Syntax
(1)
for indexVar = firstValue to lastValue [step stepValue]
[statement block]
next [indexVar]
(2)
for objectVar in collectionVar [step -1]
[statement block]
next
Description - Syntax (1)
The for statement marks the beginning of a "for-loop," which must end with a next statement. A for-loop is useful when you want to repeat the execution of a block of statements for a particular number of times. This is what happens when a for-loop is encountered:
- The value of
firstValue is assigned to indexVar (indexVar must be a simple numeric variable).
- The statements in
statementBlock are executed. statementBlock can contain any number of statements, possibly including other for-loops (but note that any for-loop that's "nested" within statementBlock should not use the same indexVar as the "outer" for-loop).
- The value of
indexVar + stepValue is assigned to indexVar. (If you omit the step stepValue clause, then incremental value defaults to 1.)
- The new value of
indexVar is compared with lastValue, to see whether the loop should be repeated:
If stepValue is positive, then repeat the loop (go to Step 2) if indexVar<=lastValue.
If stepValue is negative, then repeat the loop (go to Step 2) if indexVar>=lastValue.
For example, consider this loop:
for n = 3 to sqr(x) step 2
:
next
In the above, the sqr function is called after each iteration of the loop. Assuming that the value of x doesn't change within the loop, we are needlessly recalculating the same sqr value at each iteration. It would be much faster to do it this way:
sqrx = sqr(x)
for n = 3 to sqrx step 2
:
next
Here the sqr function is called only once.
Implementation changes:
A design mistake in FutureBasic version 4 and earlier, apparently inherited from Applesoft BASIC, has been corrected that made for/next loops always execute at least once. Compatibility with legacy FutureBasic code can be obtained by overriding a special predefined constant as shown below.
dim as long j
for j = 1 to 0
print "never get here"
next
override _forLoopsAlwaysExecuteAtLeastOnce = _true
for j = 1 to 0
print "get here" // legacy FutureBasic behavior
next
override _forLoopsAlwaysExecuteAtLeastOnce = _false
for j = 1 to 0
print "never get here"
next
Example
Sometimes it's useful to exit a for-loop "early," after some condition within statementBlock has been met. The standard way to do this is to use exit for.
for p = 1 to maxStrings
if strArray(p) = searchPascalString
found = _zTrue
theIndex = p
exit for 'force early exit from loop
end if
next
Description Syntax (2)
This syntax option provides simplified access to Core Foundation (CF) and NS collection objects such as arrays and dictionaries.
- for in marks the beginning of a fast enumeration loop. It is equivalent to Objective-C's fast enumeration and operates similarly.
- for in enumerates and supports the following Core Foundation collection objects:
CFArrayRef, CFMutableArrayRef, CFDictionaryRef, CFMutableDictionaryRef, CFSetRef, CFMutableSetRef, OrderedSetRef, MutableOrderedSetRef, CountedSetRef, and PointerArrayRef.
- for in must end with a next statement. A for in loop is useful for iterating and processing individual items within a collection.
- On each loop iteration, the
objectVar is populated with one collection object from collectionVar.
statementBlock statements are executed and the contents of collectionVar are processed from beginning to end.
- The contents of
collectionVar may be processed in reverse order by using the step -1 option. Note minus one (-1) is the only supported step increment.
Example
dim as CFArrayRef array
dim as CFStringRef string
array = @[@"One", @"Two", @"Three", @"Four", @"Five"]
for string in array
NSLog(@"%@",string)
next
Note
The while and do statements provide other useful kinds of loop structures.
See also
while; do; exit for; break; continue
