Program Errors and Debugging

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Program Errors and Debugging

• Week 10, Thursday Lab

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Debugging Programs

• Manually verify your program first
• Trace the program by hand
• With the different inputs it can take
• Use print statements
• Using the debugger

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Off-by-One Loop Errors

• Executing the loop by one more or one less time
  than it is supposed to
• If a sentinel-controlled while loop performs
  extra repetition, it may erroneously process the
  sentinel value along with the regular data
• (e.g. when reading from a file, WHILE eof).
• Checking Loop Boundaries by hand
• Evaluate initial expression and the final
  expression
• Substitute these values everywhere the counter
  variable appears in the loop body
• Verify that you get the expected results at the
  boundaries

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Conditional Errors

• Probably the most common error is not using or
  misusing BEGIN and END.
• IF ltconditiongt THEN ltstatementgtELSE
  ltstatementgt
• CASE ltselectorgt OF ltlabel-listgt
  ltstatementgt ELSE ltstatement-listgtEND

SINGLE STATEMENTS orCOMPOUND STATEMENT
ltstatementgt ltstatementgt...ltstatementgt
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Conditional Errors

• You are not testing all the conditions you should
  be testing
• Some conditions arent caught and fall through
• Your condition expressions are not correct (you
  have a logical error)

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Common Loop ErrorsOff-by-One Loop

• Example Sum of squares of numbers
• Sum 0
• FOR J K TO N-K DO
• Sum Sum Sqr(J)
• First value of J K
• Last value of J N-K
• The assignment at J K is
• Sum Sqr(K)
• The assignment at J N-K is Sum Prev.Sum
  Sqr(N-K)
• At some small value of N3 K1, trace the loop
  execution (by hand or with the debugger)

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Common Loop Errors Too Narrowly Defined
Condition

• Example
• WHILE Balance ltgt 0.0 DO
• Update (Balance)
• BETTER
• WHILE Balance gt 0.0 DO
• Update (Balance)
• Don't use inequality for testing conditions,
  especially with numbers.
• If Balance goes from negative to positive value
  without having 0.0 as a value we will get an
  infinite loop.
• When using sentinel, make sure that its value
  can't be confused with normal data item.

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Common Loop Errors Not Updating the Loop Control
Variable

• If a loop body have more than one statement,
  don't forget the BEGIN-END brackets.
• Only REPEAT-UNTIL loop doesn't need a begin-end
  brackets.
• Example (infinite loop)WHILE Power lt 10000 DO
• writeln('Next power of N is ', Power 6)
• Power Power N
• Don't use the REPEAT-UNTIL loop if you aren't
  sure that the loop will have to be executed at
  least once.
• In a FOR loop if the starting value is greater
  (for TO) or smaller than (for DOWNTO) the
  statement will not execute.

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Common Loop Errors

• Counter variable in a FOR loop should not be
  changed inside the loop body.
• Example what happens if you do(RECTBAD1.PAS)
• It is illegal to use the same counter variable in
  two nested FOR loops.
• Example what happens if you do(RECTBAD2.PAS)
• Unfortunately the Turbo Pascal environment
  doesnt check for you

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Locating Runtime Errors with PRINT Statements

• Examine program output to determine which part of
  the program is generating incorrect results
• Insert extra debugging statements (PRINT
  statements) to display intermediate results at
  different points in your program
• HOW? Insert extra writeln statements to trace the
  values of certain critical variables during
  program execution
• In order to not add any problems be careful that
  you insert extra writeln statement check if you
  need to add BEGIN-END brackets
• Focus on statements in that section to determine
  which are at fault
• When you locate the error, enclose the diagnostic
  statement with comment braces

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The Debugger

• Helps you execute your program in a controlled
  fashion to help you find errors
• Stepping through program
• Seeing output
• Watching the value of variables
• Running from breakpoint to breakpoint
• Prints diagnostics when run-time error occurs
• Indicates the statement that caused the error and
  displays the values of the selected variables

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Stepped Program Execution

• Run Menu commands
• F7 Trace into2
• F8 Step over
• F4 Go to cursor
• Run to through run to the end
• Usually used in conjunction with output or
  variable watching
• NOTE F7 and F8 are hard to distinguish sometimes

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Watching Output During Execution

• Output command (Debug Menu)
• Opens output window at same time as program
  window
• Allows seeing program running with tracing
  commands (F4, F7, F8)
• Shows output as control moves through program

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Watching Variables Expressions

• Watch
• Can select several values whose values will
  automatically be displayed after each statement
  execution.
• Can type in an expression to be evaluated based
  on variables in program

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Executing with Breakpoints

• Causes the program to stop at selected statements
• You can set several breakpoints at a program. The
  program will execute directly from one breakpoint
  to the next
• Good for large programs (when dont want to use
  F7, F8 or even F4).
• Signaled in Red/Brown then in blue-green when
  you get there.
• Run from breakpoint to breakpoint

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Executing with Breakpoints cont

• Can add a conditional breakpoint
• If you do, make sure you remove unconditional
  breakpoints at the same place.
• Breakpoints Delete

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Evaluating and Modifying Expressions