Debugging Logic Errors

CPS120Introduction to Computer ScienceLecture 6

Compiling and Debugging 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 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 A semantic error is a violation of the rules of

meaning of a programming language E.g. My refrigerator just drove a car to Chicago Overt logic errors

Something is obviously wrong even though the program is running

Convert logic errors Not so obvious something is wrong

Run things various ways to highlight these errors

It Did What?? 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 The syntax rules of C++ have been

correctly followed, but the meaning of the code is incorrect

Approaches to Correction Desk-checking Inserting Tracing Statements

Used when program "crashes" Runs to completion with incorrect

output Using an interactive debugger

Common Semantic Errors1. Infinite Loop

Created when a loop in which the expression tested never becomes false

2. Misunderstanding operator precedence3. Dangling else4. Off-By-One Error

Loop that iterates one fewer or one more than is correct

5. Code inside a loop that doesn’t belong there6. Not using a compound statement when one is

required7. Array index bounds error

Infinite Loopchar 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;

Division has a higher precedence than subtraction

Should be

milesPerGallon = (endMileage –startMileage) / gallonsUsed;

Dangling ELSEIf (relative)

if (my friend)

cout << “both“;

else

cout << “neither”; 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 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 Loopfor (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

char ch;infile.get (ch);while (infile)

cout<< ch;infile.get (ch);

Should be{cout << ch;infile.get (ch);}

Array Index Bounds Errorconst int NUM_DAYS = 7;

int temperatures[NUM_DAYS];

int count;

for (count =1;count <= NUM_DAYS; count++

{

cout << Enter a temperature: “;

cin >> temperatures [count];

} 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 Inserting print statements to display the

current status of critical variables, i.e. those being modified

//Debug trace statementcout << 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

1. What to display Variables modified in the block of code

2. Where to place the displays At logical breakpoints

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 Stops a program midstream during

debugging Cannot resume Cannot examine variables

Allows you to stop a program when a specific condition occurs Multiple asserts can be used in the same code

assert (<expression>); 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 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 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 Discover critical points by stopping

execution and taking a snapshot of the program's activity Resume activity

Check values of critical variables Observe the 'flow of control '

Run one statement at a time or in groups Order of execution

Dangling ELSEs, missing subroutine calls, infinite loops

The Step Debugging Process 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 This will start the program but only

execute the first line

Debugging View Program Pane – displays source

Has current statement indicator Variables Pane – displays list of

variables that are defined Lower Left

Watch Pane – drag variables and expressions there to see monitor their content Lower Right

Call Chain (Stack trace)

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 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 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 Breaks must be in executable statements

Watching Variables 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 double-clicking on it in the variables pane

Functions There is a pop-up menu for

switching functions above the program pane

Arrays, Structs and Classes A + next to a name in the variable

pane will show all the values of a structured variable

Preferences Alterable from the TOOLS /

OPTIONS menu

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

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