Transcript Debugging Logic Errors - Washtenaw Community College

Debugging Logic Errors
CPS120
Introduction to Computer Science
Compiling and Debugging
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Executable code will not be created
until you correct all of the syntax errors
in your source code
Then the fun (with logic errors) begins
Syntax & Logic Errors
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A syntax error is simply the violation of
the rules of a language
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These errors are detected by the compiler
A logic error is a mistake that complies
with the rules of the compiler that
causes the program to generate
incorrect output
It Did What??
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If the data is good and a program does
not do what it is supposed to do, there
must be at least one logic error present
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The syntax rules of C++ have been
correctly followed, but the meaning of the
code is incorrect
Approaches to Correction
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Desk-checking
Inserting Tracing Statements
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Used when program "crashes"
Runs to completion with incorrect output
Using an interactive debugger
Common Semantic Errors
1.
Infinite Loop
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2.
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Misunderstanding operator precedence
Dangling else
Off-By-One Error
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5.
Created when a loop in which the expression
tested never becomes false
Loop that iterates one fewer or one more than is
correct
Code inside a loop that doesn’t belong there
Infinite Loop
char response;
cout << “Please enter (y)es or (n)o ->”;
cin >> response;
while ((response !=‘y’)||(response
!=‘n’))
{
cout << “Please try again. Enter (y)es or
(n)o ->’;
Misunderstanding Operator Precedence
milesPerGallon = endMileage –
startMileage /gallonsUsed;
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Division has a higher precedence than
subtraction
Should be
milesPerGallon = (endMileage –
startMileage) / gallonsUsed;
Dangling ELSE
If (relative)
if (my friend)
cout << “both“;
else
cout << “neither”;
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When this code is run, it prints “both” correctly when both
bool variables are true
If both are false, nothing prints
If relative is true but friend is false, it prints neither
Off-by-one Error
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A loop iterates one fewer or one more than is
correct
const int NUM_VALUES = 50;
int lcv, someValue, total=0;
for (lcv=1; lcv < NUM_VALUES; lcv++)
{
cout << “Enter an integer ->”;
cin >> someValue;
total = total + someValue;
}
Bad Code Inside a Loop
for (lcv = 1; lcv <= Num_VALUES; lcv++)
{
cout <<“Enter an integer ->”;
cin >> someValue;
total = total + somevalue;
average = total / NUM_VALUES;
cout << "Total is: " << total << endl;
cout << " Average is: " << average;
}
Tracing Statements
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Inserting print statements to display the
current status of critical variables, i.e. those
being modified
//Debug trace statement
cout << endl << "Location 1:" << endl
<< "Highest values is:" << highest_value
<< "Lowest value is:" << lowest_value << endl
<< "Sum of positives is:" << sum_pos_values
<< "Sum of negatives is:" << sum_neg_values << endl
<< "Mean of positives is:" << mean_pos_values
<< "Mean of negatives is:" << mean_neg_values
<< endl << endl;
Tracing Decisions
What to display
1.
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Variables modified in the block of code
Where to place the displays
2.
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At logical breakpoints
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Before and after IF…ELSE
Before and after LOOPS and when counters
are incremented
At the end of 15 or 20 sequential statements
Interactive (Step) Debugger
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One of he best tools for uncovering logic
errors and for testing to make sure the
problems are non-existent is the step
debugger in the Visual C++ programming
environment
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Allows programmers to execute their code one line
at a time
Allows you to look at the values of variables
during the pause
Will not help with syntax problems, since it
requires an executable to work
Interactive Debugging
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Discover critical points by stopping execution
and taking a snapshot of the program's
activity
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Resume activity
Check values of critical variables
Observe the 'flow of control '
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Run one statement at a time or in groups
Order of execution
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Dangling ELSEs, missing subroutine calls, infinite loops
Stepping Options
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In the DEBUG window, you have several
options:
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GO (F5): Use this option when you have set some
breakpoints in the program and you want to get to them
or between them quickly
STEP COMMANDS: Executes one statement.
STEP INTO (F11): Tells the program to enter a function
or other command instead of just processing
STEP OVER (F10): Won’t keep a line from executing, just
steps over it
STEP OUT (Shift-F11): Returns control to the portion of
the code that called a function
RUN TO CURSOR (CNTL-F10): Will run the program to
wherever the programmer places the cursor in a program
Inserting & Removing Breakpoints
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Before you use the step debugger, you may
want to review the source code and try to
determine the general area where errors are
occurring. This is done by setting breakpoints
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This allows you to get to interesting sections of
code
Breakpoints have no impact on your program,
other than how it executes
Click on the "hand" (breakpoint) button to set
or remove breakpoints
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Breaks must be in executable statements
Watching Variables
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Use the WATCH window pane in the lower
right corner of the debugger screen to check
the value of a variable
Right click and check properties to see
variable types
A variable can also be verified at a breakpoint
by using QUICKWATCH or you can add it to
the WATCH pane
Change the value of a variable by doubleclicking on it in the variables pane