MATLAB

In previous years students have experienced various problems. We have collected some "frequently asked questions” with answers in the hope it will save you some time.

Question:

I tried to execute my M-file proj.m using >> proj but received

??? Undefined function or variable ’proj’.

or

I tried to use my function proj(x) held in proj.m but received

??? Can not find function ’proj’.

Error in ==> c:\\mat\toolbox\\mat\specgraph\fplot.m

On line 85 ==> x = xmin; y = feval(fun,x);

Reason:

You are not working in the same directory where the M-file proj.m is held.

Action:

Change directory or save the M-file into the correct directory .

******************************************************************************

Question:

I attempted to evaluate an elementary MATLAB function such as mod(101,7) (or another MATLAB function) but received the following message

??? ??? Error using ==> mod

Too many input arguments.

I then asked for help using help mod and received

No help comments found in mod.m.

Reason:

Both responses indicate that you have inadvertently named one of your own M-files mod.m previously. It has taken precedence over MATLAB ’s inbuilt function.

Action:

Change the name of your own M-file to one that does not exist in MATLAB .

******************************************************************************

Question:

Why do I receive one of the following error messages, or similar?

??? Error using ==> .*

Matrix dimensions must agree.

or

??? Error using ==> plot

Vectors must be the same lengths.

Reason:

When doing a calculation involving two arrays, or graphing using the plot command, the arrays must be of the same size.

Action:

Check the size of each array involved.

******************************************************************************

Question:

Why did I not get any numbers printed on the screen?

Reason:

Semicolons at the end of a MATLAB assignment statement prevent answers being displayed.

Action:

Leave off semicolons where you want numbers to be shown in the output.

******************************************************************************

Question:

Why did I get thousands of numbers printed on the screen?

Reason:

Semicolons at the end of a MATLAB assignment statement prevent answers

being displayed.

Action:

Place semicolons where you do not want numbers to be shown in the output.

******************************************************************************

Question:

Why do I continually get the following error message?

??? Error using ==> ^

Matrix must be square.

or

??? Error using ==> *

Inner matrix dimensions must agree.

Reason:

You have asked for something like x^2 or f(x)*g(x) where x is currently an array. It is only possible to square an array x or multiply f(x)*g(x) if x is a square matrix.

Action:

If you think x is a scalar variable, it is possible that you were also using the same symbol to represent an array and forgot to clear it. But more likely x is an array and you should be using the dot notation x.^2 or f(x).*g(x). The location might be in a script M-file or, more obscurely, in a function M-file where you did not allow for later array evaluations.

******************************************************************************

Question:

Why are parts of my graphical figures missing?

Reason:

You have probably lost graphs or graphical components.

Action:

Make sure each figure is numbered. Within a figure, use the hold on . . . hold off facility to avoid losing command outcomes.

******************************************************************************

Question:

When I access a function M-file, either by doing an evaluation after the >> prompt or else calling from another M-file, why do I get no answer back or else receive the empty answer: [ ] ?

Reason:

Although the first line in your function M-file was of the form

function y=fred(x)

you have not assigned any value to y during the file.

Action:

Make sure output variables are given values in function M-files.

******************************************************************************

Question:

When using solve on a transcendental equation in “Maple”, why do I get complex solutions, or else a real solution which is not the one I want?

Reason: Consider, for example, the equation xe^x + x³ −2 = 0. Using

syms x , ans=solve(x*exp(x)+x^3-2)

yields a complex solution only.

Action:

Use fzero in MATLAB instead. If the function has been defined in func.m, then ans=fzero(@func,[0 1]) yields ans=0.7482 which is probably the solution you were seeking within the specified interval.