### 25.6.1 Starting the debugger

The debug command starts the Xcas debugger.

- debug takes fn(arg), a function and its argument.
- debug(fn(arg)) brings up a debug window which contains a pane with the program with the current line highlighted, an eval entry box, a pane with the program including the breakpoints, a row of buttons, and a pane keeping track of the values of variables.

By default, the value of all variables in the program are in this pane. The buttons are shortcuts for entering commands in the eval box, but you can enter other commands in the eval box to change the values of variables or to run a command in the context of the program.

#### Example

With the sumprod program:

sumprod(a,b):={ local s,p; s:=a+b; p:=a*b; return [s,p]; }

input:

debug(sumprod(2,3))

which opens the debug window shown in Figure 25.2.

> Figure 25.2: The debug window in Xcas

It has the following buttons:

- sst runs the sst command, which takes no arguments and runs the highlighted line in the program before moving to the next line.
- in runs the sst_in command, which takes no argument and runs one step in the program or a user defined function used in the program.
- cont runs the cont command, which takes no arguments and runs the commands from the highlighted line to a breakpoint.
- kill runs the kill command, which exits the debugger.
- break puts the command breakpoint in the eval box, with default arguments of the current program and the current line. It sets a breakpoint at the given line of the given program. Alternatively, if you click on a line in the program in the top pane, you will get the breakpoint command with that program and the line you clicked on.

  You can set a breakpoint when you write a program with the halt command. A halt() line in the program will bring up the debugger during runtime. If you want to debug the program, though, it is still better to use the debug command. Also, you should remove any halt commands when you are done debugging.

- rmbrk puts the command rmbreakpoint in the eval box, with default arguments of the current program and the current line. It removes a breakpoint at the given line of the given program. Alternatively, you can click on the line in the program in the top pane with the bookmark you want to remove.
- watch puts the command watch in the eval box, without the arguments filled in. It takes a list of variables as arguments, and will keep track of the values of these variables in the variable pane.
- rmwtch puts the command rmwatch in the eval box without the arguments filled in. The arguments are the variables you want to remove from the watch list.