Name: return Syntax: RETURN. Description: Return from subroutine. The return command is used with a matching gosub command, to return program flow back to the main program at the end of the sub procedure. A In my experience, the following: Output ports for driving LEDs, etc. Input ports for reading logic levels. Timers that count in the background; External interrupts that trigger the interrupt routine if a port changes. Control flow - Wikipedia, the free encyclopedia. In computer science, control flow (or alternatively, flow of control) is the order in which individual statements, instructions or function calls of an imperativeprogram 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 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. 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 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). Categories. 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: 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(. If many programmers are working on a single program, subroutines are one kind of modularity that can help split up the work. Sequence. 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 B. Other research showed that control structures with one entry and one exit were much easier to understand than any other form. In other words, they were composable. This purist approach is embodied in the Pascal programming language (designed in 1. 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. You don't have to understand the basics to use the serial port But understanding it may help to determine what is wrong if you run into problems. This section not only presents new topics but also.
A 1. 98. 0 study by Henry Shapiro cited by Roberts found that using only the Pascal- provided control structures, the correct solution was given by only 2. Such languages need some way of grouping statements together. Algol 6. 0 and Pascal : begin .. ENDPython: uses indentation level (see Off- side rule)Haskell: either indentation level or curly brackets can be used, and they can be freely mixed. Lua: uses do .. The forms of the final keyword vary. Ada: final keyword is end + space + initial keyword e. ENDDOModula- 2: same final keyword END for everything. Visual Basic: every control structure has its own keyword. Wend. If- then- (else) statements. 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. Pascal: Ada: C: Shell script: Python: Lisp: ifa> 0thenwriteln(. There is usually a provision for a default action (. 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. Pascal: Ada: C: Shell script: Lisp: casesome. Charof'a': action. On. A; 'x': action. On. X; 'y','z': action. On. Yand. Z; elseaction. On. No. Match; end; casesome. Chariswhen'a'=> action. On. A; when'x'=> action. On. X; when'y'. Tail recursion is a special case of recursion which can be easily transformed to iteration. Count- controlled loops. 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. Floating- point numbers are represented imprecisely due to hardware constraints, so a loop such asfor X : = 0. 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. Condition- controlled loops. 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. A key changeable value or values are monitored within the loop and a change diverts program flow to the handling of the group event associated with the changeable value. Ada, D, Smalltalk, PHP, Perl, Object Pascal, Java, C#, Matlab, Mythryl, Visual Basic, Ruby, Python, Java. Script, Fortran 9. Haskell has do- expressions and comprehensions, which together provide similar functionality to for- expressions in Scala. General iteration. 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. 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 .. 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 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. 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. Ada. Text. IO; with. Ada. Integer. Text. IO; procedure. Print. For example,forninset. 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 usage example is searching a multi- dimensional table. This can be done either via multilevel breaks (break out of N levels), as in bash. C does not include a multilevel break, and the usual alternative is to use a goto to implement a labeled break. Rao Kosaraju refined the structured program theorem by proving that it's possible to avoid adding additional variables in structured programming, as long as arbitrary- depth, multi- level breaks from loops are allowed. There are other proposed control structures for multiple breaks, but these are generally implemented as exceptions instead. In his 2. 00. 4 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 . As commonly implemented, however, return sequencers may also carry a (return) value, whereas the break sequencer as implemented in contemporary languages usually cannot. 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.
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