Using conditional breakpoints to print out value

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Hi, I'm interested if it would be possible to use/abuse(?) conditional breakpoints to print out the value of a variable to the command window. Sometimes I work with a codebase that is read-only, so I can't insert 'disp()' statements... Thanks!

Accepted Answer

Jonathan Sullivan
Jonathan Sullivan on 12 Feb 2013
Yes sir. It takes a bit of creativity.
Here's an example. Let's print out "Hi" each time mod(ii,2) == 0.
iff(mod(ii,2),fprintf('Hi.\n') > 0,false)
This will pause the code execution. If you prefer to not pause the code execusion, try:
iff(mod(ii,2),fprintf('Hi.\n') == NaN,false)
Where iff is defined as follows:
function out = iff(cond,vtrue,vfalse)
if cond
out = vtrue;
out = vfalse;
Sean de Wolski
Sean de Wolski on 12 Feb 2013
So that it evaluates the anonymous function; that makes sense.
Eric Sampson
Eric Sampson on 12 Feb 2013
Actually here's a pretty clean way to do it without requiring the IIF function, it relies on the short-circuting behavior of the AND operator to only execute the FPRINTF when the first expression is true - just put this as the breakpoint conditional expression:
mod(i,2) && (fprintf('test.\n') < 0) % or use >=0 if you want it to pause.

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More Answers (2)

per isakson
per isakson on 12 Feb 2013
Edited: per isakson on 12 Feb 2013
Yes, that is possible. See tracer4m in the file exchange.
The function invoked at the conditional break point shall always return true and not error. Printing should be fine. If the name of variable isn't known beforehand one may use whos/who.

Matt J
Matt J on 11 Feb 2013
Edited: Matt J on 11 Feb 2013
Once the code stops at breakpoint, you can highlight a variable or expression in the editor window. Then right-click and select "Evaluate Selection".
You should also be able to see the value of a variable by hovering the mouse over any occurrence of it in the editor.
  1 Comment
Eric Sampson
Eric Sampson on 12 Feb 2013
Hi Matt, yup I understand what you say - but I'm looking to have the variable's value printed to the command window each time that the condition is true (like a trace or log), without having to manually inspect the value each time. Cheers :)

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