WHy do I get "ans = 1" in the Command Window?

In running a program I have written using the pdepe function, and sometime during the function call:

soln=pdepe(m,@solnpde,@solnic,@solnbc,r,t,options);

I get a lot of "ans = 1" scrolling in the Command Window. The solution pdepe returns appears to be correct.

What does this mean? How can it be suppressed?

Thanks, Doug

[Stephen23]

Feature added in R2024b:

https://www.mathworks.com/matlabcentral/discussions/tips/876699-find-unsuppressed-outputs-in-r2024b-using-dbstop

Answer 1

[Fangjun Jiang]

Check all of the three functions solnpde, solnic and solnbc. There must be a line somewhere that doesn't have the ; at the end, thus the output is shown in the Command Window.

Run the following line and then run your program. Hope it can help you find where the ans=1 comes from.

echo on all

[Douglas]

Thanks Fangjun,

Unfortunately this has not helped. All lines in solnpde, solnic and solnbc have the ; at the end.

Do you have any further ideas?

Doug

[Jan]

@Douglas: Please search again. One of the called function contains a line with a missing trailing semicolon. You can use the debugger to find this line: set a breakpoint in the code and step through the code line by line until the "ans = 1" appears.

Or let MLint check this for you: Open all called functions in the editor and look for the MLint warnings on the right side.

[Fangjun Jiang]

Yes. You can see from the below example how 'ans = 1' appear. You probably have some debugging line accidentally left in the code. Use break point to narrow down while file and then trace down to the line. You have to follow all the functions that are called.

>> 2-1;

>> 2-1

ans =

1

>> a=1;

>> a=1

a =

1

[Fangjun Jiang]

See updated answer for a possible way to find it out.

Answer 2

[Walter Roberson]

Try

dbstop in disp

and then run. When it stops in the debugger, look at the code. There will not be any explicit call to disp() in the situation you are seeing, but disp() is what is called internally.

Though it might be "display" instead of "disp"... I have not had reason to figure out which of the two names is used in which circumstances.

[Daniel Shub]

From the help:

DISPLAY(X) is called for the object X when the semicolon is not used to terminate a statement.

Then in the doc it says taht when overloading display: "A typical implementation of display calls disp to do most of the work ..."

While display is what is being called if there is a missing semicolon. If a hard coded disp or printf is causing the output, then it might be disp that is being called.

[Walter Roberson]

Thanks for chasing that down!

[Daniel Shub]

I had thought that a solution to this question would have been to overload display/disp to call dbstack so I was looking at what is being called. It turns out that display is super special and cannot be "overloaded". If you write a function called display.m and put it in your current directory, MATLAB ignores it.

Answer 3

[Daniel Shub]

Do you have any eval statements in your code. It is possible that all your "lines" are terminated with a semicolon, but that the eval is not.

 x = 1;
 eval('x');

compared to

 eval('x;');

Answer 4

[Jan]

There is also a difference between these two calls:

fprintf(1, '\n');    % Print a line break in the command window
fprintf('1', '\n');  % Print a '1' to the command window

The 2nd ignored the '\n' and is most likely not intented, but a typo.