it will go into debug mode and you can inspect V0 and Ve to see what their size is.

Additionally, you could also set a breakpoint at the start of your function and then step through your code line by line to see where it is going wrong.