6.2 For

1
6.2 ForNext Loops

• General Form of a ForNext Loop
• Nested ForNext Loops
• Local Type Inference

2
ForNext Loops

• Used when we know how many times we want the loop
  to execute
• A counter controlled loop

3
ForNext Loop Syntax
4
Sample

• For i As Integer 1 To 5
• lstTable.Items.Add(i " " i 2)
• Next
• The loop counter variable, i, is
• initialized to 1
• tested against the stop value, 5
• incremented by 1 at the Next statement

The loop counter variable is often used in
statements within the loop body. In this case,
displaying a sequence of numbers and their squares
5
Similar Do While Loop

• Dim i As Integer 1
• Do While i lt 5
• lstTable.Items.Add(i " " i 2)
• i 1
• Loop

Loop counter initialization
Loop counter initialization
increment
6
Example 1 Output
7
Example 1 Code

• Dim pop As Double 300000
• For yr As Integer 2010 To 2014
• lstTable.Items.Add(yr " "
• FormatNumber(pop, 0))
• pop 0.03 pop
• Next

8
Step Clause

• Normally after each pass the value of the counter
  variable increases by 1
• If the clause Step s is appended to the For
  statement, the value of s will be added to the
  counter variable after each pass.
• If the value of s is a negative number, the value
  of the counter variable will decrease after each
  pass.

9
Example with Negative Step Value

• For j As Integer 10 To 1 Step -1
• lstBox.Items.Add(j)
• Next
• lstBox.Items.Add("Blastoff")

Looping backward through the numbers. Start at
10, test to see if j gt 1, decrement by 1.
Question What would happen with Step -2?
10
Example with Negative Step Value

• For j As Integer 1 To 10 Step -1
• lstBox.Items.Add(j)
• Next
• lstBox.Items.Add("Blastoff")

Question What would happen in this case?
11
Example 2
This is a good example for testing out various
values for the loops test and step
increment/decrement.
12
Example 3
Note Here we have a separate function that is
called by the event procedure.
Function call
Parameter passing
Function declaration
More on this in Ch5.
13
Example 3
Here, the loop counter variable is used as an
index into the string. It is common practice to
use loops for retrieving or manipulating
successive positions of a string or an array or a
listboxs collection.
14
Example 3
Loop starts at the last character of the string,
and goes backward until the first.
15
Nested Loops

• Loops can be nested inside other loops.

16
Example 4
Outer loop will execute three times
Each time the outer loop executes, the inner loop
will execute three times
Therefore, the inner loop executes a total of
nine times
17
Example Nested ForNext Loops

• For i As Integer 65 To 70
• For j As Integer 1 To 25
• lstBox.Items.Add(Chr(i) j)
• Next
• Next
• Output A1
• A2
• A3

Outer loop
Inner loop
Question 1 how many times will the outer loop
execute? Question 2 how many times (total) will
the inner loop execute?
18
The Nested for Loop -- Flowchart
Statements preceding loop
No
Statements following loop
Yes
No
Yes
19
Example 4
This example produces a multiplication
table. The inner and outer loop counters are
used for doing the calculations and displaying
the results.
20
For and Next Pairs

• For and Next statements must be paired.
• If one is missing, the automatic syntax checker
  will complain with a wavy underline and a message
  such as
• A For must be paired with a Next.

21
Start, Stop, and Step values

• Consider a loop beginning with
• For i As Integer m To n Step s
• The loop will be executed exactly once if m
  equals n no matter what value s has.
• The loop will not be executed at all if m is
  greater than n and s is positive,
• or if m is less than n and s is negative.

22
Altering the Counter Variable

• The value of the counter variable should not be
  altered within the body of the loop.
• Doing so might cause the loop to repeat
  indefinitely or have an unpredictable number of
  repetitions.

23
Non-Integer Step Values

• Can lead to round-off errors with the result that
  the loop is not executed the intended number of
  times.
• We will only use Integers for all values in the
  header.