Testing and debugging approaches

Software testing: producing quality software

This requires far more than simply showing that the program "works ok" for a few typical input cases.

For all software we want to ensure many things:

• it must correctly handle all the requested functionality for all valid input data (e.g. it must compute the square root correctly, or generate the report correctly, etc)
• it must not generate any unexpected behaviour (i.e. if the user expects it to simply generate a report it should not generate the report then play the programmer's favorite song, tell jokes, or whatever)
• it must respond as specified to all anticipated invalid input data (e.g. respond appropriately if the user enters negative numbers instead of positive values, etc)
• it must have sufficient internal error checking to prevent crashes even if unanticipated invalid data is entered (no segmentation faults, no core dumps, no infinite loops, regardless of what the user enters)

This is a very tall order, particularly for large, complex software projects.

There are, however, a number of typical or predictable sources of error in most programs and a number of steps we can take to improve our code quality.

Developing Test Sets

THE GOAL OF TESTING IS TO REVEAL INADEQUACIES IN SOFTWARE

Don't write "cushy" test sets that you're pretty sure your code can pass - a good test is one that reveals a problem.

The test set developed should include, for each test:

• The exact input data and conditions under which to execute the test, since we want the test to be repeatable any time we modify or update the source code.

• The purpose of the test - every test set should have a reason, something it is meant to reveal. If you do not have a good reason for including a specific test then perhaps it is unnecessary.

• The exact output expected from the software assuming it operates correctly.

When coming up with a test set, there are three major categories to consider:

• Develop one set of tests based purely on the problem description (a.k.a. the program specifications or requirements). This is often termed black box testing, since we treat the program like a black box - we can't see what's inside, we just know what the effects should be (what goes in, what comes out).

• Develop another set of tests based on your knowledge of how the software is implemented - often this gives you greater insight as to where errors are likely to occur. This is often referred to as glass box testing, since it is assumed we can see the internal workings of the software.

Black box testing
In glass box testing we are only interested in testing the functionality of the program: we have the problem description that tells us how a correct solution is supposed to behave, so we establish test data that we think is representative of the possible input conditions the program will ultimately face.

Common black box tests:

• Range tests: if the specifications talk about ranges of values then test the limits of those ranges.

  For example, if the specifications talk about one aspect of the input having values in the range 0..1950, then include tests that, for that input, use values 0, 1, 1949, and 1950.

  As another example, if the specifications talk about handling a list of values (or a sequence of inputs) then include a test that has an empty list, and include a test that has just one element in the list.

• Standard value tests: try one or two "normal" values for each input (e.g. 975 for the 0..1950 example above)

• Out-of-range tests: test the robustness of the program by supplying values out of the described bounds. For example, in the 0..1950 case include tests that use values -1 and 1951.

• Special value tests: there may be some "special" values that could be relevant. For example, in calculating fuel efficiency it seems likely that at some point the calculation "divide distance by fuel consumed" is going to occur - what happens if the amount of fuel consumed is zero, does the program crash with a divide by zero error?

Glass box testing
As covered in the black box section, we can derive many test sets directly from the specifications.

However, it is often useful to also consider the "inside information" we have about the actual structure of the code.

The testing ideas are the same as above, but now there are more data values of interest:

• Loops: can we create test data that forces an infinite loop, or that means the loop is never executed, or is executed exactly once?

• Boolean expressions: for each expression, can we create test data that causes the expression to evaluate to true? can we create test data that causes the expression to evaluate to false?

• Path testing: for every possible sequence or order in which instructions can be executed (ignoring the specific number of times we go through loops) there should be at least one set of test data that causes things to happen in that order.

The program is supposed to read two times, each in the format hh:mm:ss, and output the time difference between the two.

The program should verify the correctness of the input, and it is not permissible to omit leading zeros in the hours segment of the time.

For example, if the times 09:59:59 and 10:01:00 are entered then the program should respond with 00:01:01.

A 24-hour clock is assumed, with times running from 00:00:00 to 23:59:59. If the first time is later than the second time then it is assumed the clock has "wrapped around" once.

For example, if the times 23:00:01 and 01:24:01 are entered then the program should respond with 02:24:00.

Now, suppose we implement the program as follows

#include <cstdio>

void PromptUser();

void ReadTime(int& hours, int& minutes, int& seconds);

void CalcTimeDifference(
             int& hrs_res, int& min_res, int& sec_res,
             int  hrs1,    int  min1,    int  sec1,
             int  hrs2,    int  min2,    int  sec2);

void DisplayTime(int hrs_res, int min_res, int sec_res);

int main()
{
    int Hours1, Minutes1, Seconds1;
    int Hours2, Minutes2, Seconds2;
    int ResultHours, ResultMinutes, ResultSeconds;

    PromptUser();
    ReadTime(Hours1, Minutes1, Seconds1);
    ReadTime(Hours2, Minutes2, Seconds2);

    CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
             Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);

    DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
    return 0;
}

void PromptUser()
{
   printf("This program calculates the difference between two times,\n");
   printf("assuming the two times are within 24 hours of one another\n");
   printf("and are entered in sequential order\n\n");
}

void ReadTime(int& hours, int& minutes, int& seconds)
{
    char ColonChar;
    printf("Please enter a time in the form hh:mm:ss\n");
    printf("assuming a 24-hour clock, and leading zeros must be included\n");
    printf("(i.e. times go from 00:00:00 to 23:59:59)\n");
    scanf("%d", &hours);
    scanf("%c", &ColonChar);
    scanf("%d", &minutes);
    scanf("%c", &ColonChar);
    scanf("%d", &seconds);
}

void CalcTimeDifference(
             int& hrs_res, int& min_res, int& sec_res,
             int  hrs1,    int  min1,    int  sec1,
             int  hrs2,    int  min2,    int  sec2)
{
   sec_res = sec2 - sec1;
   min_res = min2 - min1;
   hrs_res = hrs2 - hrs1;
   if (sec_res < 0) {
      sec_res = sec_res + 60;
      min_res = min_res - 1;
   } 
   if (min_res < 0) {
      min_res = min_res + 60;
      hrs_res = hrs_res - 1;
   }
   if (hrs_res < 0) {
      hrs_res = hrs_res + 24;
   }
}

void DisplayTime(int hrs_res, int min_res, int sec_res)
{
   if (hrs_res < 10) printf("0");
   printf("%d:", hrs_res);
   if (min_res < 10) printf("0");
   printf("%d:", min_res);
   if (sec_res < 10) printf("0");
   printf("%d:", sec_res);
}

• We note that in CalcTimeDifference the resulting (difference) time values (hours, minutes, seconds) are handled differently when they are less than 0 as opposed to when they are greater than or equal to zero.
• We also note that in DisplayTime the resulting (difference) time values (for hours, minutes, and seconds) are handled differently when they are less than 10 as opposed to when they are 10 or greater.
• As a result, we want to ensure our test cases cover hours, minutes, and seconds differences that are:
  • (a) less than 0 (e.g. 10 then 9)
  • (b) 0..9 (e.g. 2 then 11)
  • (c) 10+ (e.g. 22 then 32)
  Furthermore, to get complete path testing we want each combination of those events to be covered at least once, leading to the test combinations

Exception Handling
Exception handling is the process by which we deal with any "unusual circumstances" during execution. Typically this covers things like the entry of incorrect data or failures in other parts of a system we must use (e.g. suppose our program interacts with a database program, and for some reason the database program crashes).

Exception handling on a practical level involves inserting sufficient error checking into our source code to ensure that any possible errors are caught and an appropriate response generated (even if the "response" is just displaying an error message and shutting down the program).

The key to successful exception handling is to assume, in each and every function, that the other functions are not necessarily trustworthy - i.e. don't assume the error checking was adequately done elsewhere.
Example: suppose we have the following function

float divides(float x, float y)
{
   return (x / y);
}

float divides(float x, float y)
{
   if (y != 0) 
      return (x / y);
   else {
      printf("WARNING: DIVIDE BY ZERO\n");
      return 0;
   }
}

#include <cstdio>

void PromptUser();

int TimeCheck(int hours, int minutes, int seconds);

void ReadTime(int& hours, int& minutes, int& seconds);

void CalcTimeDifference(
             int& hrs_res, int& min_res, int& sec_res,
             int  hrs1,    int  min1,    int  sec1,
             int  hrs2,    int  min2,    int  sec2);

void DisplayTime(int hrs_res, int min_res, int sec_res);

int main()
{
    int Hours1, Minutes1, Seconds1;
    int Hours2, Minutes2, Seconds2;
    int ResultHours, ResultMinutes, ResultSeconds;

    PromptUser();
    ReadTime(Hours1, Minutes1, Seconds1);
    ReadTime(Hours2, Minutes2, Seconds2);

    CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
             Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);

    printf("\nThe time difference is: ");
    DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
    return 0;
}

void PromptUser()
{
   printf("\nThis program calculates the difference between two times,\n");
   printf("assuming the two times are within 24 hours of one another\n");
   printf("and are entered in sequential order\n");
}

void ReadTime(int& hours, int& minutes, int& seconds)
{
    char ColonChar;
    printf("\nPlease enter a time in the form hh:mm:ss\n);
    printf("assuming a 24-hour clock, and leading zeros must be included\n");
    printf("(i.e. times go from 00:00:00 to 23:59:59)\n");
    scanf("%d", &hours);
    scanf("%c", &ColonChar);
    scanf("%d", &minutes);
    scanf("%c", &ColonChar);
    scanf("%d", &seconds);
    if (TimeCheck(hours, minutes, seconds) == 0)
    {
       printf("WARNING: invalid time supplied\n");
       ReadTime(hours, minutes, seconds);
    }
}

int TimeCheck(int hours, int minutes, int seconds)
{
    if ((hours < 0) || (hours > 23)) 
       return 0;
    if ((minutes < 0) || (minutes > 59))
       return 0;
    if ((seconds < 0) || (seconds > 59))
       return 0;
    return 1;
}

void CalcTimeDifference(
             int& hrs_res, int& min_res, int& sec_res,
             int  hrs1,    int  min1,    int  sec1,
             int  hrs2,    int  min2,    int  sec2)
{
   if (TimeCheck(hrs1, min1, sec1) == 0)
      return;
   if (TimeCheck(hrs2, min2, sec2) == 0)
      return;
   sec_res = sec2 - sec1;
   min_res = min2 - min1;
   hrs_res = hrs2 - hrs1;
   if (sec_res < 0) {
      sec_res = sec_res + 60;
      min_res = min_res - 1;
   } 
   if (min_res < 0) {
      min_res = min_res + 60;
      hrs_res = hrs_res - 1;
   }
   if (hrs_res < 0) {
      hrs_res = hrs_res + 24;
   }
}

void DisplayTime(int hrs_res, int min_res, int sec_res)
{
   if (TimeCheck(hrs_res, min_res, sec_res) == 0)
   {
      printf("Invalid time supplied, hours=");
      printf("%d, minutes=", hrs_res);
      printf("%d, seconds=", min_res);
      printf("%d\n", sec_res);
      return;
   }
   if (hrs_res < 10) printf("0");
   printf("%d:", hrs_res);
   if (min_res < 10) printf("0");
   printf("%d:", min_res);
   if (sec_res < 10) printf("0");
   printf("%d\n", sec_res);
}

Incremental Development

1. first get a skeleton (outline) solution for the problem
2. get it implemented, running, and tested
3. flesh out one more part of the program
4. get it implemented, running, and tested
5. repeat steps 3 and 4 until the whole thing is done

Suppose we were going to approach the development of our "time difference" program in an incremental fashion. The steps I would advise are:

---

1. Create the main program and "empty" versions of the functions it uses, i.e.:

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       printf("\nThe time difference is: ");
       DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
       return 0;
   }
   
   void PromptUser()
   {
      printf("In PromptUser\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
      printf("In ReadTime\n");
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       printf("In TimeCheck\n");
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      printf("In CalcTimeDifference\n");
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      printf("In DisplayTime\n");
   }
   

   ---

2. Put in a body for DisplayTime, and try it with a test value

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       printf("\nThe time difference is: ");
       ResultHours = 12;
       ResultMinutes = 12;
       ResultSeconds = 12;
       DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
       return 0;
   }
   
   void PromptUser()
   {
      printf("In PromptUser\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
      printf("In ReadTime\n");
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       printf("In TimeCheck\n");
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      printf("In CalcTimeDifference\n");
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      if (hrs_res < 10) printf("0)";
      printf("%d:", hrs_res);
      if (min_res < 10) printf("0)";
      printf("%d:", min_res);
      if (sec_res < 10) printf("0)";
      printf("%d\n", sec_res);
   }
   

   ---

3. Add the body for the prompt routine

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       printf("\nThe time difference is: ");
       ResultHours = 12;
       ResultMinutes = 12;
       ResultSeconds = 12;
       DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
       return 0;
   }
   
   void PromptUser()
   {
      printf("\nThis program calculates the difference between two times,\n");
      printf("assuming the two times are within 24 hours of one another"\n");
      printf("and are entered in sequential order\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
      printf("In ReadTime\n");
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       printf("In TimeCheck\n");
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      printf("In CalcTimeDifference\n");
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      if (hrs_res < 10) printf("0");
      printf("%d:", hrs_res);
      if (min_res < 10) printf("0");
      printf("%d:", min_res);
      if (sec_res < 10) printf("0");
      printf("%d\n", sec_res);
   }
   

   ---

4. Add the body for the ReadTime routine and adjust main a bit to allow testing

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       // editing for testing, just echo hours read in
       ResultHours = Hours1;
       ResultMinutes = Minutes1;
       ResultSeconds = Seconds1;
   
       printf("\nThe time difference is: ");
       DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
       return 0;
   }
   
   void PromptUser()
   {
      printf("\nThis program calculates the difference between two times,\n");
      printf("assuming the two times are within 24 hours of one another\n");
      printf("and are entered in sequential order\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
       char ColonChar;
       printf("\nPlease enter a time in the form hh:mm:ss\n");
       printf("assuming a 24-hour clock, and leading zeros must be included\n");
       printf("(i.e. times go from 00:00:00 to 23:59:59)\n");
       scanf("%d", &hours);
       scanf("%c", &ColonChar);
       scanf("%d", &minutes);
       scanf("%c", &ColonChar);
       scanf("%d", &seconds);
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       printf("In TimeCheck\n");
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      printf("In CalcTimeDifference\n");
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      if (hrs_res < 10) printf("0");
      printf("%d:", hrs_res);
      if (min_res < 10) printf("0");
      printf("%d:", min_res);
      if (sec_res < 10) printf("0");
      printf("%d\n", sec_res);
   }
   

   ---

5. Add the routine to test data validity, and try it with the input/output routines

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       // Again, adjusted just for checking I/O, 
       //    still not really calculating time differences
       printf("\nThe time difference is: ");
       DisplayTime(Hours1, Minutes1, Seconds1);
       DisplayTime(Hours2, Minutes2, Seconds2);
       return 0;
   }
   
   void PromptUser()
   {
      printf("This program calculates the difference between two times,\n");
      printf("assuming the two times are within 24 hours of one another\n");
      printf("and are entered in sequential order\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
       char ColonChar;
       printf("Please enter a time in the form hh:mm:ss\n");
       printf("assuming a 24-hour clock, and leading zeros must be included\n");
       printf("(i.e. times go from 00:00:00 to 23:59:59)\n");
       scanf("%d", &hours);
       scanf("%c", &ColonChar);
       scanf("%d", &minutes);
       scanf("%c", &ColonChar);
       scanf("%d", &seconds);
       if (TimeCheck(hours, minutes, seconds) == 0)
       {
          printf("WARNING: invalid time supplied"\n");
          ReadTime(hours, minutes, seconds);
       }
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       if ((hours < 0) || (hours > 23)) 
          return 0;
       if ((minutes < 0) || (minutes > 59))
          return 0;
       if ((seconds < 0) || (seconds > 59))
          return 0;
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      printf("In CalcTimeDifference\n");
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      if (TimeCheck(hrs_res, min_res, sec_res) == 0)
      {
         printf("Invalid time supplied, hours=");
         printf("%d, minutes=", hrs_res);
         printf("%d, seconds=", min_res);
         printf("%d\n", sec_res);
         return;
      }
      if (hrs_res < 10) printf("0");
      printf("%d:", hrs_res);
      if (min_res < 10) printf("0");
      printf("%d:", min_res);
      if (sec_res < 10) printf("0");
      printf("%d\n", sec_res);
   }
   

   ---

6. Add the basic computations for calculating differences

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       printf("\nThe time difference is: ");
       DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
       return 0;
   }
   
   void PromptUser()
   {
      printf("This program calculates the difference between two times,\n");
      printf("assuming the two times are within 24 hours of one another\n");
      printf("and are entered in sequential order\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
       char ColonChar;
       printf("\nPlease enter a time in the form hh:mm:ss\n");
       printf("assuming a 24-hour clock, and leading zeros must be included\n");
       printf("(i.e. times go from 00:00:00 to 23:59:59)\n");
       scanf("%d", &hours);
       scanf("%c", &ColonChar);
       scanf("%d", &minutes);
       scanf("%c", &ColonChar);
       scanf("%d", &seconds);
       if (TimeCheck(hours, minutes, seconds) == 0)
       {
          printf("WARNING: invalid time supplied\n");
          ReadTime(hours, minutes, seconds);
       }
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       if ((hours < 0) || (hours > 23)) 
          return 0;
       if ((minutes < 0) || (minutes > 59))
          return 0;
       if ((seconds < 0) || (seconds > 59))
          return 0;
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      sec_res = sec2 - sec1;
      min_res = min2 - min1;
      hrs_res = hrs2 - hrs1;
      if (sec_res < 0) {
         sec_res = sec_res + 60;
         min_res = min_res - 1;
      } 
      if (min_res < 0) {
         min_res = min_res + 60;
         hrs_res = hrs_res - 1;
      }
      if (hrs_res < 0) {
         hrs_res = hrs_res + 24;
      }
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      if (TimeCheck(hrs_res, min_res, sec_res) == 0)
      {
         printf("Invalid time supplied, hours=");
         printf("%d, minutes=", hrs_res);
         printf("%d, seconds=", min_res);
         printf("%d\n=", sec_res);
         return;
      }
      if (hrs_res < 10) printf("0");
      printf("%d:", hrs_res);
      if (min_res < 10) printf("0");
      printf("%d:", min_res);
      if (sec_res < 10) printf("0");
      printf("%d\n", sec_res);
   }
   

   ---

7. Add the error checking for calculating time differences

   #include <cstdio>
   
   void PromptUser();
   
   int TimeCheck(int hours, int minutes, int seconds);
   
   void ReadTime(int& hours, int& minutes, int& seconds);
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2);
   
   void DisplayTime(int hrs_res, int min_res, int sec_res);
   
   int main()
   {
       int Hours1, Minutes1, Seconds1;
       int Hours2, Minutes2, Seconds2;
       int ResultHours, ResultMinutes, ResultSeconds;
   
       PromptUser();
       ReadTime(Hours1, Minutes1, Seconds1);
       ReadTime(Hours2, Minutes2, Seconds2);
   
       CalcTimeDifference( ResultHours, ResultMinutes, ResultSeconds,
                Hours1, Minutes1, Seconds1, Hours2, Minutes2, Seconds2);
   
       printf("\nThe time difference is: ");
       DisplayTime(ResultHours, ResultMinutes, ResultSeconds);
       return 0;
   }
   
   void PromptUser()
   {
      printf("This program calculates the difference between two times,\n");
      printf("assuming the two times are within 24 hours of one another\n");
      printf("and are entered in sequential order\n");
   }
   
   void ReadTime(int& hours, int& minutes, int& seconds)
   {
       char ColonChar;
       printf("Please enter a time in the form hh:mm:ss\n");
       printf("assuming a 24-hour clock, and leading zeros must be included\n");
       printf("(i.e. times go from 00:00:00 to 23:59:59)\n");
       scanf("%d", &hours);
       scanf("%c", &ColonChar);
       scanf("%d", &minutes);
       scanf("%c", &ColonChar);
       scanf("%d", &seconds);
       if (TimeCheck(hours, minutes, seconds) == 0)
       {
          printf("WARNING: invalid time supplied\n");
          ReadTime(hours, minutes, seconds);
       }
   }
   
   int TimeCheck(int hours, int minutes, int seconds)
   {
       if ((hours < 0) || (hours > 23)) 
          return 0;
       if ((minutes < 0) || (minutes > 59))
          return 0;
       if ((seconds < 0) || (seconds > 59))
          return 0;
       return 1;
   }
   
   void CalcTimeDifference(
                int& hrs_res, int& min_res, int& sec_res,
                int  hrs1,    int  min1,    int  sec1,
                int  hrs2,    int  min2,    int  sec2)
   {
      if (TimeCheck(hrs1, min1, sec1) == 0)
         return;
      if (TimeCheck(hrs2, min2, sec2) == 0)
         return;
      sec_res = sec2 - sec1;
      min_res = min2 - min1;
      hrs_res = hrs2 - hrs1;
      if (sec_res < 0) {
         sec_res = sec_res + 60;
         min_res = min_res - 1;
      } 
      if (min_res < 0) {
         min_res = min_res + 60;
         hrs_res = hrs_res - 1;
      }
      if (hrs_res < 0) {
         hrs_res = hrs_res + 24;
      }
   }
   
   void DisplayTime(int hrs_res, int min_res, int sec_res)
   {
      if (TimeCheck(hrs_res, min_res, sec_res) == 0)
      {
         printf("Invalid time supplied, hours=");
         printf("%d, minutes=", hrs_res);
         printf("%d, seconds=", min_res);
         printf("%d\n", sec_res);
         return;
      }
      if (hrs_res < 10) printf("0");
      printf("%d:", hrs_res);
      if (min_res < 10) printf("0");
      printf("%d:", min_res);
      if (sec_res < 10) printf("0");
      printf("%d\n", sec_res);
   }
   

   ---

The debugging process

If you have thought out your test plan as per the discussion above, and you're building your software in an incremental fashion, then you already have an excellent start on debugging: you have a focus on which segment of the source code is (most likely) the source of any current problems, and you have a rationale for each test set that gives you a basis for investigating further.

One of the key goals in debugging is to constantly narrow the sources of your current error - visual inspection of the code may not be sufficient. Try to identify new test values you can use to precisely determine when and why your program crashes, hangs, or simply gives incorrect data.

Debugging software is helpful, but often is not enough to track down the cause of a problem unless you also make careful (and sometimes creative) deductions about what your program is doing and why.

In addition to fuse, we also have available a debugger named gdb that can be run from a terminal window. To invoke this debugger, follow the steps below (example is shown assuming pex2.C is the source code file):

g++ -g pex2.C -o pex2
gdb pex2

(gdb)

• run the program until it hits a certain point in the program then stop and wait for further commands
• step through program execution one command at a time
• examine or alter variable values
• etc