Control flow

Many control commands have been devised for programming languages. This section describes notable constructs; it is organized by functionality.

A function provides for control flow in that when called, execution jumps to the start of the function's code and when it completes, control returns the calling point. In the following C code, control jumps from line 6 to 2 in order to call function then keyword, this can be called an if statement.

if-then-else

Like if-then, but with a second action to be performed if the condition is false. In a language that does not include the when and if(a>0){ puts("yes"); }else{ puts("no"); }

In bash:

(princ (if(pluspa) "yes" "no"))

Multiway branch

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A multiway branch jumps control based on matching values. There is usually a provision for a default action if no match is found. A switch statement 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.

The following Pascal code shows a relatively simple switch statement. Pascal uses the casesomeCharof 'a':actionOnA; 'x':actionOnX; 'y','z':actionOnYandZ; elseactionOnNoMatch; end;

In Ada:

actionOnA;\n when 'x' => actionOnX;\n when 'y' | 'z' => actionOnYandZ;\n when others => actionOnNoMatch;\nend;\n"}}'>case someChar is when 'a' => actionOnA; when 'x' => actionOnX; when 'y' | 'z' => actionOnYandZ; when others => actionOnNoMatch; end;

In C:

(casesome-char ((#\a)action-on-a) ((#\x)action-on-x) ((#\y#\z)action-on-y-and-z) (elseaction-on-no-match))

In Fortran:

In a loop, a sequence of statements—frequently called the loop body—gets re-executed (at least in part) after a conditional branch or jump to a previous step in the sequence. The number of times a loop gets executed usually depends on runtime state, for example: the body is executed once for each item of a collection (definite iteration), until a condition is met (indefinite iteration). Some loops, on the other hand, may repeat infinitely. It is possible for a loop to be written as an infinite loop but for loop exit to be controlled by a test within the loop body that transfers control outside the loop. Early exit from a loop may be supported via a break statement.

A loop inside the loop body of another loop is called a nested loop.

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

Numeric

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FORI=1TON xxx NEXTI

Example in Pascal:

repeat xxx untiltest;

Example in C family of pre-test:

for keyword, the three-part C-style loop is a condition-based construct, not a numeric-based one. The second part, the condition, is evaluated before each loop, so the loop is pre-test. The first part is a place to initialize state, and the third part is for incrementing for the next iteration, but both aspects can be performed elsewhere. The following C code implements the logic of a numeric loop that iterates for i from 0 to n-1.

Some loop constructs enumerate the items of a collection; iterating for each item.

Example in Smalltalk:

foreach (item; myCollection) { xxx }

Example in TCL:

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

Example in C#:

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.

Infinite

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n⁠ times, but the second part is only executed loop Get(X); if X = 0 then exit; end if; DoSomething(X); end loop;

A more idiomatic style uses n, there exists a program containing a multi-level break of depth while(statement_1,statement_2,condition){ statement_a; statement_b; }

While legal, this is marginal, and it is primarily used, if at all, only for short modify-then-test cases, as in:

0) {\n // ...\n}\n"}}'>while(read_string(s),strlen(s)>0){ // ... }

Loop variants and invariants

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

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 sublanguage

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

Loop system cross-reference table

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Programming language	Conditional			Loop				Early exit	Loop continuation	Redo	Retry	Correctness facilities
	Begin	Middle	End	Numeric	Collection	General	Infinite					Variant	Invariant
Ada	Yes	No	No
APL	Yes	Yes		No
C	Yes	No	deep nested	No	No		Yes		No	No
C#	Yes	Yes	deep nested	No	No	No	deep nested	Yes	builtin only Yes	Yes	Yes	Yesdeep nested	No		No	No		Yes
F#	No	No		Yes	Yes	No	No	No	No	deep nested	No
Fortran 95 and later	No	No	deep nested	Yes	Yes	Yes	No
Haskell	No	No		Yes	No Yes	deep nested		Yes	No Yes	deep nested	No
Kotlin	Yes	No	deep nested	No	Yes	No	Yes	No
OCaml	No	No		No No	builtin only		Yes	No Yes No	No	No
Perl	Yes		No	Yes
Python	No	Yes	deep nested	No	No	No	one level per type of loop	No	No Yes	No one level No
Ruby	Yes	No		Yes	No		Yes	No	No
Standard ML	No	No		Yes	No	No	No	No
Visual Basic .NET	Yes	No	one level per type of loop	No	No		No	No	No
Zig	No	No	deep nested	No	There is no special construct, since the while function can be used for this. Numeric looping is effected by iteration across an integer interval; early exit by including an additional condition for exit. Requires loop variants to be integers; transfinite variants are not supported. Eiffel: Why loop variants are integers Common Lisp predates the concept of generic collection type. doSomething(), then control jumps from line 2 to 4 and the message "Caught runtime_error" is printed and then control jumps to after the throw, exception to introduce an exception handler and View original on Wikipedia | Back | FazBrowse Home