Calculation is not working for Textbox text change

Question

1.00/5 (2 votes)

See more:

I have a web form with three textboxes on it. Textbox1, Textbox2 and Textbox3. I have a textbox text change on Textbox2 to do a calculation but it is not doing it right. Textbox3 will display the answer. I am trying to get the percentage difference between two numbers. Here is my code:

C#

protected void TextBox2_TextChanged(object sender, EventArgs e)
        {
            int a = Convert.ToInt32(TextBox1.Text);
            int b = Convert.ToInt32(TextBox2.Text);
            TextBox3.Text = Convert.ToString((a - b) / ((a + b) / 2)* 100);
        }

What did I do wrong in the code for this not to work?

Posted 20-Oct-14 5:27am

Computer Wiz99

Updated 20-Oct-14 5:34am

v2

Add a Solution

Comments

PIEBALDconsult 20-Oct-14 11:36am

Try using floating point numbers rather than integers.

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2014-10-20T05:44:00

Solution 2

Percentage difference is

(ABS(difference between A and B) / ((A + B) / 2)) * 100

Which is what your code does...except your code uses integer math.
And since (A - B) will always be smaller than (A + B) for positive numbers, your code will always generate zero.

Change all the values to float or double and it'll work fine.

Posted 20-Oct-14 5:44am

OriginalGriff

Comments

Manfred Rudolf Bihy 20-Oct-14 11:47am

Which is exactly what I said. Anything new in this solution?

OriginalGriff 20-Oct-14 12:04pm

You hadn't posted it when I started typing! :laugh:

Computer Wiz99 20-Oct-14 12:05pm

First thing is that the link I showed you was what I was looking for. If you enter in two numbers on that link you will get back the same answer. Not a negative. Check it out:

Calculate percentage difference
between V1 = 100 and V2 = 1000

( | V1 - V2 | / ((V1 + V2)/2) ) * 100

= ( | 100 - 1000 | / ((100 + 1000)/2) ) * 100
= ( | -900 | / (1100/2) ) * 100
= ( 900 / 550 ) * 100
= 1.636364 * 100

= 163.6364% difference

And if I do it like this:

Calculate percentage difference
between V1 = 1000 and V2 = 100

( | V1 - V2 | / ((V1 + V2)/2) ) * 100

= ( | 1000 - 100 | / ((1000 + 100)/2) ) * 100
= ( | 900 | / (1100/2) ) * 100
= ( 900 / 550 ) * 100
= 1.636364 * 100

= 163.6364% difference

Same answer with no negative.

OriginalGriff 20-Oct-14 12:14pm

Well yes - but your code doesn't take the absolute value of the difference, so any negative you get (and 100 - 1000 will give you a negative) will (rightly) be propagated to the final value.
Use http://msdn.microsoft.com/en-us/library/system.math.abs(v=vs.110).aspx on the result of the subtraction and you will lose the negative throughout.

Manfred Rudolf Bihy 20-Oct-14 11:49am

Also note that OP is using the TextChanged event.
So there are likely to be some conversion exceptions.

Manfred Rudolf Bihy · Answer 2 · 2014-10-20T05:36:00

Considering that a and b are positive numbers then (a - b) will always be less than (a + b). In integer arithmetic an smaller integer divided by a larger integer (compared to one another of course) will always be zero. So if the difference of both integers is smaller than half the sum you will always end up with zero.

Just change your calculation like this:

C#

TextBox3.Text = Convert.ToString((a - b) * 100 / ((a + b) / 2));

or even like this.

TextBox3.Text = Convert.ToString((a - b) * 100.0 / ((a + b) / 2));

Cheers!

[Edit]
I just noticed that you based your calculation on the TextChanged event. That is not appropriate, as this event is being fired for every edit that is made and you're not making sure that every text field is properly filled and you're also not checking that the conversion is done properly. For all I know it may throw exceptions based on what exactly your input was.

C#

    try
    {
        int a = Convert.ToInt32(TextBox1.Text);
        int b = Convert.ToInt32(TextBox2.Text);
        TextBox3.Text = Convert.ToString((a - b) * 100.0 / ((a + b) / 2));
    }
    catch()
    {
        TextBox3.Text = "Conversion failed!";
    }

This would be more appropriate.

[/Edit]