Transcript Debugging Logic Errors - Washtenaw Community College

Debugging Logic Errors
CPS120
Introduction to Computer Science
Lecture 6
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; misuse of structure and
form in programming or a violation of the
compiler’s rules. 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
Semantic Errors
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A semantic error is a violation of the rules of
meaning of a programming language
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E.g. My refrigerator just drove a car to Chicago
Overt logic errors
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Something is obviously wrong even though the program
is running
Convert logic errors
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Not so obvious something is wrong
Run things various ways to highlight these errors
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.
3.
4.
Misunderstanding operator precedence
Dangling else
Off-By-One Error
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5.
6.
7.
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
Not using a compound statement when one is
required
Array index bounds error
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
cont int NUM_VALUES = 50;
int lcv,someValue, total=o;
for (lcv=1; lcv < NUM_VLUES; 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;
}
Missing Compound Statement
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char ch;
infile.get (ch);
while (infile)
cout<< ch;
infile.get (ch);
Should be
{
cout << ch;
infile.get (ch);
}
Array Index Bounds Error
const int NUM_DAYS = 7;
int temperatures[NUM_DAYS];
int count;
for (count =1;count <= NUM_DAYS; count++
{
cout << Enter a temperature: “;
cin >> temperatures [count];
}
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This code contains a fundamental error – the array’s correct indices range
from 0 through 6, while the array indices used in the for loop range from
1 to 7
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
ASSERT Macro
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Stops a program midstream during debugging
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Allows you to stop a program when a specific
condition occurs
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Cannot resume
Cannot examine variables
Multiple asserts can be used in the same code
assert (<expression>);
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If the expression is true (has a non-zero value),
the program continues. If false (has a zero value),
execution terminates with an error message that
the assert failed
Requires: #include <cassert.h1>
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
The Step Debugging Process
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Start out in the Visual C++ editor
window
Click on BUILD, SET ACTIVE
CONFIGURATIONS, WIN 32 DEBUG
Click on the BUILD menu, START
DEBUG and STEP INTO your program
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This will start the program but only
execute the first line
Debugging View
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Program Pane – displays source
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Variables Pane – displays list of variables that
are defined
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Lower Left
Watch Pane – drag variables and expressions
there to see monitor their content
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Has current statement indicator
Lower Right
Call Chain (Stack trace)
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
Functions
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There is a pop-up menu for switching
functions above the program pane
Arrays, Structs and Classes
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A + next to a name in the variable
pane will show all the values of a
structured variable
Preferences
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Alterable from the TOOLS / OPTIONS
menu
Program Testing
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Sampling: Testing some of the functions
of software and making inferences on
the rest of the functionality based on
these observations
Population Testing: Testing every
possible option before software is
considered complete
Step Debugger

One of he bet 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



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
The Step Debugging Process



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
Start out in the Visual C++ editor window
Click on the BUILD menu, START DEBUG and
STEP INTO your program

This will start the program but only execute the
first line
Stepping Options

In the DEBUG window, you have several
options:





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 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

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



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
Watching Variables


Use the WATCH window pane in the
lower right corner of the debugger
screen to check the value of a variable
A variable can also be verified at a
breakpoint by using QUICKWATCH or
you can add it to the WATCH pane
Program Testing


Sampling: Testing some of the functions
of software and making inferences on
the rest of the functionality based on
these observations
Population Testing: Testing every
possible option before software is
considered complete