Control flow

Not to be confused with Flow control.

In computer science, control flow (or alternatively, flow of control) refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated.

Within an imperative programming language, a control flow statement is a statement whose execution results in a choice being made as to which of two or more paths should be followed. For non-strict functional languages, functions and language constructs exist to achieve the same result, but they are not necessarily called control flow statements.

The kinds of control flow statements supported by different languages vary, but can be categorized by their effect:

Interrupts and signals are low-level mechanisms that can alter the flow of control in a way similar to a subroutine, but usually occur as a response to some external stimulus or event (that can occur asynchronously), rather than execution of an 'in-line' control flow statement. Self-modifying code can also be used to affect control flow through its side effects, but usually does not involve an explicit control flow statement (an exception being the ALTER verb in COBOL).

At the level of machine or assembly language, control flow instructions usually work by altering the program counter. For some CPUs the only control flow instructions available are conditional or unconditional branch instructions (also called jumps).

Contents

Primitives

Labels

Main article: Label (programming language)

A label is an explicit name or number assigned to a fixed position within the source code, and which may be referenced by control flow statements appearing elsewhere in the source code. Other than marking a position within the source code a label has no effect.

Line numbers are an alternative to a named label (and used in some languages such as Fortran and BASIC), that are whole numbers placed at the beginning of each line of text within the source code. Languages which use these often impose the constraint that the line numbers must increase in value in each subsequent line, but may not require that they be consecutive. For example, in BASIC:

10 LET X = 3
20 PRINT X

In other languages such as C and Ada a label is an identifier, usually appearing at the beginning of a line and immediately followed by a colon. For example, in C:

Success: printf ("The operation was successful.\n");

The Algol 60 language allowed both whole numbers and identifiers as labels (both attached by colons to the following statement), but few if any other variants of Algol allowed whole numbers.

Goto

Main article: GOTO

The goto statement (a combination of the English words go and to, and pronounced accordingly) is the most basic form of unconditional transfer of control.

Although the keyword may either be in upper or lower case depending on the language, it is usually written as:

   goto label

The effect of a goto statement is to cause the next statement to be executed to be the statement appearing at (or immediately after) the indicated label.

Goto statements have been considered harmful by many computer scientists, notably Dijkstra.

Subroutines

Main article: Subroutine

The terminology for subroutines varies; they may alternatively be known as routines, procedures, functions (especially if they return results) or methods (especially if they belong to classes or type classes).

In the 1950s, computer memories were very small by current standards so subroutines were used primarily to reduce program size; a piece of code was written once and then used many times from various other places in the program.

Nowadays, subroutines are more frequently used to help make a program that is more structured, e.g. by isolating some particular algorithm or hiding some particular data access method. If many programmers are working on a single program, subroutines are one kind of modularity that can help split up the work.

Minimal structured control flow

See also: Structured program theorem

In May 1966, Böhm and Jacopini published an article[1] in Communications of the ACM which showed that any program with gotos could be transformed into a goto-free form involving only choice (IF THEN ELSE) and loops (WHILE condition DO xxx), possibly with duplicated code and/or the addition of Boolean variables (true/false flags). Later authors have shown that choice can be replaced by loops (and yet more Boolean variables).

The fact that such minimalism is possible does not necessarily mean that it is desirable; after all, computers theoretically only need one machine instruction (subtract one number from another and branch if the result is negative), but practical computers have dozens or even hundreds of machine instructions.

What Böhm and Jacopini's article showed was that all programs could be goto-free. Other research showed that control structures with one entry and one exit were much easier to understand than any other form, primarily because they could be used anywhere as a statement without disrupting the control flow. In other words, they were composable. (Later developments, such as non-strict programming languages - and more recently, composable software transactions - have continued this line of thought, making components of programs even more freely composable.)

Control structures in practice

Most programming languages with control structures have an initial keyword which indicates the type of control structure involved. Languages then divide as to whether or not control structures have a final keyword.

Choice

If-then-(else) statements

Main article: Conditional (programming)

Conditional expressions and conditional constructs are features of a programming language which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false.

Pascal: C: Shell script: Python: 
if a > 0 then begin
      writeln("yes")
end else begin
      writeln("no")
end

if (a > 0) { 
      printf("yes");
} else {
      printf("no");
}

if [ $a -gt 0 ] 
then
      echo "yes"
else
      echo "no"
fi

if a > 0: 
      print "yes"
else:
      print "no"

Less common variations include:-

Case and switch statements

Main article: Switch statement

Switch statements (in some languages, case statements or multiway branches) compare a given value with specified constants and take action according to the first constant to match. There is usually a provision for a default action ('else','otherwise') to be taken if no match succeeds. Switch statements can allow compiler optimizations, such as lookup tables. In dynamic languages, the cases may not be limited to constant expressions, and might extend to pattern matching, as in the shell script example on the right, where the '*)' implements the default case as a regular expression matching any string. Case logic can also be implemented in functional form, as in SQL's decode statement.

Pascal: C: Shell script: 
case someChar of
  'a': actionOnA;
  'x': actionOnX;
  'y','z':actionOnYandZ;
  else actionOnNoMatch;
end;

switch (someChar) {
  case 'a': actionOnA; break;
  case 'x': actionOnX; break;
  case 'y':
  case 'z': actionOnYandZ; break;
  default: actionOnNoMatch;
}

case $someChar in 
   a)    actionOnA ;;
   x)    actionOnX ;;
   [yz]) actionOnYandZ ;;
  *)     actionOnNoMatch  ;;
esac

Loops

A loop is a sequence of statements which is specified once but which may be carried out several times in succession. The code "inside" the loop (the body of the loop, shown below as xxx) is obeyed a specified number of times, or once for each of a collection of items, or until some condition is met, or indefinitely.

In functional programming languages, such as Haskell and Scheme, loops can be expressed by using recursion or fixed point iteration rather than explicit looping constructs. Tail recursion is a special case of recursion which can be easily transformed to iteration.

Count-controlled loops

Main article: For loop
See also: Loop counter

Most programming languages have constructions for repeating a loop a certain number of times. Note that if N is less than 1 in these examples then the language may specify that the body is skipped completely, or that the body is executed just once with N = 1. In most cases counting can go downwards instead of upwards and step sizes other than 1 can be used.

   FOR I = 1 TO N            for I := 1 to N do begin
       xxx                       xxx
   NEXT I                    end;

   DO I = 1,N                for ( I=1; I<=N; ++I ) {
       xxx                       xxx
   END DO                    }

In many programming languages, only integers can be reliably used in a count-controlled loop. Floating-point numbers are represented imprecisely due to hardware constraints, so a loop such as

   for X := 0.1 step 0.1 to 1.0 do

might be repeated 9 or 10 times, depending on rounding errors and/or the hardware and/or the compiler version. Furthermore, if the increment of X occurs by repeated addition, accumulated rounding errors may mean that the value of X in each iteration can differ quite significantly from the expected sequence 0.1, 0.2, 0.3, ..., 1.0.

Condition-controlled loops

Main article: While loop
See also: Do-while loop

Most programming languages have constructions for repeating a loop until some condition changes. Note that some variations place the test at the start of the loop, while others have the test at the end of the loop. In the former case the body may be skipped completely, while in the latter case the body is always executed at least once.

   DO WHILE (test)           repeat 
       xxx                       xxx 
   LOOP                      until test;

   while (test) {            do
       xxx                       xxx
   }                         while (test);

Collection-controlled loops

Main article: Foreach

Several programming languages (e.g. Ada, D, Smalltalk, Perl, Object Pascal, Java, C#, Mythryl, Visual Basic, Ruby, Python, JavaScript, Fortran 95 and later) have special constructs which allow implicitly looping through all elements of an array, or all members of a set or collection.

   someCollection do: [:eachElement |xxx].
   
   for Item in Collection do begin xxx end;

   foreach (item; myCollection) { xxx }

   foreach someArray { xxx }

   Collection<String> coll; for (String s : coll) {}

   foreach (string s in myStringCollection) { xxx }

   $someCollection | ForEach-Object { $_ }
   
   forall ( index = first:last:step... )

General iteration

General iteration constructs such as C's for statement and Common Lisp's do form can be used to express any of the above sorts of loops, as well as others -- such as looping over a number of collections in parallel. Where a more specific looping construct can be used, it is usually preferred over the general iteration construct, since it often makes the purpose of the expression more clear.

Infinite loops

Infinite loops are used to assure a program segment loops forever or until an exceptional condition arises, such as an error. For instance, an event-driven program (such as a server) should loop forever handling events as they occur, only stopping when the process is terminated by an operator.

Often, an infinite loop is unintentionally created by a programming error in a condition-controlled loop, wherein the loop condition uses variables that never change within the loop.

Continuation with next iteration

Sometimes within the body of a loop there is a desire to skip the remainder of the loop body and continue with the next iteration of the loop. Some languages provide a statement such as continue (most languages), skip, or next (Perl and Ruby), which will do this. The effect is to prematurely terminate the innermost loop body and then resume as normal with the next iteration. If the iteration is the last one in the loop, the effect is to terminate the entire loop early.

Redo current iteration

Some languages, like Perl and Ruby, have a redo statement that restarts the current iteration from the beginning.

Restart loop

Ruby has a retry statement that restarts the entire loop from the initial iteration.

Early exit from loops

When using a count-controlled loop to search through a table, it might be desirable to stop searching as soon as the required item is found. Some programming languages provide a statement such as break (most languages), exit, or last (Perl), whose effect is to terminate the current loop immediately and transfer control to the statement immediately following that loop. One can also return out of a subroutine executing the looped statements, breaking out of both the nested loop and the subroutine. Things can get a bit messy if searching a multi-dimensional table using nested loops (see #Proposed control structures below).

The following example is done in Ada which supports both early exit from loops and loops with test in the middle. Both features are very similar and comparing both code snippets will show the difference: early exit needs to be combined with an if statement while a condition in the middle is a self contained construct.

with Ada.Text IO;
with Ada.Integer Text IO;
 
procedure Print_Squares is 
    X : Integer;
begin
    Read_Data : loop
        Ada.Integer Text IO.Get(X);
    exit Read_Data when X = 0;
        Ada.Text IO.Put (X * X);
        Ada.Text IO.New_Line;
    end loop Read_Data;
end Print_Squares;

Python supports conditional execution of code depending on whether a loop was exited early (with a break statement) or not by using a else-clause with the loop. For example,

for n in set_of_numbers:
    if isprime(n):
        print "Set contains a prime number"
        break
else:
    print "Set did not contain any prime numbers"

Note that the else clause in the above example is attached to the for statement, and not the inner if statement. Both Python's for and while loops support such an else clause, which is executed only if early exit of the loop has not occurred.

Loop variants and invariants

Loop variants and loop invariants are used to express correctness of loops.[2]

In practical terms, a loop variant is an integer expression which has an initial non-negative value. The variant's value must decrease during each loop iteration but must never become negative during the correct execution of the loop. Loop variants are used to guarantee that loops will terminate.

A loop invariant is an assertion which must be true before the first loop iteration and remain true after each iteration. This implies that when a loop terminates correctly, both the exit condition and the loop invariant are satisfied. Loop invariants are used to monitor specific properties of a loop during successive iterations.

Some programming languages, such as Eiffel contain native support for loop variants and invariants. In other cases, support is an add-on, such as the Java Modeling Language's specification for loop statements in Java.

Loop system cross reference table

Programming language conditional loop early exit continuation redo retry correctness facilities
begin middle end count collection general infinite [1] variant invariant
Ada Yes Yes Yes Yes arrays No Yes deep nested No
C Yes No Yes No [2] No Yes No deep nested [3] deep nested [3] No
C++ Yes No Yes No [2] Yes [9] Yes No deep nested [3] deep nested [3] No
C# Yes No Yes No [2] Yes Yes No deep nested [3] deep nested [3]
Common Lisp Yes Yes Yes Yes Yes Yes Yes deep nested No
Eiffel Yes No No Yes [10] Yes Yes No one level [10] No No No [11] integer only [13] Yes
F# Yes No No Yes Yes No No No [6] No No
FORTRAN 77 Yes No No Yes No No No one level Yes
Fortran 90 Yes No No Yes No No Yes deep nested Yes
Fortran 95 and later Yes No No Yes arrays No Yes deep nested Yes
Haskell No No No No Yes No Yes No [6] No No
Java Yes No Yes No [2] Yes Yes No deep nested deep nested No non-native [12] non-native [12]
JavaScript Yes No Yes No [2] Yes Yes No deep nested deep nested No
OCaml Yes No No Yes arrays,lists No No No [6] No No
PHP Yes No Yes No [2] [5] Yes [4] Yes No deep nested deep nested No
Perl Yes No Yes No [2] [5] Yes Yes No deep nested deep nested Yes
Python Yes No No No [5] Yes No No deep nested [6] deep nested [6] No
REBOL No [7] Yes Yes Yes Yes No [8] Yes one level [6] No No
Ruby Yes No Yes Yes Yes No Yes deep nested [6] deep nested [6] Yes Yes
Standard ML Yes No No No arrays,lists No No No [6] No No
Visual Basic .NET Yes No Yes Yes Yes No Yes one level per type of loop one level per type of loop
Windows PowerShell Yes No Yes No [2] Yes Yes No ? Yes
  1. a while (true) does not count as an infinite loop for this purpose, because it is not a dedicated language structure.
  2. a b c d e f g h C's for (init; test; increment) loop is a general loop construct, not specifically a counting one, although it is often used for that.
  3. a b c Deep breaks may be accomplished in C, C++ and C# through the use of labels and gotos.
  4. a Iteration over objects was added in PHP 5.
  5. a b c A counting loop can be simulated by iterating over an incrementing list or generator, for instance, Python's range().
  6. a b c d e Deep breaks may be accomplished through the use of exception handling.
  7. a There is no special construct, since the while function can be used for this.
  8. a There is no special construct, but users can define general loop functions.
  9. a The C++11 standard introduced the range-based for. In the STL there is an std::for_each template function which can iterate on STL containers and call an unary function for each element.[3] The functionality also can be constructed as macro on these containers.[4]
  10. a Count controlled looping is effected by iteration across an integer interval; early exit by including an additional condition for exit.
  11. a Eiffel supports a reserved word retry, however it is used in exception handling, not loop control.
  12. a Requires Java Modeling Language (JML) behavioral interface specification language.
  13. a Requires loop variants to be integers; transfinite variants are not supported. [1]

Structured non-local control flow

Many programming languages, particularly those which favor more dynamic styles of programming, offer constructs for non-local control flow. These cause the flow of execution to jump out of a given context and resume at some predeclared point. Conditions, exceptions, and continuations are three common sorts of non-local control constructs.

Conditions

PL/I has some 22 standard conditions (e.g. ZERODIVIDE SUBSCRIPTRANGE ENDFILE) which can be RAISEd and which can be intercepted by: ON condition action; Programmers can also define and use their own named conditions.

Like the unstructured if only one statement can be specified so in many cases a GOTO is needed to decide where flow of control should resume.

Unfortunately, some implementations had a substantial overhead in both space and time (especially SUBSCRIPTRANGE), so many programmers tried to avoid using conditions.

Common Syntax examples:

 ON condition GOTO label

Exceptions

Main article: Exception handling

Modern languages have a structured construct for exception handling which does not rely on the use of GOTO:

try {
    xxx1                                  // Somewhere in here
    xxx2                                  //     use: '''throw''' someValue;
    xxx3
} catch (someClass& someId) {             // catch value of someClass
    actionForSomeClass 
} catch (someType& anotherId) {           // catch value of someType
    actionForSomeType
} catch (...) {                           // catch anything not already caught
    actionForAnythingElse
}

Any number and variety of catch clauses can be used above. In D, Java, C#, and Python a finally clause can be added to the try construct. No matter how control leaves the try the code inside the finally clause is guaranteed to execute. This is useful when writing code that must relinquish an expensive resource (such as an opened file or a database connection) when finished processing:

FileStream stm = null;                    // C# example
try {
    stm = new FileStream ("logfile.txt", FileMode.Create);
    return ProcessStuff(stm);             // may throw an exception
} finally {
    if (stm != null)
        stm. Close();
}

Since this pattern is fairly common, C# has a special syntax:

using (FileStream stm = new FileStream ("logfile.txt", FileMode.Create)) {
    return ProcessStuff(stm);             // may throw an exception
}

Upon leaving the using-block, the compiler guarantees that the stm object is released. Python's with statement and Ruby's block argument to File.open are used to similar effect.

All these languages define standard exceptions and the circumstances under which they are thrown. Users can throw exceptions of their own (in fact C++ allows users to throw and catch almost any type).

If there is no catch matching a particular throw, then control percolates back through subroutine calls and/or nested blocks until a matching catch is found or until the end of the main program is reached, at which point the program is forcibly stopped with a suitable error message.

The AppleScript scripting programming language provides several pieces of information to a "try" block:

try
    set myNumber to myNumber / 0
on error e  number n  from f  to t  partial result pr
    if ( e = "Can't divide by zero" ) then display dialog "You must not do that"
end try

Continuations

Main article: Continuation

Non-local control flow cross reference

Programming language conditions exceptions
Ada No Yes
C No No
C++ No Yes
C# No Yes
D No Yes
Eiffel No Yes
Haskell No Yes
Java No Yes
Mythryl Yes Yes
Objective-C No Yes
PHP No Yes
PL/I Yes No
Python No Yes
REBOL Yes Yes
Ruby No Yes
Visual Basic .NET Yes Yes
Windows PowerShell No Yes

Proposed control structures

In a spoof Datamation article[5] in 1973, R. Lawrence Clark suggested that the GOTO statement could be replaced by the COMEFROM statement, and provides some entertaining examples. This was actually implemented in INTERCAL, a deliberately esoteric programming language language.

In his 1974 article "Structured Programming with go to Statements",[6] Donald Knuth identified two situations which were not covered by the control structures listed above, and gave examples of control structures which could handle these situations. Despite their utility, these constructions have not yet found their way into mainstream programming languages.

Loop with test in the middle

The following was proposed by Dahl in 1972:[7]

   loop                           loop
       xxx1                           read(char);
   while test;                    while not atEndOfFile;
       xxx2                           write(char);
   repeat;                        repeat;

If xxx1 is omitted we get a loop with the test at the top. If xxx2 is omitted we get a loop with the test at the bottom. If while is omitted we get an infinite loop. Hence this single construction can replace several constructions in most programming languages. A possible variant is to allow more than one while test; within the loop, but the use of exitwhen (see next section) appears to cover this case better.

Languages lacking this construct generally emulate it using an equivalent infinite-loop-with-break idiom:

while (true) {
    xxx1
    if (not test)
        break
    xxx2
}

In Ada, the above loop construct (loop-while-repeat) can be represented using a standard infinite loop (loop - end loop) that has an exit when clause in the middle (not to be confused with the exitwhen statement in the following section).

with Ada.Text_IO;
with Ada.Integer_Text_IO;
 
procedure Print_Squares is 
    X : Integer;
begin
    Read_Data : loop
        Ada.Integer_Text_IO.Get(X);
    exit Read_Data when X = 0;
        Ada.Text IO.Put (X * X);
        Ada.Text IO.New_Line;
    end loop Read_Data;
end Print_Squares;

Naming a loop (like Read_Data in this example) is optional but permits leaving the outer loop of several nested loops.

Multiple early exit/exit from nested loops

This was proposed by Zahn in 1974.[8] A modified version is presented here.

   exitwhen EventA or EventB or EventC;
       xxx
   exits
       EventA: actionA
       EventB: actionB
       EventC: actionC
   endexit;

exitwhen is used to specify the events which may occur within xxx, their occurrence is indicated by using the name of the event as a statement. When some event does occur, the relevant action is carried out, and then control passes just after endexit. This construction provides a very clear separation between determining that some situation applies, and the action to be taken for that situation.

exitwhen is conceptually similar to exception handling, and exceptions or similar constructs are used for this purpose in many languages.

The following simple example involves searching a two-dimensional table for a particular item.

   exitwhen found or missing;
       for I := 1 to N do
           for J := 1 to M do
               if table[I,J] = target then found;
       missing;
   exits
       found:   print ("item is in table");
       missing: print ("item is not in table");
   endexit;

See also

References

  1. ^ Böhm, Jacopini. "Flow diagrams, turing machines and languages with only two formation rules" Comm. ACM, 9(5):366-371, May 1966.
  2. ^ Meyer, Bertrand (1991). Eiffel: The Language. Prentice Hall. pp. 129–131. 
  3. ^ for_each. Sgi.com. Retrieved on 2010-11-09.
  4. ^ Chapter 1. Boost.Foreach. Boost-sandbox.sourceforge.net (2009-12-19). Retrieved on 2010-11-09.
  5. ^ We don't know where to GOTO if we don't know where we've COME FROM. This (spoof) linguistic innovation lives up to all expectations. By R. Lawrence Clark* From DATAMATION, December, 1973
  6. ^ Knuth, Donald E. "Structured Programming with go to Statements" ACM Computing Surveys 6(4):261-301, December 1974.
  7. ^ Dahl & Dijkstra & Hoare, "Structured Programming" Academic Press, 1972.
  8. ^ Zahn, C. T. "A control statement for natural top-down structured programming" presented at Symposium on Programming Languages, Paris, 1974.
  • Hoare, C. A. R. "Partition: Algorithm 63," "Quicksort: Algorithm 64," and "Find: Algorithm 65." Comm. ACM 4, 321-322, 1961.

External links