Help with programming in C++ - Page 3

LastWish

2013 Posts

October 10 2010 15:15 GMT

#41

On October 08 2010 06:33 Adeny wrote:
This works fine...


int x = 0;
cin >> x;
if (cin.good() > 0)
{
    cout << "user input is: " << x << endl;
}
else
{
    cout << "user input is bad" << endl;
}

LOLOL DISREGARD THIS ENTIRE MESSAGE

+ Show Spoiler +

Ok, let's validate the input so that only digits between 0 and 9 will be accepted. If you use cin >> on an integer, your program will blow up if someone starts typing anything but strictly numbers. Because of that we have to jump hoops...

HUH, DISREGARD THIS LOL. It seems if you cin >> to an int, with characters and nonsense, the int stays 0... God damnit it.
+ Show Spoiler +


#include <string>
#include <iostream>
#include <conio.h>

using namespace std;

int main()
{
	cout << "Input number or w/e\n";
	string input;
	cin >> input;
	for (int x = 0; x < input.size(); x++)
	{
		if ( input[x] < 48 && input[x] > 57 )
		{
			cout << "Only 0-9";
			break;
		}
	}

	int validated = atoi(input.c_str());
	if (validated > 0)
        {
		cout << validated;
        }
        else
        {
            cout << "Number must be over 0\n";
        }

	getch();
	return 0;
}

What it does is take the users input, places it in to a string. Now it dissects the string, and checks each character seperately if its ASCII value is above or below 48-57, which is 0-9 on an ASCII table (ASCII Chart). Then convers the string to an integer using atoi(), and finally checks if the ints value is above 0.

Chances are though, your professor doesn't expect that of you and just poorly worded the assignment, maybe he wants you to assume that a user will always input numbers, in which case simply doing the following should work:

+ Show Spoiler +


int input;
cin >> input;
if (input > 0)
{
	// do whatever here
}
else
{
	// tell the user his input was bad.
}

Also god damn conversions in C++, jeez.

Seems to me you need some help aswell.

You can use :


if(cin.fail()){
   // print bad stuff
   return -1;
}

Alternatively you can set cin.exceptions(istream::failbit) and catch istream::failure exception.


if (input[x] < 48 && input[x] > 57 )

You don't have to use 48 or 57 but :


if (input[x] < '0' && input[x] > '9' )

does the same.

Edit : Oh ok, you have edited the former message..

Adeny

Norway1233 Posts

October 10 2010 16:52 GMT

#42

^Yeah, it's been a while since I C++'d, the manual method is probably from C I dunno, lately trying to learn C# has left my head spinning, too many C's.

KiLL_ORdeR

United States1518 Posts

October 10 2010 17:02 GMT

#43

On October 09 2010 09:11 Spazer wrote:
My god Wascana Eclipse is bloated. Dev C++ is 9MB compared to this 150MB monster.

Anyway, I thought I'd show you how to debug simple programs. I'm gonna use your ocean levels program as an example and go over your mistake in there at the same time.

+ Show Spoiler [Warning: really long] +

To begin with, your screen should look something like this:

Click on the text editor window (the window with all your code in it). Now go to Run > Debug As > Local C/C++ Application

This message box might come up. Choose yes to change perspectives.
[image loading]

Your screen should now look like this:

Here's an overview of the various important screens:

This kinda tells you what the system's doing. Right now it's saying that the program is suspended at a breakpoint. I'll explain this in a minute. You can probably ignore this for the most part.
This is the variables screen. It tells you about variables are used in the program as they are declared and changed.
This is your program. Note the line I've circled, and how there's a tiny blue arrow on the left hand side. This means the program is currently stopped at that line.
This is the console. It will show you the output of your program as it would normally appear in the command prompt. You've probably used this section before.

So here's what debug mode does: it stops your program at certain locations called breakpoints, and allows you to step through your program one line at a time. This way, you can observe the changes in your variables and locate any problems easier. Normally, you'd be able to set breakpoints wherever you'd like in your source file, but I can't figure out how to properly use this stupid program. Even the online help wasn't any use. -_-

When you enter debug mode using the steps I've described above, the breakpoint is automatically set at the first line of the program. Thus, your program is currently stopped at the line int years = 0;
Look at the variables window. Notice how years has shown up because it has been declared, but that the value is all screwed up. This is because the value has not been assigned to the variable.

Let's hit step into to advance the code one line:
[image loading]

Notice that the little blue arrow has gone down one line. Also notice that the variable years now has a value of 0.

I'm gonna keep going.

Now we're at the loop. Take a look at the variable screen. years is 0, which is less than or equal to 25, so we enter the loop.

Stepping further...

Look at the console and notice that we now have output. The reason why the first two cout statements didn't show up before this is that text is sometimes stored in the buffer instead of immediately being displayed. Suffice it to say that this is nothing to worry about.

Anyway, this is the part where your program screws up. This line:

cout << years << "\t\t" << (years + 1.5) <<endl;

Since years is 0, (years +1.5) gives you an output of 1.5 instead of 0.
As others have mentioned in this thread, having (years * 1.5) would fix this.

Strictly speaking, the expression (years +1.5) is bad since type int isn't meant to contain decimal values. However, the program is smart enough to typecast the integer as a float or double to compensate for this.

Stepping some more...

years has now incremented, as you can see.
The while loop has also ended, so we're now reevaluating the condition right now. years is still under 25, so we'll end up entering the loop again.

Stepping more...

Here we see that the output for 1 year is 2.5 instead of 1.5
Again, using the expression (years * 1.5) would solve this problem, giving an output of 1.5

Hit resume (outlined in green) to make the program run to the end without further stoppages. Hit terminate (outlined in red) to make the program stop running completely (means no more stepping) at its current location.
[image loading]

So that's a basic overview of how to debug your programs. Using debug mode will give you a better understanding of what your program is doing at every step, and help you locate those nasty bugs. It's a really nice feature you should learn to use.

Edit: By the way, I really hate Wascana Eclipse. It's not very intuitive at all. Given a choice, I'd use another development environment, but you guys might not get a choice in that. =/

Wow man, just wow.

We went over the debugging process for about half of one 50 minute class period, and haven't touched it since. This is really, really informative though, i'm really glad that you went through the effort of doing this, thank you very much.

We don't get a choice of which program to use. There is a list of programs we can use on the course webpage, but most of them cost money, and Wascana is free, and what we use in class

Spazer

Canada8031 Posts

October 10 2010 17:19 GMT

#44

On October 11 2010 02:02 KiLL_ORdeR wrote:

Show nested quote +

To begin with, your screen should look something like this:

Click on the text editor window (the window with all your code in it). Now go to Run > Debug As > Local C/C++ Application

This message box might come up. Choose yes to change perspectives.
[image loading]

Your screen should now look like this:

Here's an overview of the various important screens:

This kinda tells you what the system's doing. Right now it's saying that the program is suspended at a breakpoint. I'll explain this in a minute. You can probably ignore this for the most part.
This is the variables screen. It tells you about variables are used in the program as they are declared and changed.
This is your program. Note the line I've circled, and how there's a tiny blue arrow on the left hand side. This means the program is currently stopped at that line.
This is the console. It will show you the output of your program as it would normally appear in the command prompt. You've probably used this section before.

Notice that the little blue arrow has gone down one line. Also notice that the variable years now has a value of 0.

I'm gonna keep going.

Now we're at the loop. Take a look at the variable screen. years is 0, which is less than or equal to 25, so we enter the loop.

Stepping further...

cout << years << "\t\t" << (years + 1.5) <<endl;

For whatever reason, programming classes usually don't teach you how to use the debugging features. I don't know why - it's just so useful. Debugging tells you so much about how your program operates that I don't know how I got by without it before. Anyway, enjoy.

TossFloss

Canada606 Posts

October 12 2010 05:29 GMT

#45

On October 08 2010 12:29 Tenrou wrote:
My advice is to read your book and every time you see something new, create a test program to use the new code. Check to see what it does and can't do.

Most textbooks are trash. You should read a book but make sure it's a good one.

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