This originally was in a text file, gdb.txt. It was moved into an HTML document so comments could be embedded in a red font.

The code is given below (but can also be seen in gdb by using the list command).

// Swap two char variable values
// allowing for repeats
// Example for demo of gdb
//JWB
#include <iostream>
using namespace std;
int main () {
int i;
char a,b, more;
do {
cout << "Input character values for a and b : ";
cin >> a;
cout << "\nOriginally a is " << a << " and b is " << b << endl;

//do swap
a = b;
b = a;

//output results
cout << "\nAfter the swap a is now " << a << " and b is now " << b << endl;

cout << "Input c to continue; s to stop : ";
cin >> more;
cout << endl;
}
while (more = 'c');
}

Originally a is A and b is ¿ Oops- b doesn't seem to have a value

After the swap a is now ¿ and b is now ¿ Value of a seems lost
Input c to continue; s to stop : Program doesn't wait for input here

Input character values for a and b : X Y Note- it loops without any input
Originally a is X and b is ¿ Same problems occur

After the swap a is now ¿ and b is now ¿
Input c to continue; s to stop :
Input character values for a and b :
Program received signal SIGINT, Interrupt. CTRL-C to abort the run
0x006db410 in __kernel_vsyscall ()
(gdb) l List the lines at the beginning
1 // Swap two char variable values
2 // allowing for repeats
3 // Example for demo of gdb
4 //JWB
5 #include <iostream>
6 using namespace std;
7 int main () {
8 int i;
9 char a,b, more;
10 do {
11 cout << "Input character values for a and b : ";

(gdb) l List more of the program
12 cin >> a; Oops- looks as if b is not input

13 cout << "\nOriginally a is " << a << " and b is " << b << endl; Break here
14 to examine a and b
15 //do swap
16 a = b;
17 b = a;
18
19 //output results
20 cout << "\nAfter the swap a is now " << a << " and b is now " << b << endl;
21
(gdb) b 13 Set a break at line 13
Breakpoint 1 at 0x80487f0: file swap.cpp, line 13.
(gdb) r Redo the run
The program being debugged has been started already.
Start it from the beginning? (y or n) y

Starting program: /users/cs/faculty/jbaker/cs1-alt/swap
Input character values for a and b : AB

Breakpoint 1, main () at swap.cpp:13
13 cout << "\nOriginally a is " << a << " and b is " << b << endl;
(gdb) p a Look at value for a-it's ok
$1 = 65 'A'
(gdb) p b Look at value for b-
$2 = -65 '¿' it's incorrect
(gdb) p b='B' Ignore input and set b
$3 = 66 'B'
(gdb) c
Continuing. Continue the run with the new value
Originally a is A and b is B Now this output is correct
After the swap a is now B and b is now B OOps-swapping code must be wrong
Input c to continue; s to stop : This line is not excuting
Input character values for a and b :
Program received signal SIGINT, Interrupt.
0x00829410 in __kernel_vsyscall () Abort
(gdb) l
8 int i;
9 char a,b, more;
10 do {
11 cout << "Input character values for a and b : ";
12 cin >> a;
13 cout << "\nOriginally a is " << a << " and b is " << b << endl;
14
15 //do swap

16 a = b; Tracing this by hand we see a is set to b's value

17 b = a; B and b is set to a's value which is B
We see a is set to B and b is set to a's value which is B
(gdb) l
18
19 //output results
20 cout << "\nAfter the swap a is now " << a << " and b is now " << b << endl;
21
22 cout << "Input c to continue; s to stop : ";
23 cin >> more;
24 cout << endl;
25 }
26 while (more = 'c');
27 }
(gdb) b 24
(gdb) clear 13
Deleted breakpoint 1
(gdb) b 24 Set a breakpoint at line 24 to see why the loop is not handled correctly
Breakpoint 2 at 0x80488d3: file swap.cpp, line 24.
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y

Starting program: /users/cs/faculty/jbaker/cs1-alt/swap
Input character values for a and b : AB

Originally a is A and b is ¿

After the swap a is now ¿ and b is now ¿
Input c to continue; s to stop :
Breakpoint 2, main () at swap.cpp:24
24 cout << endl;
(gdb) p more Check value of more
$4 = 66 'B' This was the value for b which was not stored earlier in the program
(gdb) p more='c' Set more to 'c'
(gdb) c
Continuing.

Input character values for a and b : So, it looks as if the loop is ok- or is it?
Program received signal SIGINT, Interrupt.
0x00293410 in __kernel_vsyscall ()
(gdb) quit
The program is running. Exit anyway? (y or n) y
[loki.cs.kent.edu]{3}-> exit

Script done on Mon 07 Sep 2009 12:55:39 AM EDT

Observe we have found the following errors:
1) b does not receive a value
2) the swap code needs to be corrected.

By correcting 1) we also correct the failure to ask the user to decide whether or not to continue.

However, we did not try to run when more is 's'. There is an error there as more = 'c' in the bottom while statement causes more to be ASSIGNED the code for 'c' and being nonzero, it is true even when more is 'c'.To correct, change to more == 'c'.

This is a good lesson in realizing that debugging is one of the hardest parts of programming - but also a fun part as you are acting like a detective. Don't let your ego get in the way - everyone has errors in their programs initially.