Control explanations .


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Control statements. Simple statements Basic structured statements Sequence Selection Iteration The jump statement. Simple statements in imperative languages. These are atomic (all or nothing) operations: assignment, the empty statement, a procedure call, exit, next, break, continue
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Control articulations Simple statements Basic organized explanations Sequence Selection Iteration The hop proclamation

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Simple proclamations in basic dialects These are nuclear (win big or bust) operations: task, the void proclamation, a methodology call, exit, next, break, proceed go to (hop). A piece is likewise a win or bust operation.

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Structured proclamations Three key components permit us to gathering straightforward explanations into organized articulations. succession , or the compound proclamation: { S1 S2 } determination , or the restrictive articulation: if (C) S1 else S2 cycle , or the circle explanation: while (C) S

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Structured articulations (2) All other control structures can be effectively gotten from the three fundamental systems. on the off chance that ( C ) S  if ( C ) S else {} do S while ( C )  S while ( C ) S switch (i) { if (i == C1 ) S1 case C1 : S1 …  else … et cetera.

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Sequence (1) Languages Mechanisms Algol, Pascal, Ada, ... begin ... end C, Java, Perl { ... } Fortran IV nothing Prolog implicit a :- b, c, d. This implies assessing b, then c, then d. Scheme (begin ...)

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single section, single leave Sequence (2) A compound proclamation is dealt with as a straightforward explanation. This depends on a vital deliberation standard . The inward structure can be "disconnected away".

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Selection The if-else build is available in all programming dialects (Prolog is the main real special case). Modula and Ada were the first to have legitimately sectioned if-then-else, with four watchwords around three components of the choice: if C then S1 else S2 end if Nested choice if-elsif-...- else was additionally presented in Ada: if C1 then S1 elsif C2 then S2 ...... elsif Cn then Sn else Sn+1 end if

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Special types of choice in Fortran IV Computed GO TO. GO TO (mark 1 , ..., name n ), expression Assigned GO TO. Dole out name i TO variable GO TO variable(label 1 , ..., name n )

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Special types of determination (2) The switch proclamation in C and Java has been presumably roused by figured GO TO. switch(expression){ case const 1 : S 1 ; ... case const n : S n ; default: S n+1 ;} After S i has been executed, control "falls through" to the resulting case: S i+1 is executed next. Fall-through can be maintained a strategic distance from by including break articulations.

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Special types of determination (3) Case proclamation in Pascal, Ada and other comparative dialects: every case is particular, there is no "fall-through". In Ada: case expression is when constantList 1 => S 1 ; ... at the point when constantList n => S n ; when others => S n+1 ; end case;

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Special types of choice (4) Selection in Prolog is driven by achievement and disappointment, not by the genuine false restriction. Choice is understood in backtracking: on the off chance that you succeed, stop; if not, attempt another decision . union( [Elem | S1], S2, S1_S2 ) :- part( Elem, S2 ), union( S1, S2, S1_S2 ). union( [Elem | S1], S2, [Elem | S1_S2] ) :- \+ part( Elem, S2 ), union( S1, S2, S1_S2 ). union( [], S2, S2 ).

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Y N Graphical representation flowgraphs — stream charts — flowcharts if–then–else if–then C S 1 S 2 S

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Y N C S 1 S 2 Graphical representation (2) The deliberation guideline: if ( C ) S1 else S2 is a basic articulation. Single section, single exit

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N C 1 C 2 C 3 C n … Y S 1 S 2 S 3 S n S n+1 Graphical representation (3) if–then–elsif-… elsif-then-else

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N e=v 1 e=v 2 e=v 3 e=v n … Y S 1 S 2 S 3 S n S n+1 Graphical representation (4) case e of v1: S1; ... else Sn+1 end

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Iteration Variations: pre test cycle or pos t test emphasis. while C do S Pascal rehash S until C while ( C ) S Java do S while ( C ) while C circle S end circle; Ada (no posttest cycle)

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Iteration (2) In Ada, the prefix while C is an augmentation of the essential iterative proclamation: circle S end circle; Another prefix: for i in range The uncovered circle articulation must be halted from inside the circle. Constrained leave shuts the closest cycle: exit; unconditional leave when C ;

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Iteration (3) The while prefix is a condensing. The development while C circle S end circle; is equal to circle exit when not C ; S end circle;

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SUM := 0; circle get(X); exit when X = 0; SUM := SUM + X; end circle; Simpler, more instinctive SUM := 0; get(X); while X/= 0 circle SUM := SUM + X; get(X); end circle; Condition turned around, get(X) shows up twice Example: utilization of exit

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S Y N C S ¬C S N Y C Graphical representation while-do rehash until rehash until

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S 1 Y C N S 2 Graphical representation (2) circle - exit - end circle

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for circles For-circles ("counter-controlled") are truly prior and less broad than condition-controlled iterative structures. DO 1000 var = lo , hello there Fortran IV ... 1000 CONTINUE DO name var = lo , hello , incr for var := expr do S Algol 60 for var := low stride incr until high do S for var := expr while C do S Iterators can be joined: for i := 0, i+1 while i ≤ n do S(i)

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for circles (2) for var in range Ada circle S end circle; for var in invert extend circle S end circle; for ( e1 ; e2 ; e3 ) S C, Java, Perl (enough said J ) What is this? for (;;) S

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for circles 32) Iteration in Prolog and in Scheme is communicated by recursion. The same, obviously, is conceivable in most other run of the mill dialects.

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Jump (the goto articulation) Unconstrained exchange of control is the main system accessible in low-level dialects — however they are exceptionally broad. One-branch choice and goto permit us to express all other control structures. The hop system is risky, may hurt coherence, and ought to be maintained a strategic distance from — propelled control structures function admirably for all run of the mill and for most less average employments.

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Jump (the goto statemen)t (2) Some dialects limit goto (don\'t permit hopping inside a cycle or determination) and make it difficult to utilize. Ada makes names obvious from far away (so that your manager can see it!).: SUM := 0; loop get(X); if X = 0 then goto DONE ; end if; SUM := SUM + X; end circle; <<DONE>> put(SUM); goto may leave "unfinished business" — dynamic control structures that must be "folded" immediately.

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