In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an imperative programming language from a declarative programming language.

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

A set of statements is in turn generally structured as a block, which in addition to grouping, also defines a lexical scope.

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.

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

Categories

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

• Continuation at a different statement (unconditional branch or jump)
• Executing a set of statements only if some condition is met (choice - i.e., conditional branch)
• Executing a set of statements zero or more times, until some condition is met (i.e., loop - the same as conditional branch)
• Executing a set of distant statements, after which the flow of control usually returns (subroutines, coroutines, and continuations)
• Stopping the program, preventing any further execution (unconditional halt)

Primitives

Labels

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. A label marks a position within source code and has no other effect.

Line numbers are an alternative to a named label used in some languages (such as BASIC). They are whole numbers placed at the start of each line of text in the source code. Languages which use these often impose the constraint that the line numbers must increase in value in each following 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 start of a line and immediately followed by a colon. For example, in C:

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

The language ALGOL 60 allowed both whole numbers and identifiers as labels (both linked by colons to the following statement), but few if any other ALGOL variants allowed whole numbers. Early Fortran compilers only allowed whole numbers as labels. Beginning with Fortran-90, alphanumeric labels have also been allowed.

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

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 mainly^{[citation needed]} to reduce program size. A piece of code was written once and then used many times from various other places in a program.

Today, subroutines are more often used to help make a program more structured, e.g., by isolating some algorithm or hiding some data access method. If many programmers are working on one program, subroutines are one kind of modularity that can help divide the work.

Sequence

In structured programming, the ordered sequencing of successive commands is considered one of the basic control structures, which is used as a building block for programs alongside iteration, recursion and choice.

Minimal structured control flow

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 showed that choice can be replaced by loops (and yet more Boolean variables).

That such minimalism is possible does not mean that it is necessarily desirable; after all, computers theoretically need only 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,^{[citation needed]} mainly 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 strategy, making components of programs even more freely composable.)

Some academics took a purist approach to the Böhm–Jacopini result and argued that even instructions like break and return from the middle of loops are bad practice as they are not needed in the Böhm–Jacopini proof, and thus they advocated that all loops should have a single exit point. This purist approach is embodied in the language Pascal (designed in 1968–1969), which up to the mid-1990s was the preferred tool for teaching introductory programming in academia.^[2] The direct application of the Böhm–Jacopini theorem may result in additional local variables being introduced in the structured chart, and may also result in some code duplication.^[3] Pascal is affected by both of these problems and according to empirical studies cited by Eric S. Roberts, student programmers had difficulty formulating correct solutions in Pascal for several simple problems, including writing a function for searching an element in an array. A 1980 study by Henry Shapiro cited by Roberts found that using only the Pascal-provided control structures, the correct solution was given by only 20% of the subjects, while no subject wrote incorrect code for this problem if allowed to write a return from the middle of a loop.^[2]

Control structures in practice

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

• No final keyword: ALGOL 60, C, C++, Haskell, Java, Pascal, Perl, PHP, PL/I, Python, PowerShell. Such languages need some way of grouping statements together:
  • ALGOL 60 and Pascal: begin ... end
  • C, C++, Java, Perl, PHP, and PowerShell: curly brackets { ... }
  • PL/I: DO ... END
  • Python: uses indent level (see Off-side rule)
  • Haskell: either indent level or curly brackets can be used, and they can be freely mixed
  • Lua: uses do ... end
• Final keyword: Ada, APL, ALGOL 68, Modula-2, Fortran 77, Mythryl, Visual Basic. The forms of the final keyword vary:
  • Ada: final keyword is end + space + initial keyword e.g., if ... end if, loop ... end loop
  • APL: final keyword is :End optionally + initial keyword, e.g., :If ... :End or :If ... :EndIf, Select ... :End or :Select ... :EndSelect, however, if adding an end condition, the end keyword becomes :Until
  • ALGOL 68, Mythryl: initial keyword spelled backwards e.g., if ... fi, case ... esac
  • Fortran 77: final keyword is END + initial keyword e.g., IF ... ENDIF, DO ... ENDDO
  • Modula-2: same final keyword END for everything
  • Visual Basic: every control structure has its own keyword. If ... End If; For ... Next; Do ... Loop; While ... Wend

Choice

If-then-(else) statements

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.

• IF..GOTO. A form found in unstructured languages, mimicking a typical machine code instruction, would jump to (GOTO) a label or line number when the condition was met.
• IF..THEN..(ENDIF). Rather than being restricted to a jump, any simple statement, or nested block, could follow the THEN key keyword. This a structured form.
• IF..THEN..ELSE..(ENDIF). As above, but with a second action to be performed if the condition is false. This is one of the most common forms, with many variations. Some require a terminal ENDIF, others do not. C and related languages do not require a terminal keyword, or a 'then', but do require parentheses around the condition.
• Conditional statements can be and often are nested inside other conditional statements. Some languages allow ELSE and IF to be combined into ELSEIF, avoiding the need to have a series of ENDIF or other final statements at the end of a compound statement.

if a > 0 then
  writeln("yes")
else
  writeln("no");

if a > 0 then
      Put_Line("yes");
else
      Put_Line("no");
end if;

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

if ; then
      echo "yes"
else
      echo "no"
fi

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

(princ
  (if (plusp a)
      "yes"
      "no"))

Less common variations include:

• Some languages, such as Fortran, have a three-way or arithmetic if, testing whether a numeric value is positive, negative or zero.
• Some languages have a functional form of an if statement, for instance Lisp's cond.
• Some languages have an operator form of an if statement, such as C's ternary operator.
• Perl supplements a C-style if with when and unless.
• Smalltalk uses ifTrue and ifFalse messages to implement conditionals, rather than any fundamental language construct.

Case and switch statements

Switch statements (or 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 glob matching any string. Case logic can also be implemented in functional form, as in SQL's decode statement.

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

case someChar is
  when 'a' => actionOnA;
  when 'x' => actionOnX;
  when 'y' | 'z' => actionOnYandZ;
  when others => 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 ;;
   ) actionOnYandZ ;;
   *)    actionOnNoMatch  ;;
esac

(case some-char
  ((#\a)     action-on-a)
  ((#\x)     action-on-x)
  ((#\y #\z) action-on-y-and-z)
  (else      action-on-no-match))

select case (someChar)
  case ('a')
    actionOnA
  case ('x')
    actionOnX
  case ('y','z')
    actionOnYandZ
  case default
    actionOnNoMatch
end select

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. When one of those items is itself also a loop, it is called a "nested loop".^[4][5][6]

In functional programming languages, such as Haskell and Scheme, both recursive and iterative processes are expressed with tail recursive procedures instead of looping constructs that are syntactic.

Count-controlled loops

Most programming languages have constructions for repeating a loop a certain number of times. In most cases counting can go downwards instead of upwards and step sizes other than 1 can be used.

FOR I = 1 TO N
   xxx
NEXT I

for I := 1 to N do begin
   xxx
end;

DO I = 1,N
    xxx
END DO

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

In these examples, if N < 1 then the body of loop may execute once (with I having value 1) or not at all, depending on the programming language.

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

Most programming languages have constructions for repeating a loop until some condition changes. Some variations test the condition at the start of the loop; others test it at the end. If the test is at the start, the body may be skipped completely; if it is at the end, the body is always executed at least once.

DO WHILE (test)
    xxx
LOOP

repeat
    xxx
until test;

while (test) {
    xxx
}

do
    xxx
while (test);

A control break is a value change detection method used within ordinary loops to trigger processing for groups of values. Values are monitored within the loop and a change diverts program flow to the handling of the group event associated with them.

   DO UNTIL (End-of-File)
      IF new-zipcode <> current-zipcode
         display_tally(current-zipcode, zipcount)
         
         current-zipcode = new-zipcode
         zipcount = 0
      ENDIF
      
      zipcount++
   LOOP

Collection-controlled loops

Several programming languages (e.g., Ada, D, C++11, Smalltalk, PHP, Perl, Object Pascal, Java, C#, MATLAB, Visual Basic, Ruby, Python, JavaScript, Fortran 95 and later) have special constructs which allow implicit looping through all elements of an array, or all members of a set or collection.

   someCollection do: .

   for Item in Collection do begin xxx end;

   foreach (item; myCollection) { xxx }

   foreach someArray { xxx }

   foreach ($someArray as $k => $v) { xxx }

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

   foreach (string s in myStringCollection) { xxx }

   someCollection | ForEach-Object { $_ }

   forall ( index = first:last:step... )

Scala has for-expressions, which generalise collection-controlled loops, and also support other uses, such as asynchronous programming. Haskell has do-expressions and comprehensions, which together provide similar function to for-expressions in Scala.

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, and others, such as looping over some 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 clearer.

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.

Infinite loops can be implemented using other control flow constructs. Most commonly, in unstructured programming this is jump back up (goto), while in structured programming this is an indefinite loop (while loop) set to never end, either by omitting the condition or explicitly setting it to true, as while (true) .... Some languages have special constructs for infinite loops, typically by omitting the condition from an indefinite loop. Examples include Ada (loop ... end loop),^[7] Fortran (DO ... END DO), Go (for { ... }), and Ruby (loop do ... end).

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,^[8] cycle (Fortran), 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^[9] and Ruby,^[10] have a redo statement that restarts the current iteration from the start.

Restart loop

Ruby has a retry statement that restarts the entire loop from the initial iteration.^[11]

Early exit from loopsedit

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 (Visual Basic), or last (Perl), which effect is to terminate the current loop immediately, and transfer control to the statement immediately after that loop. Another term for early-exit loops is loop-and-a-half.

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 must 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 an 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")

The else clause in the above example is linked 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.

Some languages support breaking out of nested loops; in theory circles, these are called multi-level breaks. One common use example is searching a multi-dimensional table. This can be done either via multilevel breaks (break out of N levels), as in bash^[12] and PHP,^[13] or via labeled breaks (break out and continue at given label), as in Java and Perl.^[14] Alternatives to multilevel breaks include single breaks, together with a state variable which is tested to break out another level; exceptions, which are caught at the level being broken out to; placing the nested loops in a function and using return to effect termination of the entire nested loop; or using a label and a goto statement. C does not include a multilevel break, and the usual alternative is to use a goto to implement a labeled break.^[15] Python does not have a multilevel break or continue – this was proposed in PEP 3136, and rejected on the basis that the added complexity was not worth the rare legitimate use.^[16]

The notion of multi-level breaks is of some interest in theoretical computer science, because it gives rise to what is today called the Kosaraju hierarchy.^[17] In 1973 S. Rao Kosaraju refined the structured program theorem by proving that it is possible to avoid adding additional variables in structured programming, as long as arbitrary-depth, multi-level breaks from loops are allowed.^[18] Furthermore, Kosaraju proved that a strict hierarchy of programs exists: for every integer n, there exists a program containing a multi-level break of depth n that cannot be rewritten as a program with multi-level breaks of depth less than n without introducing added variables.^[17]

One can also return out of a subroutine executing the looped statements, breaking out of both the nested loop and the subroutine. There are other proposed control structures for multiple breaks, but these are generally implemented as exceptions instead.

In his 2004 textbook, David Watt uses Tennent's notion of sequencer to explain the similarity between multi-level breaks and return statements. Watt notes that a class of sequencers known as escape sequencers, defined as "sequencer that terminates execution of a textually enclosing command or procedure", encompasses both breaks from loops (including multi-level breaks) and return statements. As commonly implemented, however, return sequencers may also carry a (return) value, whereas the break sequencer as implemented in contemporary languages usually cannot.^[19]

Loop variants and invariantsedit

Loop variants and loop invariants are used to express correctness of loops.^[20]

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 sublanguageedit

Some Lisp dialects provide an extensive sublanguage for describing Loops. An early example can be found in Conversional Lisp of Interlisp. Common Lisp^[21] provides a Loop macro which implements such a sublanguage.

Loop system cross-reference tableedit

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