The first type of loop we will cover is the For...Next
loop. The operation of this type of structure is that it uses a variable to countr from a start value to an end value. The
format of this loop is shown below:
For <Variable>=<Expression1> To <Expression2>
Code sequence
Next [ <Variable> ]
The variable must be a numeric type but can be any legal variable name. I do not want to catch
any of you callit it "variable" ;). However, it is optional to put it after the
Next keyword. Before the start of the loop, the variable will
be assigned the value of Expression1. This is now the start of the loop and if the value of the
variable is less than or equal to (this can be changed and will be explained below) the value of
Expression2, the code sequence inside the loop will be preformed. If the value is greater than
Expression2, the program continues executing from the line after the Next
keyword associated with the For statement. At the end of the
loop, the value of the variable is increased and the program returns to the start of the loop
(where it compares the variable to expression2). Here are some examples showing what happens
with different values for the expressions:
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DefType.w i
NPrint "For loop from 0 to 10"
For i=0 To 10
NPrint "Loop 1, counting: ",i
Next i
NPrint "For loop from 5 to 5"
For i=5 To 5
NPrint "Second loop, counting: ",i
Next ; See, no variable name here
NPrint "For loop from 1 to 0"
For i=1 To 0
NPrint "Loop 3, counting: ",i
Next
MouseWait
End
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As you will see if you run the program, nothing ever gets printed from the third
For...Next loop. This is
because the start value of the variable is more than the value of the second expression. This
feature can be useful, but what if you want to count backwards? In that case, you would need to use
the For...Next loop
and include the Step keyword:
For <Variable>=<Expression1> To <Expression2> [ Step <Expression3> ]
Code sequence
Next [ <Variable> ]
This works in the same way as the previous For...Next
loops except that instead of increasing the value of the variable at the end of every loop, the
value of Expression3 is added to the variable. If the value of Expression3 is negative, the loop will
begin if the end value is less than or equal to the start value (which if you remember is the
opposite of the original method described previously). So in the previous example (where you needed to count
backwards), you could use -1 and it would count down instead of up. Here are some examples of
using the Step keyword with For...Next loops.
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DefType.w i
NPrint "For loop from 5 to 0 with step -1"
For i=5 To 0 Step -1
NPrint "Loop 1, counting: ",i
Next
NPrint "For loop from 0 to 10 with step 3"
For i=0 To 10 Step 3
NPrint "Second loop, counting: ",i
Next
NPrint "For loop from 0 to 2*Pi with fractional step"
For angle.q=0 To 2*Pi-0.1 Step 0.1
; This could be used for e.g. processing a circle using radians instead of
; degrees. The -0.1 in the second expression is so we do not reach the value 2*Pi
; (which would be repeating 0 anyway). Oh, and Pi is a Blitz command which returns
; the mathematical value of Pi (3.141....)
NPrint "Counting fractional values: ",i
Next
NPrint "A little something can count in any direction - change me and see :)"
start.w = 0
end.w = 10
stepsize.w=1
If start > end Then stepsize=-1
For i=start To end Step stepsize
NPrint "Final loop, counting: ",i
Next
MouseWait
End
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