Procedural and Object-oriented Programming 6

Trace (Debug)

TRACE – How Does Your Code Execute?

Understanding what your Rexx program does

Sometimes coding errors cannot be understood easily
Keyword statement TRACE and built-in function (BIF) TRACE()
- Allows to temporarily have Rexx show in detail how it executes your statements
- Allows to enter interactive mode that allows to inspect a specific location in your code

Most important options

N	normal trace setting, only traces failures in commands
A	show all statements that get executed
R	show all statements and their final results
I	show in detail all intermediate steps when executing a statement

Prefixing an option with a question mark (?) causes TRACE to become interactive and to trace statements step by step at each press of the <enter> key
- Enter "trace n" to return to normal execution and stop interactive trace

TRACE N – Example, 1

Simple program, run twice with different output, default option N
- Values change per run because of using the Rexx random()-BIF

/* default: "trace normal" in effect */
a=100+20
b=100+random(10,99)
say "a*b:" a*b

Output of run # 1:

a*b: 16800

Output of run # 2:

a*b: 17400

TRACE A – Example, 2

Running the program with TRACE A
- Option A traces all statements (clauses) before execution

trace a     /* show all statements */
a=100+20
b=100+random(10,99)
say "a*b:" a*b

Output:

     2 *-* a=100+20
     3 *-* b=100+random(10,99)
     4 *-* say "a*b:" a*b
a*b: 17040

TRACE R – Example, 3

Running the program with TRACE R
- Option R traces all statements (clauses) and their final results

trace r     /* show results */
a=100+20
b=100+random(10,99)
say "a*b:" a*b

Output:

     2 *-* a=100+20
       >>>   "120"
     3 *-* b=100+random(10,99)
       >>>   "113"
     4 *-* say "a*b:" a*b
       >>>   "a*b: 13560"
a*b: 13560

TRACE I – Example, 4

Running the program with TRACE I
- Option I traces all intermediate results

trace i     /* show intermediates */
a=100+20
b=100+random(10,99)
say "a*b:" a*b

Output:

     2 *-* a=100+20
       >L>   "100"
       >L>   "20"
       >O>   "+" => "120"
       >>>   "120"
       >=>   A <= "120"
     3 *-* b=100+random(10,99)
       >L>   "100"
       >L>   "10"
       >A>   "10"
       >L>   "99"
       >A>   "99"
       >F>   RANDOM => "98"
       >O>   "+" => "198"
       >>>   "198"
       >=>   B <= "198"
     4 *-* say "a*b:" a*b
       >L>   "a*b:"
       >V>   A => "120"
       >V>   B => "198"
       >O>   "*" => "23760"
       >O>   " " => "a*b: 23760"
       >>>   "a*b: 23760"
a*b: 23760

TRACE I, TRACE N – Example, 5

Running the program with TRACE I for tracing a single statement
- TRACE N gets used to reset tracing to the default

a=100+20
trace i     /* show intermediates */
b=100+random(10,99)
trace n     /* reset to normal    */
say "a*b:" a*b

Output:

     3 *-* b=100+random(10,99)
       >L>   "100"
       >L>   "10"
       >A>   "10"
       >L>   "99"
       >A>   "99"
       >F>   RANDOM => "80"
       >O>   "+" => "180"
       >>>   "180"
       >=>   B <= "180"
     4 *-* trace n     /* reset to normal    */
a*b: 21600

TRACE ?A – Example, 6

Using TRACE ?A for tracing all statements in interactive mode
- Interactive tracing allows for inspecting, changing values, tracing step by step

trace ?a     /* show all statements, enter interactive mode */
a=100+20
b=100+random(10,99)
say "a*b:" a*b

Output:

       +++ "WindowsNT COMMAND E:\tmp\06_ooRexx\19_trace_r_interactive.rex"
     2 *-* a=100+20
+++ Interactive trace. "Trace Off" to end debug, ENTER to continue. +++
say a
120
a=100; trace r
    3 *-*     b=100+random(10,99)
       >>>       "198"
trace off
a*b: 19800

Execution stops, manually adding a statement to display the current value of variable a,
pressing <ENTER> continues.

Execution stops, manually adding two statements: changing variable a to 100 and
changing tracing to trace results from now on, pressing <ENTER> continues.

Execution stops, manually adding a single statement:
set interactive tracing off, pressing <ENTER> continues.

Contact

WU

Prof. Mag. Dr. Rony G. Flatscher

Welthandelsplatz 1

1020 Wien/Vienna

Austria/Europe

www: www.wu.ac.at