Executing a loop for a range of cells - vba

Question

This is a very basic question and I'm sure it has been answered but I can't seem to find it elsewhere. I have a portion of vba code that works fine for a single cell. However I want to expand it to work for a range of cells (all within the same column).
Basically its a goalseeking loops that changes a value in the "b" column until the value in the "w" column matches (comes within 99%) the value in the "x" column.
What works:
Sub Goalseeker()
Do Until Range("w32").Value / Range("x32").Value > 0.99
Range("b32").Value = Range("b32").Value - 1
Loop
End Sub
I want to extend this to work for rows 32 to 107.
What I've tried:
Edit: I've made adjustment based on the comments received and tweaked and few things before it worked. If anyone is interested in the process:
Option Explicit
Sub Goalseeker()
Dim i As Integer
Dim targetcell As Double
Dim outputcell As Double
Dim variablecell As Range
For i = 32 To 107
targetcell = Cells(i, "x")
outputcell = Cells(i, "w")
Set variablecell = Range("B" & i)
Do Until outputcell / targetcell > 0.99
variablecell = variablecell - 1
targetcell = Cells(i, "x")
outputcell = Cells(i, "w")
Loop
Next
End Sub
The bit I had to tweak was
Do Until outputcell / targetcell > 0.99
variablecell = variablecell - 1
targetcell = Cells(i, "x")
outputcell = Cells(i, "w")
Loop
Redefining (i apologize if that's the wrong term) targetcell and outputcell was necessary to prevent an infinite loop.
Thanks all. I will work on making this function for relative references instead of absolute.

Answer 1

Few problems here. Change your For i loop to the format For i = x to y not For i = x to i = y
You can refer to targetcell and outputcell as double but the variablecell needs to be a range. And if it's a range it needs Set
You should declare ALL your variables, as below.
And finally, you might want to put in a catch to get out of infinite looping (in case the target never reaches above 0.99 ?)
Sub Goalseeker()
Dim i As Integer
Dim targetcell As Double
Dim outputcell As Double
Dim variablecell As Range
For i = 32 To 107
targetcell = Cells(i, "x")
outputcell = Cells(i, "w")
Set variablecell = Range("B" & i)
Do Until outputcell / targetcell > 0.99
variablecell = variablecell - 1
Loop
Next
End Sub

Answer 2

Consider the following example table:
Use the code below to find the correct values in the "B" column (as shown) in order to minimize the error between the result (next column) and the goal (two columns over).
Option Explicit
Public Sub GoalSeekMyValues()
' Call GoalSeek with inputvalues "B2:B16", having the result
' at column offset 1, and the goal in column offset 2.
' Note that Range("B2").Resize(15, 1) = Range("B2:B16"),
' But I prefer the top cell and row count of this syntax.
GoalSeek Range("B2").Resize(15, 1), 1, 2
End Sub
Public Sub GoalSeek(ByVal variables As Range, ByVal result_offset As Long, ByVal goal_offset As Long)
Dim n As Long, i As Long, pct_error As Double, last_error As Double
'x is the input value (variable)
'y is the result
'g is the goal for y
Dim x As Double, y As Double, g As Double
' Read the number of rows in the input values
n = variables.Rows.Count
Dim r As Range
' Loop through the rows
For i = 1 To n
'Set a range reference at the i-th input cell
Set r = variables.Cells(i, 1)
' Read the value, the result and the goal
x = r.Value
y = r.Offset(0, result_offset).Value
g = r.Offset(0, goal_offset).Value
pct_error = Abs(y / g - 1)
Do
'Set the next value
r.Value = x - 1
' Read the result (assume goal doesn't change)
y = r.Offset(0, result_offset).Value
' Keep last error, and calculate new one
last_error = pct_error
pct_error = Abs(y / g - 1)
' If new error is more than last then exit the loop
' and keep the previous value (with less error).
If pct_error > last_error Then
' Keep last value
r.Value = x
Exit Do
End If
' read the input value
x = r.Value
' Assume inputs must be positive so end the loop
' on zero on negative numbers
Loop Until x <= 0
Next i
End Sub
Your code has lots of points of failure.
Your code might not ever reach a solution and excel will hang (until Ctrl-Break is pressed). I have a hard break when the inputs become zero or negative. Other problems require other ways to tell that there isn't a solution.
The first time the result comes within 1% of the solution might not produce the least error. I solve this by tracking the absolute value of the relative error. Only when the error starts increasing I terminate the loop. This assumes that decreasing the input by one unit will improve the solution (at least initially). If this is not the case the code will fail.
You use absolute referencing (like reading the 12th cell down and 4th across) and that is not very reusable style of programming. I always try to use relative referencing. I start from the top left referenced cell (in this case B2) and move down and right from there using the following methods:
Range("B2").Cells(5,1) - Reference 5th row and 1st column from B2.
Range("B2").Resize(15, 1) - Expand the range to include 15 rows and one column.
Range("B2).Cells(i,1).Offset(0, 3) - From the i-th row use column offset of 3 (meaning the 4th column in the table).
I suggest to use one of the common goal seeking methods (like bisection), or better yet, use the built-in goal seek function
example:
Range("B2").Cells(i,2).GoalSeek Goal:=Range("B2").Cells(i,3).Value, ChangingCell:=Range("B2").Cells(i,1)