How to draw a intersection point on two cures?

[liangchu]

hi,I want to draw the point of intersection on two cures.For example,the point of inersection on Line y=x and parabola y=x*x (-2<=x<=2).But it is failure for the following program segment:

   For x=-2 to 2 step 0.01
     Picture1.ForeColor=vbBlack
     If x=x*x then
       Picture1.Forecolor=vbRed
       Picture1.Pset(x,x)
     Else 
       Picture1.Pset(x,x*x)
     Endif
   Next x

What's wrong with it? Hope to explain it.Thank you a lot.

[Hack]

For one thing, x never gets incremented or decremented so it always has a value of -2

If you put a break on your code and step through it you will see that is ALWAYS hits the Else part, and x continues to equal -2

[mstraf]

For one thing, x never gets incremented or decremented so it always has a value of -2

Not if x is defined as Double (or Float), in that case the loop works ok
This is a classic rounding problem: the step is not a multiple of 2, therefore the test (x = x*x) will never be true (because incrementing by 0.01 x will never be zero)
This is one of the many ways (and not the best, but I did not want to change the code) to fix the problem:

    Picture1.Scale (-2, 10)-(2, -10)
    
    Dim x As Double
    Dim dstep As Double
    dstep = 2 ^ -6
    
    For x = -2 To 2 Step dstep
    
        If x = x * x Then
            Picture1.ForeColor = vbRed
        Else
            Picture1.ForeColor = vbBlack
        End If
        Picture1.PSet (x, x * x)
    
    Next x