Computers are good at doing things repeatedly. Often, we will want to loop - run a series of statements either a set number of times, or until some condition is met.

The for loop is the first type. It is good when you want to run some statements a known number of times (either when you write the program, or based on user input). Take the following example: you want to calculate compounding interest over a number of years. Although there is a formula for this, it serves as a good example of a loop, so we will take the "brute force" approach to solving it.

Our algorithm for this process might look like this:

• For testing, assume the following values:
  • 1000 starting balance
  • 5 percent interest
  • 10 years
  • How much will the user have at the end, using compounding interest?
• Build form that would allow the end user to enter these 3 values... for algorithm purposes, use fixed values stated above
• If user entered 5 for percent, convert to number usable in formula by dividing by 100
• Simulate the interest growth each year, 10 times, by:
  • Multiply the starting value (1000) by the interest rate (.05) and get 50 - this is the interest for the first year
  • Add this to the starting balance
  • Move to the next year. The next year, the starting value is 1050, times .05, to get an interest of 52.50; add this to the starting balance of 1050 to get 1102.50.
  • Continue for the next 8 years

So we plan to run this cycle of startbalance=(startbalance*.05)+startbalance 10 times.

Here is the code I wrote for this:

 

 Private Sub cmdCalc_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdCalc.Click
        Dim interestRate As Decimal
        Dim balance As Decimal
        Dim numberOfYears As Integer
        interestRate = CDec(txtInterestRate.Text) / 100
        balance = CInt(txtStartingBalance.Text)
        numberOfYears = CInt(txtNumberOfYears.Text)
        Dim i As Integer
        For i = 1 To numberOfYears
            balance = balance * interestRate + balance
        Next
        balance = (Math.Floor(balance * 100)) / 100
        lblAnswer.Text = CStr(balance)
    End Sub 

In this logic, we use the for...next loop. Quite simply - we declare a variable (i is frequently used). We choose a starting point (1 in my case) and an ending point (number of years). The i variable acts as a counter - for each lap (iteration) we use the current value of i. At the end of each lap, we add 1 to the value of i... the loop will end when the counter variable is no longer equal to or less than numberOfYears.

It is possible to use a step variable, to change how our loop functions - like so:

for i=1 to 10 step 2
'statements
'next

In this case, for each lap, we would have values of 1,3,5,7,9. The loop would add 2 to i at the last lap, and get 11 for i. During the next check of the counter, it would realize i was outside the boundaries of the starting value and the ending value, and exit the loop (resume statements found below the loop).