Visual Basic: For Loop step problem

leecho · (This post was last modified: Apr 18, 2018 07:13 PM by leecho.)

Hi guys,

In short I've created a For loop for a program that I'm making in which the loop needs to start from i = 0 and end at i = 1, stepping 0.01 at a time.
As such I wrote this...

Assume the following:

Code 
Dim range as integer = 1

Dim a as integer = 5

Dim b as integer = 2

Dim c as integer = 2

Dim funcY(100) as Double 'store all Y values

Dim funcX(100) as Double 'store all X values

(Yes I know these can be written as constants, but they're actually passed into a sub routine. I'm only using dim to demonstrate in this post...
Additionally the funcY and funcX array's are bound to 100 indexes for a reason, it's irrelevant to this problem I'm presenting.)

Code 
For i = 0 to range Step 0.01

     Console.WriteLine("i = " & i & ". range = " & range)

     funcY(i) = (a*(Pow(i, 2))+b*i+c)

     funcX(i) = i

     Console.WriteLine(inputFunction & ": x = " & funcX(i) & " y = " & funcY(i))    'NOT FULLY WORKING FOR ALL VALS OF A, B AND C

Next

Some of you may have picked up that this code is to output all the x and y values for a quadratic equation in the form ax^2+bx+c.

So, ideally what would happen is the program finds y for all values of x in the range 0 to 1, stepping by 0.01.
Ultimately the above code provided actually does work, but there's a hiccup. For some reason when I hit i = 0.81 to i = 0.82, the results start to act very strange (see below)

So, with a = 5, b = 2, c = 2 and range = 1, stepping by 0.01 between 0 and range(which is 1 in this case) this is what the program outputs
Focus on when i = 0.81. Infact any value of i > 0.81

Code 
With the given equation: y=5x^2+2x+2

i = 0. range = 1

y=5x^2+2x+2: x = 0 y = 2

i = 0.01. range = 1

y=5x^2+2x+2: x = 0.01 y = 2.0205

i = 0.02. range = 1

y=5x^2+2x+2: x = 0.02 y = 2.042

i = 0.03. range = 1

y=5x^2+2x+2: x = 0.03 y = 2.0645

i = 0.04. range = 1

y=5x^2+2x+2: x = 0.04 y = 2.088

i = 0.05. range = 1

y=5x^2+2x+2: x = 0.05 y = 2.1125

i = 0.06. range = 1

y=5x^2+2x+2: x = 0.06 y = 2.138

i = 0.07. range = 1

y=5x^2+2x+2: x = 0.07 y = 2.1645

i = 0.08. range = 1

y=5x^2+2x+2: x = 0.08 y = 2.192

i = 0.09. range = 1

y=5x^2+2x+2: x = 0.09 y = 2.2205

i = 0.1. range = 1

y=5x^2+2x+2: x = 0.1 y = 2.25

i = 0.11. range = 1

y=5x^2+2x+2: x = 0.11 y = 2.2805

i = 0.12. range = 1

y=5x^2+2x+2: x = 0.12 y = 2.312

i = 0.13. range = 1

y=5x^2+2x+2: x = 0.13 y = 2.3445

i = 0.14. range = 1

y=5x^2+2x+2: x = 0.14 y = 2.378

i = 0.15. range = 1

y=5x^2+2x+2: x = 0.15 y = 2.4125

i = 0.16. range = 1

y=5x^2+2x+2: x = 0.16 y = 2.448

i = 0.17. range = 1

y=5x^2+2x+2: x = 0.17 y = 2.4845

i = 0.18. range = 1

y=5x^2+2x+2: x = 0.18 y = 2.522

i = 0.19. range = 1

y=5x^2+2x+2: x = 0.19 y = 2.5605

i = 0.2. range = 1

y=5x^2+2x+2: x = 0.2 y = 2.6

i = 0.21. range = 1

y=5x^2+2x+2: x = 0.21 y = 2.6405

i = 0.22. range = 1

y=5x^2+2x+2: x = 0.22 y = 2.682

i = 0.23. range = 1

y=5x^2+2x+2: x = 0.23 y = 2.7245

i = 0.24. range = 1

y=5x^2+2x+2: x = 0.24 y = 2.768

i = 0.25. range = 1

y=5x^2+2x+2: x = 0.25 y = 2.8125

i = 0.26. range = 1

y=5x^2+2x+2: x = 0.26 y = 2.858

i = 0.27. range = 1

y=5x^2+2x+2: x = 0.27 y = 2.9045

i = 0.28. range = 1

y=5x^2+2x+2: x = 0.28 y = 2.952

i = 0.29. range = 1

y=5x^2+2x+2: x = 0.29 y = 3.0005

i = 0.3. range = 1

y=5x^2+2x+2: x = 0.3 y = 3.05

i = 0.31. range = 1

y=5x^2+2x+2: x = 0.31 y = 3.1005

i = 0.32. range = 1

y=5x^2+2x+2: x = 0.32 y = 3.152

i = 0.33. range = 1

y=5x^2+2x+2: x = 0.33 y = 3.2045

i = 0.34. range = 1

y=5x^2+2x+2: x = 0.34 y = 3.258

i = 0.35. range = 1

y=5x^2+2x+2: x = 0.35 y = 3.3125

i = 0.36. range = 1

y=5x^2+2x+2: x = 0.36 y = 3.368

i = 0.37. range = 1

y=5x^2+2x+2: x = 0.37 y = 3.4245

i = 0.38. range = 1

y=5x^2+2x+2: x = 0.38 y = 3.482

i = 0.39. range = 1

y=5x^2+2x+2: x = 0.39 y = 3.5405

i = 0.4. range = 1

y=5x^2+2x+2: x = 0.4 y = 3.6

i = 0.41. range = 1

y=5x^2+2x+2: x = 0.41 y = 3.6605

i = 0.42. range = 1

y=5x^2+2x+2: x = 0.42 y = 3.722

i = 0.43. range = 1

y=5x^2+2x+2: x = 0.43 y = 3.7845

i = 0.44. range = 1

y=5x^2+2x+2: x = 0.44 y = 3.848

i = 0.45. range = 1

y=5x^2+2x+2: x = 0.45 y = 3.9125

i = 0.46. range = 1

y=5x^2+2x+2: x = 0.46 y = 3.978

i = 0.47. range = 1

y=5x^2+2x+2: x = 0.47 y = 4.0445

i = 0.48. range = 1

y=5x^2+2x+2: x = 0.48 y = 4.112

i = 0.49. range = 1

y=5x^2+2x+2: x = 0.49 y = 4.1805

i = 0.5. range = 1

y=5x^2+2x+2: x = 0.5 y = 4.25

i = 0.51. range = 1

y=5x^2+2x+2: x = 0.51 y = 4.3205

i = 0.52. range = 1

y=5x^2+2x+2: x = 0.52 y = 4.392

i = 0.53. range = 1

y=5x^2+2x+2: x = 0.53 y = 4.4645

i = 0.54. range = 1

y=5x^2+2x+2: x = 0.54 y = 4.538

i = 0.55. range = 1

y=5x^2+2x+2: x = 0.55 y = 4.6125

i = 0.56. range = 1

y=5x^2+2x+2: x = 0.56 y = 4.688

i = 0.57. range = 1

y=5x^2+2x+2: x = 0.57 y = 4.7645

i = 0.58. range = 1

y=5x^2+2x+2: x = 0.58 y = 4.842

i = 0.59. range = 1

y=5x^2+2x+2: x = 0.59 y = 4.9205

i = 0.6. range = 1

y=5x^2+2x+2: x = 0.6 y = 5

i = 0.61. range = 1

y=5x^2+2x+2: x = 0.61 y = 5.0805

i = 0.62. range = 1

y=5x^2+2x+2: x = 0.62 y = 5.162

i = 0.63. range = 1

y=5x^2+2x+2: x = 0.63 y = 5.2445

i = 0.64. range = 1

y=5x^2+2x+2: x = 0.64 y = 5.328

i = 0.65. range = 1

y=5x^2+2x+2: x = 0.65 y = 5.4125

i = 0.66. range = 1

y=5x^2+2x+2: x = 0.66 y = 5.498

i = 0.67. range = 1

y=5x^2+2x+2: x = 0.67 y = 5.5845

i = 0.68. range = 1

y=5x^2+2x+2: x = 0.68 y = 5.672

i = 0.69. range = 1

y=5x^2+2x+2: x = 0.69 y = 5.7605

i = 0.7. range = 1

y=5x^2+2x+2: x = 0.7 y = 5.85

i = 0.71. range = 1

y=5x^2+2x+2: x = 0.71 y = 5.9405

i = 0.72. range = 1

y=5x^2+2x+2: x = 0.72 y = 6.032

i = 0.73. range = 1

y=5x^2+2x+2: x = 0.73 y = 6.1245

i = 0.74. range = 1

y=5x^2+2x+2: x = 0.74 y = 6.218

i = 0.75. range = 1

y=5x^2+2x+2: x = 0.75 y = 6.3125

i = 0.76. range = 1

y=5x^2+2x+2: x = 0.76 y = 6.408

i = 0.77. range = 1

y=5x^2+2x+2: x = 0.77 y = 6.5045

i = 0.78. range = 1

y=5x^2+2x+2: x = 0.78 y = 6.602

i = 0.79. range = 1

y=5x^2+2x+2: x = 0.79 y = 6.70050000000001

i = 0.8. range = 1

y=5x^2+2x+2: x = 0.8 y = 6.8

i = 0.81. range = 1

y=5x^2+2x+2: x = 0.81 y = 6.9005

i = 0.820000000000001. range = 1

y=5x^2+2x+2: x = 0.820000000000001 y = 7.002

i = 0.830000000000001. range = 1

y=5x^2+2x+2: x = 0.830000000000001 y = 7.10450000000001

i = 0.840000000000001. range = 1

y=5x^2+2x+2: x = 0.840000000000001 y = 7.20800000000001

i = 0.850000000000001. range = 1

y=5x^2+2x+2: x = 0.850000000000001 y = 7.31250000000001

i = 0.860000000000001. range = 1

y=5x^2+2x+2: x = 0.860000000000001 y = 7.41800000000001

i = 0.870000000000001. range = 1

y=5x^2+2x+2: x = 0.870000000000001 y = 7.52450000000001

i = 0.880000000000001. range = 1

y=5x^2+2x+2: x = 0.880000000000001 y = 7.63200000000001

i = 0.890000000000001. range = 1

y=5x^2+2x+2: x = 0.890000000000001 y = 7.74050000000001

i = 0.900000000000001. range = 1

y=5x^2+2x+2: x = 0.900000000000001 y = 7.85000000000001

i = 0.910000000000001. range = 1

y=5x^2+2x+2: x = 0.910000000000001 y = 7.96050000000001

i = 0.920000000000001. range = 1

y=5x^2+2x+2: x = 0.920000000000001 y = 8.07200000000001

i = 0.930000000000001. range = 1

y=5x^2+2x+2: x = 0.930000000000001 y = 8.18450000000001

i = 0.940000000000001. range = 1

y=5x^2+2x+2: x = 0.940000000000001 y = 8.29800000000001

i = 0.950000000000001. range = 1

y=5x^2+2x+2: x = 0.950000000000001 y = 8.41250000000001

i = 0.960000000000001. range = 1

y=5x^2+2x+2: x = 0.960000000000001 y = 8.52800000000001

i = 0.970000000000001. range = 1

y=5x^2+2x+2: x = 0.970000000000001 y = 8.64450000000001

i = 0.980000000000001. range = 1

y=5x^2+2x+2: x = 0.980000000000001 y = 8.76200000000001

i = 0.990000000000001. range = 1

y=5x^2+2x+2: x = 0.990000000000001 y = 8.88050000000001

VIEW NEXT POST FOR CONTINUATION

The specific problem shows up here

Code 
i = 0.8. range = 1

y=5x^2+2x+2: x = 0.8 y = 6.8

i = 0.81. range = 1

y=5x^2+2x+2: x = 0.81 y = 6.9005

i = 0.820000000000001. range = 1

y=5x^2+2x+2: x = 0.820000000000001 y = 7.002

i = 0.830000000000001. range = 1

y=5x^2+2x+2: x = 0.830000000000001 y = 7.10450000000001

i = 0.840000000000001. range = 1

y=5x^2+2x+2: x = 0.840000000000001 y = 7.20800000000001

i = 0.850000000000001. range = 1

y=5x^2+2x+2: x = 0.850000000000001 y = 7.31250000000001

Why all of a sudden has i gone from 0.81 to 0.820000000000001, with this trend continuing throughout the rest of the values for i.
As you can see this affects my x and y results which is no Bueno.
Of course I could limit the decimal places that i could be, but I'm more interested as to WHY this happens more than anything else.

Any ideas HF?
Tah, Leech

m0noxide · Apr 18, 2018 07:57 PM

"Floating-point decimal values generally do not have an exact binary representation. This is a side effect of how the CPU represents floating point data. For this reason, you may experience some loss of precision, and some floating-point operations may produce unexpected results."

- https://msdn.microsoft.com/en-us/library/c151dt3s.aspx

SIM-2362662 · (This post was last modified: Apr 19, 2018 04:09 AM by SIM-2362662.)

As the user above me said, double variables are just ESTIMATIONS. They cant store the exact value.
Thats why the output is correct at the beginning. The error is just too small. But as you keep making more operations, the estimation becomes more and more imprecise.
So how can you avoid this?
The main problem we have here is re-calculating the i value based on the previous value of it. So we are basically estimating a value based on a estimation. We need to get rid of that. We want to calculate (estimate) i based on a true value (integer). (True value = a value that is not estimated)

To do that, replace :

Code 
For i = 0 to range Step 0.01

With:

Code 
       For k = 0 To Range * 100 Step 1

            Dim i As Double = k * 0.01

Doing this, we re-calculate the i value based on the current k value. (not based on the previous i value).

Your final code will be :

Code 
Dim Range As Integer = 1

        Dim a As Integer = 5

        Dim b As Integer = 2

        Dim c As Integer = 2

        Dim funcY(100) As Double 'store all Y values

        Dim funcX(100) As Double 'store all X values

        For k = 0 To Range * 100 Step 1

            Dim i As Double = k * 0.01

            Console.WriteLine("i = " & i & ". range = " & Range)

            funcY(i) = (a * (Pow(i, 2)) + b * i + c)

            funcX(i) = i

            Console.WriteLine(inputFunction & ": x = " & funcX(i) & " y = " & funcY(i))    'NOT FULLY WORKING FOR ALL VALS OF A, B AND C

        Next

leecho · Apr 19, 2018 05:57 PM

Replies are much appreciated.

I guess that makes sense then, to me it was quite weird just to see crop up but now I understand...

(Apr 19, 2018 04:07 AM)SIM-2362662 Wrote: As the user above me said, double variables are just ESTIMATIONS. They cant store the exact value.
Thats why the output is correct at the beginning. The error is just too small. But as you keep making more operations, the estimation becomes more and more imprecise.
So how can you avoid this?
The main problem we have here is re-calculating the i value based on the previous value of it. So we are basically estimating a value based on a estimation. We need to get rid of that. We want to calculate (estimate) i based on a true value (integer). (True value = a value that is not estimated)

To do that, replace :

Code
For i = 0 to range Step 0.01
With:

Code
For k = 0 To Range * 100 Step 1 Dim i As Double = k * 0.01

Doing this, we re-calculate the i value based on the current k value. (not based on the previous i value).

Your final code will be :

Code
Dim Range As Integer = 1 Dim a As Integer = 5 Dim b As Integer = 2 Dim c As Integer = 2 Dim funcY(100) As Double 'store all Y values Dim funcX(100) As Double 'store all X values For k = 0 To Range * 100 Step 1 Dim i As Double = k * 0.01 Console.WriteLine("i = " & i & ". range = " & Range) funcY(i) = (a * (Pow(i, 2)) + b * i + c) funcX(i) = i Console.WriteLine(inputFunction & ": x = " & funcX(i) & " y = " & funcY(i)) 'NOT FULLY WORKING FOR ALL VALS OF A, B AND C Next

Yeah, that's the exact method I initially wrote down and thought that I could just use a step of 0.01 instead (to which I stumbled across the double problem). So I actually have this already implemented, though the response is still appreciated.

Many thanks people!

PolarisGang · (This post was last modified: Apr 22, 2018 08:41 AM by PolarisGang.)

I mean this:

Code 
Public Function QuadricEquation(ByVal A As Integer, ByVal B As Integer, ByVal C As Integer, ByVal Range As Single) As Vec2()

        Dim storedAnswers(Decimal.Round(Range, 2, MidpointRounding.AwayFromZero) * 100) As Vec2

        For i As Integer = 0 To Decimal.Round(Range, 2, MidpointRounding.AwayFromZero) * 100

            Dim temp_i As Single = i / 100.0F

            Console.WriteLine("i = " & temp_i & ". range = " & Range)

            storedAnswers(i) = New Vec2(temp_i, (A * (Math.Pow(temp_i, 2)) + B * temp_i + C))

            Console.WriteLine("y=5x^2+2x+2" & ": " & storedAnswers(i).ToString)

        Next

        Console.WriteLine("")

        Console.WriteLine("Quadric equation completed.")

        Console.ReadKey(True)

        Return storedAnswers

    End Function

Gave me this:

Code 
'Too many chars to keep whole log

y=5x^2+2x+2: x: 0,65  y:5,4125

i = 0,66. range = 1

y=5x^2+2x+2: x: 0,66  y:5,498

i = 0,67. range = 1

y=5x^2+2x+2: x: 0,67  y:5,5845

i = 0,68. range = 1

y=5x^2+2x+2: x: 0,68  y:5,672

i = 0,69. range = 1

y=5x^2+2x+2: x: 0,69  y:5,7605

i = 0,7. range = 1

y=5x^2+2x+2: x: 0,7  y:5,85

i = 0,71. range = 1

y=5x^2+2x+2: x: 0,71  y:5,9405

i = 0,72. range = 1

y=5x^2+2x+2: x: 0,72  y:6,032

i = 0,73. range = 1

y=5x^2+2x+2: x: 0,73  y:6,1245

i = 0,74. range = 1

y=5x^2+2x+2: x: 0,74  y:6,218

i = 0,75. range = 1

y=5x^2+2x+2: x: 0,75  y:6,3125

i = 0,76. range = 1

y=5x^2+2x+2: x: 0,76  y:6,408

i = 0,77. range = 1

y=5x^2+2x+2: x: 0,77  y:6,5045

i = 0,78. range = 1

y=5x^2+2x+2: x: 0,78  y:6,602

i = 0,79. range = 1

y=5x^2+2x+2: x: 0,79  y:6,7005

i = 0,8. range = 1

y=5x^2+2x+2: x: 0,8  y:6,8

i = 0,81. range = 1

y=5x^2+2x+2: x: 0,81  y:6,9005

i = 0,82. range = 1

y=5x^2+2x+2: x: 0,82  y:7,002

i = 0,83. range = 1

y=5x^2+2x+2: x: 0,83  y:7,1045

i = 0,84. range = 1

y=5x^2+2x+2: x: 0,84  y:7,208

i = 0,85. range = 1

y=5x^2+2x+2: x: 0,85  y:7,3125

i = 0,86. range = 1

y=5x^2+2x+2: x: 0,86  y:7,418

i = 0,87. range = 1

y=5x^2+2x+2: x: 0,87  y:7,5245

i = 0,88. range = 1

y=5x^2+2x+2: x: 0,88  y:7,632

i = 0,89. range = 1

y=5x^2+2x+2: x: 0,89  y:7,7405

i = 0,9. range = 1

y=5x^2+2x+2: x: 0,9  y:7,85

i = 0,91. range = 1

y=5x^2+2x+2: x: 0,91  y:7,9605

i = 0,92. range = 1

y=5x^2+2x+2: x: 0,92  y:8,072001

i = 0,93. range = 1

y=5x^2+2x+2: x: 0,93  y:8,1845

i = 0,94. range = 1

y=5x^2+2x+2: x: 0,94  y:8,298

i = 0,95. range = 1

y=5x^2+2x+2: x: 0,95  y:8,412499

i = 0,96. range = 1

y=5x^2+2x+2: x: 0,96  y:8,528

i = 0,97. range = 1

y=5x^2+2x+2: x: 0,97  y:8,644501

i = 0,98. range = 1

y=5x^2+2x+2: x: 0,98  y:8,762

i = 0,99. range = 1

y=5x^2+2x+2: x: 0,99  y:8,8805

i = 1. range = 1

y=5x^2+2x+2: x: 1  y:9

Quadric equation completed.

If that's what you're looking for.

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