Ch 5 P 163

flapjack · April 21st, 2010, 04:34 PM

Hi Imar,

I'm having a little problem understanding the use of the ! operator in the following expression...

bool success = false;
while (!success)
{
success = SendEmailMessage();
}

My understanding is that the While loop is used to repeat a statement while a condition is true.

Therefore if the SendEmailMessage(); method fails it will return false so success will be false. When success is tested in the While loop it will be set to true and therefore will drop out of the loop.

Ahhh this is doing my head in! Could you step me through this please. I know that I am not grasping something that is fundamental.

Thank you

Bill

jack_hilary · April 23rd, 2010, 06:13 AM

Hello there,
Well the concept is very simple and sweet. Let me explain it.

Quote:

My understanding is that the While loop is used to repeat a statement while a condition is true.

Yes you are right. While loop continues until condition is false.
So this is the base for the while loop. What actually is going there is that until a message is sent successfully, the code keep trying to send the message again and again.

Quote:

bool success = false;
while (!success)
{
success = SendEmailMessage();
}

now as success is initialized with false (bool variable) so !suceess will become true and hence the while loop will execute for the first time and the function SendEmailMessage() is called.
You must know that a function returns the value where it was called from so SendEmailMessage() function will return the value true if it sends messages successfully otherwise false and will assign it to success variable in loop body.
If messages are sent successfully then it does not make sense to recall the loop so as now the success contains true, it will be inverted and will become false and hence loop will not execute any longer. And if messages sending fails then function will return false and hence success contains false which will be inverted again to true and loop starts executing again and will continue until it sends messages successfully.

Quote:

When success is tested in the While loop it will be set to true and therefore will drop out of the loop.

I think you know now where you were wrong to understand it.
Hope this helps.

flapjack · April 23rd, 2010, 08:45 AM

Many thanks for that Jack. I understand it perfectly now.

Regards Bill

jack_hilary · April 23rd, 2010, 09:55 AM

Hello Bill,
Glad to know it helped you. You may also use the following code.

Code:

bool success = true;
while (success)
{
success = !SendEmailMessage();
}

Both code (this one and the previous one) will work same. And hope this time you are familiar with this one and can easily understand what is going on behind scene.

Cheers,

Jack

Imar · April 23rd, 2010, 11:46 AM

Quote:

Both code (this one and the previous one) will work same.

But the second one is very confusing to read. Don't use in production code please....

Imar

jack_hilary · April 23rd, 2010, 11:56 AM

Quote:

But the second one is very confusing to read. Don't use in production code please....

what makes it confusing and why not to use in production. Please let me know the reason behind scene.

Regards,

Jack

Imar · April 23rd, 2010, 12:02 PM

Success implies you're done.

Code:

 
bool success = true;
while (success)

So while we're having success, we sending a message? And not being able to send a message is a success? And then when we can send the message we call it a failure and exit the loop?

Pretty confusing if you ask me.

Imar

jack_hilary · April 23rd, 2010, 12:12 PM

OK I see.
The way we think it may be really confusing. For the first time we read the code it may completely confuse us that and reason being is really straight forward that you have explained. Really thanks for that.

But the code I wrote just because it is programeticlly correct and implements the same functionality.
Any ways thank you again.

Regards,

Jack

Imar · April 23rd, 2010, 12:31 PM

I know. But writing correct code is not enough. Writing correct and understandable code is what makes you a better prgrammer.

Imar

jack_hilary · April 23rd, 2010, 12:34 PM

Quote:

Writing correct and understandable code is what makes you a better prgrammer.

I do agree with you. Many thanks for that.

Regards,

Jack