how many control structures? - tunipopl/lecture-slides-pdf/09-control-structures.pdf · how many...

1Principles of programming languagesMaarit Harsu / Matti Rintala / Henri Hansen

TUT Software Systems

How many control structures?

• 1: – Conditional jump [goto-statement]

• 2: – Choice of two control-flows [if-statement]

– Logically controlled iteration [while-statement]

• Many languages have several– To improve readability and writability

– If the language is large, only some subset is often used



Choice• 1-way choice (Fortran IV)

• 2-way choice (Algol60)

• multi-way (e.g. Ada)

IF ( cond ) stmt IF ( FLAG .NE. 1 ) GO TO 20I = 1J = 2

20 CONTINUE

if cond then stmtelse stmt

if ( x < 10 ) then class = ”small”;elsif ( x < 100 ) then class = ”medium”;elsif ( x < 1000 ) then class = ”large”;else class = ”huge”;end if;



case-structure

• Conditional statements based on equality– case labels (possible values)

• Design issues:– Type of the expression?

– Values of the case labels?• constants?

• Enumerated types?

• Mutually exclusive?

– Other options executed?• default, else, others -branching

case E of L1: S1; L2: S2; ... Ln: Sn;end

switch ( E ) { case L1: S1; case L2: S2; break; ... case Ln: Sn;}

Pascal:

C code:

Variablerecordoptions



Implementing choice• If-statement compiling:

– Condition truth value used in conditional jump

• case-statement:– Small number of case-tags

• Compile as if-then-elsif

– Reasonable number of tags that cover the set of values• Jump address table• Empty values use the default address

– Large number of tags, and even larger set of values• Hash-table or other dictionary



Implementing choices

r1 := Ar2 := Br1 := r1 > r2r2 := Cr3 := Dr2 := r2 > r3r1 := r1 & r2r2 := Er3 := Fr2 := r2 <> r3r1 := r1 | r2if r1 = 0 goto L2

L1: then_clausegoto L3

L2: else_clauseL3:

if ( ( A > B ) and ( C > D ) ) or ( E <> F ) then then_clauseelse else_clause

Pascal-koodia:



Jump address table

• Each case-tag corresponds to an index

• Index points to an address for jumping when condition evaluates as the case-tag

• Code is found in the addresscase E of 1: S1; 2: S2; 3: S3; 4: S4;end

1234

...S1S2S3S4...



Peculiarities of choice

• ML and Haskell: piecewise functions– fun fact(0) = 1

| fact(n : int) : int = n * fact(n-1);– fact 0 = 1

fact n = n * fact(n-1)

• Smalltalk boolean objects– Boolean inherited classes True and False

– Their methods ifTrue and ifFalse, conditional code as parameter, either executed or not

– i < 2 ifTrue: [ Transcript show: 'It is small' ].



Repetition statements• Definite/bounded iteration [counter]

– The number of repetitions is known before starting

• Indefinite/unbounded iteration [logical condition]– Number of iterations is determined during the iteration

– Test of ending at the end or the beginning

loop -- statements exit when cond; -- statementsend loop;

generalisation (Ada):



Definite/bounded iteration

• Contains loop index (loop counter)for index from expr1 to expr2 do stmt end

• Semantics vary between languages– Is the index value defined after execution?

– Can the index value be changed within the body?

– Are upper and lower values evaluated only once?

Iteratingan array



<for_stmt> ::= for var := <list_elem> {, <list_elem>} do <stmt><list_elem> ::= <expr> | <expr> step <expr> until <expr> | <expr> while <bool_expr>

for count := 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 do list [ count ] := 0

for count := 1 step 1 until 10 do list [ count ] := 0

for count := 1, count + 1 while ( count <= 10 ) do list [ count ] := 0

for index := 1, 4, 13, 41 step 2 until 47, 3 * index while index < 1000, 34, 2, -24 do sum := sum + index

1, 4, 13, 41, 43, 45, 47, 147, 441, 34, 2, -24

Algol60:for-stmt:

for-stmtuses:

complexuse:

Indexvalues:



Alternative ways of definite iteration

for variable := init_value ( to | downto ) final_value do stmt

Pascal:

for variable in [ reverse ] discrete_range loop ...end loop;

i: Float := 3.14;for i in 1..10 loop sum := sum + i;end loop;

Ada:

The scope of the index (Ada):



C for-statement

for ( index = 0; index <= 10; index++ ) sum += list [ index ];

• No actual loop index

• Three expressions– Any expression can be omitted

– Each expression may be compound

for ( count1 = 0, count2 = 1.0; count1 <= 10 && count2 <= 100.0; sum = ++count + count2, count2 *= 2.5 );



C++ and Java for-statements

• C++– ”loop indes” definition

– No effect on visibility

• Java– Truth-valued conditions

– ”loop index” is visible only inside the loop

for ( int i = 0; i < MAX; i++ )

int i;for ( i = 0; i < MAX; i++ )



Value of the index after the loop

• Safe semantics:– Not visible

• If visible:– undefine

• Pascal

– Last given value(first value ”over” the limit)• esim. Fortran, Algol60

var c: ’a’..’z’;...for c := ’a’ to ’z’ do begin ...end;

Pascal:



Indefinite/unbounded iteration

• End condition evaluation– beginning

• 0 – n iterations

– end• 1 – n iterations

• Ending logic:– while: continue if true

– until: end if true

s := 0;Read ( nm );while nm >= 0 do begin s := s + nm; Read ( nm );end;

s := 0;Read ( nm );repeat s := s + nm; Read ( nm );until luku < 0;

Pascal-koodia:



Peculiarities of iteration• Combination of definite and indefinite

iteration (C for)

for ( E1; E2; E3 ) S; E1; while ( E2 ) { S; E3 };

• One general structure (Algol68)– Only mandatory parts: do and od

for index from expr1 by expr2 to expr3while expr4 do statement od



Peculiarities of iteration

• Smalltalk iteration is an intermediate method, code block as parameter, with the value of the index as its parameter– Interval from: 1 to: 10 do: [ i | Transcript show: i ]

• Undefined iteration is a method of a code block

– [ Transcript show i. i := i-1. i > 0 ] whileTrue.

– [ i > 0 ] whileTrue: [ Transcript show i. i := i-1. ]



Loop controlOUTER_LOOP: for row in 1..max_rows loopINNER_LOOP: for col in 1..max_cols loop sum := sum + mat ( row, col ); exit OUTER_LOOP when sum > 1000; end loop INNER_LOOP; end loop OUTER_LOOP;

while ( sum < 1000 ) { getnext ( value ); if ( value < 0 ) continue; sum += value;}

while ( sum < 1000 ) { getnext ( value ); if ( value < 0 ) break; sum += value;}

Ada:

C jaC++:



Loops and data structurestype Days is ( Mon, Tue, Wed, Thu, Fri, Sat, Sun );for Index in Days loop ... end loop;

int[] tbl = {1, 2, 3}; // C++: int tbl[] = {1, 2, 3};for (int i : tbl) { ... }

String [ ] strList = { ”Bob”, ”Carol”, ”Ted” };foreach ( String name in strList ) Console.WriteLine ( ”Name: ”, name );

Ada:

C#:

Lambdas and library functions used as loops

Haskell: let tbl = [1, 2, 3] in map (\x -> x*x + 1) tbl

int[] tbl = {1, 2, 3}; // C++: int tbl[] = {1, 2, 3};for (int i : tbl) { ... }

C++Java:



Generators (Python)● Functions are "single-use": parameters in, return

value out● Generators are "multi-use": they can produce

multiple values (when needed)● Generator is an iterator, which is used to access (and

calculate) the values● Generator functions return generators (iterators)● (Or: normal data structures can be viewed as

generator functions with pre-calculated values, accessed through iterators)



Generators, example

def firstn(n): num = 0 while num < n: yield num # "return", ready to continue num += 1

i = firstn(10)print(next(i)) ; print(next(i))j = (x*x for x in i)print(next(j))



Benefits of generators

• Avoid creating big data structures, create values when necessary

• Allow lazy evaluation (almost)

• Allow pipeline programming: actions (generators) are chained together, feeding values to next action (generator)

how many control structures? - tunipopl/lecture-slides-pdf/09-control-structures.pdf · how many...

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