Development Environment

Setting Breakpoints

Set breakpoints to pause execution of the function so you can examine values where you think the problem might be. You can only set breakpoints at executable lines in saved files that are in the current directory or in directories on the search path. When you create a new M-file, save it before setting breakpoints. You cannot set breakpoints while MATLAB is busy, for example, running an M-file.

Breakpoints for the Example.   It is unclear whether the problem in the example is in collatzplot or collatz. To start, set breakpoints in collatzplot.m at lines 10, 11, and 12. The breakpoint at line 10 allows you to step into collatz to see if the problem might be there. The breakpoints at lines 11 and 12 stop the program where you can examine the interim results.

Setting Breakpoints Using the Editor/Debugger.   To set a breakpoint using the Editor/Debugger, click in the breakpoint alley at the line where you want to set the breakpoint. The breakpoint alley is the column to the right of the line number. You can only set breakpoints at lines that are preceded by a - (dash). Lines not preceded by a dash, such as comments, are not executable. Other ways to set a breakpoint are to position the cursor in the line and then click the set/clear breakpoint button on the toolbar, or select Set/Clear Breakpoint from the Breakpoints menu or the context menu.

A breakpoint icon appears, as in the following illustration for line 10.

Function Equivalent.   To set a breakpoint using the debugging functions, use dbstop. For the example, type

dbstop in collatzplot at 10
dbstop in collatzplot at 11
dbstop in collatzplot at 12

Some useful related functions are:

• dbtype - Lists the M-file with line numbers in the Command Window.
• dbstatus - Lists breakpoints.

Setting Stops for Conditions.   Use these items on the Breakpoints menu or the dbstop function equivalents to instruct the program to stop when it encounters a problem:

• Stop If Error, or dbstop if error
• Stop If Warning, or dbstop if warning
• Stop If NaN Or Inf (for not-a-number or infinite value), or dbstop if naninf or dbstop if infnan

Using Debugging Features

Running an M-File