Wrong result if variables are not declared to 0 in calculator - vb.net

Question

everyone. I'm trying to make a basic calculator and as of now I only have the addition, sqrt, and % working. What confuses me is that, if I don't declare the first number to 0 after performing an operation, it gives me the wrong result.
For example:
num2 += Val(displayPanel.Text)
result = num1 + num2
displayPanel.Text = result
num1 = 0
num2 = 0
This certain code will give me correct results while
num2 += Val(displayPanel.Text)
result = num1 + num2
displayPanel.Text = result
will give me the answer '5' if I put 1+1 = 2 + 1. Same goes for my other classes where:
num1 += Val(displayPanel.Text)
result = Math.Sqrt(num1)
displayPanel.Text = result
num1 = 0
gives me the right result after doing sqrt of 9 + 1 which returns 4. Whereas if I removed the num1 = 0, doing sqrt of 9 + 1 will be 13. It seems that it's ignoring the operand and instead joins the two numbers as one string for the sqrt method.
My program is working fine now but can someone explain to me how not declaring num1 or num2 to 0 gives me an incorrect calculation?
Public Class Form1
Dim num1 As Single
Dim num2 As Single
Dim result As Single
addBtn_Click(sender As Object, e As EventArgs) Handles addBtn.Click
num1 += Val(displayPanel.Text)
displayPanel.Text = ""
equalBtn_Click(sender As Object, e As EventArgs) Handles equalBtn.Click
num2 += Val(displayPanel.Text)
result = num1 + num2
displayPanel.Text = result
num1 = 0
num2 = 0
sqrt_Click(sender As Object, e As EventArgs) Handles sqrt.Click
num1 += Val(displayPanel.Text)
result = Math.Sqrt(num1)
displayPanel.Text = result
num1 = 0

Answer 1

I think you might have problem with +=. Maybe change it to a simple =.
It's a bit hard to know everything that is going on here since I don't see all the code. Assuming num1 is equal to 0 at the beginning of the problem.
Also, let's convert += to something equivalent but easier to understand.
num1 = num1 + Val(displayPanel.Text)
result = Math.Sqrt(num1)
displayPanel.Text = result
If I write 9 in the textbox, num1 would be equal to 9. But if I execute it a second time, num1 would be equal to 18 since it's adding itself to the current value.

Answer 2

Your question "explain to me how not declaring num1 or num2 to 0 gives me an incorrect calculation?"
You have declared num1 and num2 at the class level (Form level) They will hold there assigned values until the entire form goes out of scope. You need them to hold there value so you can refer to these values in more than one method.
When you use the += you are asking the compiler to take the existing value and add a new value to it. with a simple assignment = you overwrite the value contained in the varaible.
Since += adds to the existing value instead of overwriting it completely you need to reset it to zero after each calculation.
If you used = you would not need to reset to zero because it would be overwritten.

Answer 3

The Val function in Vb.net is not operating as you are expecting it to.
Val attempts to convert textual information to a double or an integer.
See the documentation for it here.
Note that it explicitly states this:
(The Val function) stops converting at the first character that cannot be interpreted as a numeric digit, numeric modifier, numeric punctuation, or white space.
That is why it completely skips the = operator, and should skip parts like sqrt of; they are not a numeric modifiers, punctuation, digits, or whitespace.
Also of note, you may be seeing incorrect values because of multiple test cases in succession. Since your num1 and num2 variables have values that are not cleared, each additional evaluation will reuse the variables as they are.