Version: 19

Looping structures

Looping structures repeat a sequence of statements until a condition is met or a number of times is reached.

While...End while

The formal syntax of the While...End while control flow structure is:

 While(Boolean_Expression)
    statement(s)
 End while

A While...End while loop executes the statements inside the loop as long as the Boolean expression is TRUE. It tests the Boolean expression at the beginning of the loop and does not enter the loop at all if the expression is FALSE.

It is common to initialize the value tested in the Boolean expression immediately before entering the While...End while loop. Initializing the value means setting it to something appropriate, usually so that the Boolean expression will be TRUE and While...End while executes the loop.

The Boolean expression must be set by something inside the loop or else the loop will continue forever. The following loop continues forever because NeverStop is always TRUE:

 NeverStop:=True
 While(NeverStop)
 End while

If you find yourself in such a situation, where a method is executing uncontrolled, you can use the trace facilities to stop the loop and track down the problem. For more information about tracing a method, see the Error handling page.

Example

 CONFIRM("Add a new record?") //The user wants to add a record?
 While(OK=1) //Loop as long as the user wants to
    ADD RECORD([aTable]) //Add a new record
 End while //The loop always ends with End while

In this example, the OK system variable is set by the CONFIRM command before the loop starts. If the user clicks the OK button in the confirmation dialog box, the OK system variable is set to 1 and the loop starts. Otherwise, the OK system variable is set to 0 and the loop is skipped. Once the loop starts, the ADD RECORD command keeps the loop going because it sets the OK system variable to 1 when the user saves the record. When the user cancels (does not save) the last record, the OK system variable is set to 0 and the loop stops.

Repeat...Until

The formal syntax of the Repeat...Until control flow structure is:

 Repeat
    statement(s)
 Until(Boolean_Expression)

A Repeat...Until loop is similar to a While...End while loop, except that it tests the Boolean expression after the loop rather than before. Thus, a Repeat...Until loop always executes the loop once, whereas if the Boolean expression is initially False, a While...End while loop does not execute the loop at all.

The other difference with a Repeat...Until loop is that the loop continues until the Boolean expression is TRUE.

Example

Compare the following example with the example for the While...End while loop. Note that the Boolean expression does not need to be initialized—there is no CONFIRM command to initialize the OK variable.

 Repeat
    ADD RECORD([aTable])
 Until(OK=0)

For...End for

The formal syntax of the For...End for control flow structure is:

 For(Counter_Variable;Start_Expression;End_Expression{;Increment_Expression})
    statement(s)
 End for

The For...End for loop is a loop controlled by a counter variable:

The counter variable Counter_Variable is a numeric variable (Real or Long Integer) that the For...End for loop initializes to the value specified by Start_Expression.
Each time the loop is executed, the counter variable is incremented by the value specified in the optional value Increment_Expression. If you do not specify Increment_Expression, the counter variable is incremented by one (1), which is the default.
When the counter variable passes the End_Expression value, the loop stops.

Important: The numeric expressions Start_Expression, End_Expression and Increment_Expression are evaluated once at the beginning of the loop. If these expressions are variables, changing one of these variables within the loop will not affect the loop.

Tip: However, for special purposes, you can change the value of the counter variable Counter_Variable within the loop; this will affect the loop.

Usually Start_Expression is less than End_Expression.
If Start_Expression and End_Expression are equal, the loop will execute only once.
If Start_Expression is greater than End_Expression, the loop will not execute at all unless you specify a negative Increment_Expression. See the examples.

Basic examples

The following example executes 100 iterations:

 For(vCounter;1;100)
  //Do something
 End for

The following example goes through all elements of the array anArray:

 For($vlElem;1;Size of array(anArray))
  //Do something with the element
    anArray{$vlElem}:=...
 End for

The following example goes through all the characters of the text vtSomeText:

 For($vlChar;1;Length(vtSomeText))
  //Do something with the character if it is a TAB
    If(Character code(vtSomeText[[$vlChar]])=Tab)
  //...
    End if
 End for

The following example goes through the selected records for the table [aTable]:

 FIRST RECORD([aTable])
 For($vlRecord;1;Records in selection([aTable]))
  //Do something with the record
    SEND RECORD([aTable])
  //...
  //Go to the next record
    NEXT RECORD([aTable])
 End for

Most of the For...End for loops you will write in your projects will look like the ones listed in these examples.

Decrementing variable counter

In some cases, you may want to have a loop whose counter variable is decreasing rather than increasing. To do so, you must specify Start_Expression greater than End_Expression and a negative Increment_Expression. The following examples do the same thing as the previous examples, but in reverse order:

The following example executes 100 iterations:

 For(vCounter;100;1;-1)
  //Do something
 End for

The following example goes through all elements of the array anArray:

 For($vlElem;Size of array(anArray);1;-1)
  //Do something with the element
    anArray{$vlElem}:=...
 End for

The following example goes through all the characters of the text vtSomeText:

 For($vlChar;Length(vtSomeText);1;-1)
  //Do something with the character if it is a TAB
    If(Character code(vtSomeText[[$vlChar]])=Tab)
  //...
    End if
 End for

The following example goes through the selected records for the table [aTable]:

 LAST RECORD([aTable])
 For($vlRecord;Records in selection([aTable]);1;-1)
  //Do something with the record
    SEND RECORD([aTable])
  //...
  //Go to the previous record
    PREVIOUS RECORD([aTable])
 End for

Incrementing the counter variable by more than one

If you need to, you can use an Increment_Expression (positive or negative) whose absolute value is greater than one.

The following loop addresses only the even elements of the array anArray:

 For($vlElem;2;Size of array(anArray);2)
  //Do something with the element #2,#4...#2n
    anArray{$vlElem}:=...
 End for

Comparing looping structures

Let's go back to the first For...End for example. The following example executes 100 iterations:

 For(vCounter;1;100)
  //Do something
 End for

It is interesting to see how the While...End while loop and Repeat...Until loop would perform the same action. Here is the equivalent While...End while loop:

 $i:=1 //Initialize the counter
 While($i<=100) //Loop 100 times
  //Do something
    $i:=$i+1 //Need to increment the counter
 End while

Here is the equivalent Repeat...Until loop:

 $i:=1 //Initialize the counter
 Repeat
  //Do something
    $i:=$i+1 //Need to increment the counter
 Until($i=100) //Loop 100 times

Tip: The For...End for loop is usually faster than the While...End while and Repeat...Until loops, because 4D tests the condition internally for each cycle of the loop and increments the counter. Therefore, use the For...End for loop whenever possible.

Optimizing the execution of the For...End for loops

You can use Real and Long Integer variables as well as interprocess, process, and local variable counters. For lengthy repetitive loops, especially in compiled mode, use local Long Integer variables.

Here is an example:

 C_LONGINT($vlCounter) //use local Long Integer variables
 For($vlCounter;1;10000)
  //Do something
 End for

Nested For...End for looping structures

You can nest as many control structures as you (reasonably) need. This includes nesting For...End for loops. To avoid mistakes, make sure to use different counter variables for each looping structure.

Here are two examples:

The following example goes through all the elements of a two-dimensional array:

 For($vlElem;1;Size of array(anArray))
  //...
  //Do something with the row
  //...
    For($vlSubElem;1;Size of array(anArray{$vlElem}))
  //Do something with the element
       anArray{$vlElem}{$vlSubElem}:=...
    End for
 End for

The following example builds an array of pointers to all the date fields present in the database:

 ARRAY POINTER($apDateFields;0)
 $vlElem:=0
 For($vlTable;1;Get last table number)
    If(Is table number valid($vlTable))
       For($vlField;1;Get last field number($vlTable))
          If(Is field number valid($vlTable;$vlField))
             $vpField:=Field($vlTable;$vlField)
             If(Type($vpField->)=Is date)
                $vlElem:=$vlElem+1
                INSERT IN ARRAY($apDateFields;$vlElem)
                $apDateFields{$vlElem}:=$vpField
             End if
          End if
       End for
    End if
 End for

For each...End for each

The formal syntax of the For each...End for each control flow structure is:

 For each(Current_Item;Expression{;begin{;end}}){Until|While}(Boolean_Expression)}
    statement(s)
 End for each

The For each...End for each structure iterates a specified Current_item over all values of the Expression. The Current_item type depends on the Expression type. The For each...End for each loop can iterate through three Expression types:

collections: loop through each element of the collection,
entity selections: loop through each entity,
objects: loop through each object property.

The following table compares the three types of For each...End for each:

	Loop through collections	Loop through entity selections	Loop through objects
Current_Item type	Variable of the same type as collection elements	Entity	Text variable
Expression type	Collection (with elements of the same type)	Entity selection	Object
Number of loops (by default)	Number of collection elements	Number of entities in the selection	Number of object properties
Support of begin / end parameters	Yes	Yes	No

The number of loops is evaluated at startup and will not change during the processing. Adding or removing items during the loop is usually not recommended since it may result in missing or redundant iterations.
By default, the enclosed statement(s) are executed for each value in Expression. It is, however, possible to exit the loop by testing a condition either at the begining of the loop (While) or at the end of the loop (Until).
The begin and end optional parameters can be used with collections and entity selections to define boundaries for the loop.
The For each...End for each loop can be used on a shared collection or a shared object. If your code needs to modify one or more element(s) of the collection or object properties, you need to use the Use...End use keywords. Depending on your needs, you can call the Use...End use keywords:
- before entering the loop, if items should be modified together for integrity reasons, or
- within the loop when only some elements/properties need to be modified and no integrity management is required.

Loop through collections

When For each...End for each is used with an Expression of the Collection type, the Current_Item parameter is a variable of the same type as the collection elements. By default, the number of loops is based on the number of items of the collection.

The collection must contain only elements of the same type, otherwise an error will be returned as soon as the Current_Item variable is assigned the first mismatched value type.

At each loop iteration, the Current_Item variable is automatically filled with the matching element of the collection. The following points must be taken into account:

If the Current_Item variable is of the object type or collection type (i.e. if Expression is a collection of objects or of collections), modifying this variable will automatically modify the matching element of the collection (because objects and collections share the same references). If the variable is of a scalar type, only the variable will be modified.
The Current_Item variable must be of the same type as the collection elements. If any collection item is not of the same type as the variable, an error is generated and the loop stops.
If the collection contains elements with a Null value, an error will be generated if the Current_Item variable type does not support Null values (such as longint variables).

Example

You want to compute some statistics for a collection of numbers:

 C_COLLECTION($nums)
 $nums:=New collection(10;5001;6665;33;1;42;7850)
 C_LONGINT($item;$vEven;$vOdd;$vUnder;$vOver)
 For each($item;$nums)
    If($item%2=0)
       $vEven:=$vEven+1
    Else
       $vOdd:=$vOdd+1
    End if
    Case of
       :($item<5000)
          $vUnder:=$vUnder+1
       :($item>6000)
          $vOver:=$vOver+1
    End case
 End for each
  //$vEven=3, $vOdd=4
  //$vUnder=4,$vOver=2

Loop through entity selections

When For each...End for each is used with an Expression of the Entity selection type, the Current_Item parameter is the entity that is currently processed.

The number of loops is based on the number of entities in the entity selection. On each loop iteration, the Current_Item parameter is automatically filled with the entity of the entity selection that is currently processed.

Note: If the entity selection contains an entity that was removed meanwhile by another process, it is automatically skipped during the loop.

Keep in mind that any modifications applied on the current entity must be saved explicitly using entity.save( ).

Example

You want to raise the salary of all British employees in an entity selection:

 C_OBJECT(emp)
 For each(emp;ds.Employees.query("country='UK'"))
    emp.salary:=emp.salary*1,03
    emp.save()
 End for each

Loop through object properties

When For each...End for each is used with an Expression of the Object type, the Current_Item parameter is a text variable automatically filled with the name of the currently processed property.

The properties of the object are processed according to their order of creation. During the loop, properties can be added to or removed from the object, without modifying the number of loops that will remain based on the original number of properties of the object.

Example

You want to switch the names to uppercase in the following object:

{
    "firstname": "gregory",
    "lastname": "badikora",
    "age": 20
}

You can write:

 For each(property;vObject)
    If(Value type(vObject[property])=Is text)
       vObject[property]:=Uppercase(vObject[property])
    End if
 End for each

{
    "firstname": "GREGORY",
    "lastname": "BADIKORA",
    "age": 20
}

begin / end parameters

You can define bounds to the iteration using the optional begin and end parameters.

Note: The begin and end parameters can only be used in iterations through collections and entity selections (they are ignored on object properties).

In the begin parameter, pass the element position in Expression at which to start the iteration (begin is included).
In the end parameter, you can also pass the element position in Expression at which to stop the iteration (end is excluded).

If end is omitted or if end is greater than the number of elements in Expression, elements are iterated from begin until the last one (included). If the begin and end parameters are positive values, they represent actual positions of elements in Expression. If begin is a negative value, it is recalculed as begin:=begin+Expression size (it is considered as the offset from the end of Expression). If the calculated value is negative, begin is set to 0. Note: Even if begin is negative, the iteration is still performed in the standard order. If end is a negative value, it is recalculed as end:=end+Expression size

For example:

a collection contains 10 elements (numbered from 0 to 9)
begin=-4 -> begin=-4+10=6 -> iteration starts at the 6th element (#5)
end=-2 -> end=-2+10=8 -> iteration stops before the 8th element (#7), i.e. at the 7th element.

Example

 C_COLLECTION($col;$col2)
 $col:=New collection("a";"b";"c";"d";"e")
 $col2:=New collection(1;2;3)
 C_TEXT($item)
 For each($item;$col;0;3)
    $col2.push($item)
 End for each
  //$col2=[1,2,3,"a","b","c"]
 For each($item;$col;-2;-1)
    $col2.push($item)
 End for each
  //$col2=[1,2,3,"a","b","c","d"]

Until and While conditions

You can control the For each...End for each execution by adding an Until or a While condition to the loop. When an Until(condition) statement is associated to the loop, the iteration will stop as soon as the condition is evaluated to True, whereas when is case of a While(condition) statement, the iteration will stop when the condition is first evaluated to False.

You can pass either keyword depending on your needs:

The Until condition is tested at the end of each iteration, so if the Expression is not empty or null, the loop will be executed at least once.
The While condition is tested at the beginning of each iteration, so according to the condition result, the loop may not be executed at all.

Example

 $colNum:=New collection(1;2;3;4;5;6;7;8;9;10)
 
 $total:=0
 For each($num;$colNum)While($total<30) //tested at the beginning
    $total:=$total+$num
 End for each
 ALERT(String($total)) //$total = 36 (1+2+3+4+5+6+7+8)
 
 $total:=1000
 For each($num;$colNum)Until($total>30) //tested at the end
    $total:=$total+$num
 End for each
 ALERT(String($total)) //$total = 1001 (1000+1)

While...End while​

Example​

Repeat...Until​

Example​

For...End for​

Basic examples​

Decrementing variable counter​

Incrementing the counter variable by more than one​

Comparing looping structures​

Optimizing the execution of the For...End for loops​

Nested For...End for looping structures​

For each...End for each​

Loop through collections​

Example​

Loop through entity selections​

Example​

Loop through object properties​

Example​

begin / end parameters​

Example​

Until and While conditions​

Example​

While...End while

Example

Repeat...Until

Example

For...End for

Basic examples

Decrementing variable counter

Incrementing the counter variable by more than one

Comparing looping structures

Optimizing the execution of the For...End for loops

Nested For...End for looping structures

For each...End for each

Loop through collections

Example

Loop through entity selections

Example

Loop through object properties

Example

begin / end parameters

Example

Until and While conditions

Example