CSE 452: Programming Languages

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CSE 452 Programming Languages

• Control Flow

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Outline

• Control Structures
• Selection Statements
• Iterative Statements
• Unconditional Branches
• Subprograms and Procedures

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Iterative Statements

• Repeated execution of a statement or compound
  statement
• accomplished either by iteration or recursion
• here we look at iteration, because recursion is a
  unit-level control (e.g., using functions)
• General design issues for iteration control
  statements
• How is the iteration controlled?
• Counter-controlled vs logical-controlled
• Where should the control mechanism appear in the
  loop?
• pretest (before loop body is executed) vs
  posttest (after loop body is executed)

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Iterative Statements

• Counter-Controlled Loops
• for (i init_value i lt terminal_value
  istepsize)
• Design Issues
• What are the type and scope of the loop variable?
• What is the value of the loop variable at loop
  termination?
• Should it be legal for the loop variable or loop
  parameters to be changed in the loop body, and if
  so, does the change affect loop control?
• Should the loop parameters be evaluated only
  once, or once for every iteration?

Loop parameters
Loop variable
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Counter-Controlled Loops

• FORTRAN 95
• Syntax Do label var initial, terminal ,
  stepsize
• stepsize can be any value but zero
• parameters can be expressions
• Design choices
• Loop var must be integer loop parameters can be
  expressions
• The loop var cannot be changed in the loop, but
  loop parameters can because they are evaluated
  only once, it does not affect loop control
• Single entry structure loop can be entered
  through Do statement

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Counter-Controlled Loops

• Fortran 95
• Do label var init_expression,
  terminal_expression , step_expression
• Operational Semantics for Fortran 95 Do statement
• init_value init_expression
• terminal_value terminal_expression
• step_value step_expression
• do_var init_value
• iteration_count max(int((terminal_valueinit_v
  aluestep_value)/step_value), 0)
• loop
• if iteration_count ? 0 goto out
• loop body
• do_var do_var step_value
• iteration_count interation_count 1
• goto loop
• out

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Counter-Controlled Loops

• Another form of Do statement for FORTRAN 95
• name DO variable initial, terminal ,
  stepsize
• END DO name
• Uses a special word for closing END DO

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Counter-Controlled Loops

• Ada
• Syntax
• for var in reverse discrete_range loop
• ...
• end loop
• reverse indicates that values of discrete range
  are assigned in reverse order
• Step size is always one (or next element in
  discrete_range)

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Counter-Controlled Loops

• Ada Design choices
• Type of the loop var is that of the discrete
  range
• discrete range is subrange of integer or
  enumeration type, such as 1..10 or Monday..Friday
• Scope of loop var is the loop body (it is
  implicitly declared) loop var does not exist
  outside the loop
• Count Float 1.35
• for Count in 1..10 loop
• Sum Sum Count
• end loop
• Count gets the value of 1.35
• The loop var cannot be changed in the loop, but
  the discrete range can it does not affect loop
  control
• The discrete range is evaluated just once

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Counter-Controlled Loops

• C-based languages
• Syntax
• for (expr_1 expr_2 expr_3)
• loop body
• Loop body can be single, compound, or null
  statement
• Expressions can be whole statements, or even
  statement sequences, with the statements
  separated by commas
• The value of a multiple-statement expression is
  the value of the last statement in the expression
• for (i 0, j 10 j i i)
• All expressions of Cs for statement are optional
  
• If expr_2 is absent, it is an infinite loop

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Counter-Controlled Loops

• for (expr_1 expr_2 expr_3)
• loop body
• Operational Semantics
• expr_1 initialization (evaluate once)
• loop
• if expr_2 0 goto out loop control (each
  iter)
• loop_body
• expr_3 increment loop counter?
• goto loop
• out

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Counter-Controlled Loops

• Design choices for C
• There is no explicit loop variable or loop
  parameters
• All involved variables can be changed in the loop
  body
• It is legal to branch into loop body
• The first expression is evaluated once, but the
  other two are evaluated with each iteration

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Counter-Controlled Loops

• include ltstdio.hgt
• main()
• int count, i 2
• count 2
• / count 5 /
• if (count 2 0)
• goto loop
• for (i0 ilt5 i)
• loop printf("id\n", i)

count 2 i 2 i 3 i 4 count
5 i 0 i 1 i 2 i 3 i 4
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Counter-Controlled Loops

• C
• Differs from C in two ways
• The control expression can also be Boolean
• The initial expression can include variable
  definitions (scope is from the definition to the
  end of the loop body)
• for (int i0 i lt len i)
• Java
• Differs from C in that the control expression
  must be Boolean

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Logically-Controlled Loops

• Repetition control is based on a Boolean
  expression, rather than a counter
• More general than counter-controlled loops
• Every counting loop can be built with a logical
  loop, but not vice-versa
• Design Issues
• Pretest or postest?
• Should logically controlled loop be a special
  form of counting loop statement or a separate
  statement?

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Logically-Controlled Loops

• Pascal has separate pretest and posttest logical
  loop statements (while-do and repeat-until)
• C and C also have both
• while (count gt 0)
• do
• while (count gt 0)
• Legal to branch into both while and do loop bodies

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Logically-Controlled Loops

• Ada has a pretest version, but no posttest
• FORTRAN 77, 90, and 95 have neither
• Perl has two pretest logical loops, while and
  until, but no posttest logical loop
• while (count gt 0)
• until (count 0)

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User-Located Loop Control

• User modify the control flow of program
• Design issues
• Should the conditional mechanism be an integral
  part of the exit?
• Should only one loop body be exited or can
  enclosing loops also be exited?

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User-Located Loop Control

• Exit statement
• Unconditional unlabeled exit break (C, C)
• for (index0 indexlt10 index)
• if (value lt 0) break
• Unconditional labeled exit break (Java, C),
  last (Perl)
• C outerLoop for (row0 rowltnumRows row)
• for (col 0 col lt numCols
  col)
• sum matrixrowcol
• if (sum gt 1000)
• break outerLoop
• Perl LINE while (ltSTDINgt)
• last LINE if // ...

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User-Located Loop Control

• Skip the rest of loop body
• C/C have unlabeled control statement (continue)
• while (sum lt 1000)
• value getNextValue()
• if (value lt 0) continue
• sum value
• Java, Perl, and C have statements similar to
  continue, except they can include labels that
  specify which loop is continued

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Iteration Based on Data Structures

• Loops are controlled by number of elements in a
  data structure
• Perl, Javascript, PHP, and C have such
  statements
• Perl _at_values (1, 2, 3, 4, 5)
• foreach value (_at_values)
• print Value is value\n
• \
• C String strList Bob, John,
  Carol
• foreach (String name in strList)

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Iteration Based on Data Structures

• More general approach
• uses a user-defined data structure and a
  user-defined function (called an iterator) to go
  through the structures elements
• Java has a Collection interface that contains two
  methods
• boolean add(Object obj) - adds elements to
  collection
• Iterator iterator() - used to visit the elements
  in the collection one by one.

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Unconditional Branching

• Transfers execution control to a specified
  location in the program
• goto label
• Problem readability
• Some languages do not have them e.g., Java
• Loop exit statements are restricted and somewhat
  camouflaged gotos
• Label forms
• Unsigned int constants Pascal (with colon)
  FORTRAN (no colon)
• Identifiers with colons ALGOL 60, C
• Variables as labels PL/I

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Subprograms

• Two fundamental abstraction facilities in
  programming language
• Process abstraction represented by subprograms
• Data abstraction
• General characteristics of subprograms
• A subprogram has a single entry point
• The caller is suspended during execution of the
  called subprogram
• Control always returns to the caller when the
  called subprograms execution terminates

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Subprograms

• A subprogram definition is a description of the
  actions of the subprogram abstraction
• A subprogram call is an explicit request that the
  subprogram be executed
• A subprogram is active if, after being called, it
  has begun execution but has not yet completed
  that execution
• A subprogram header is the first line of the
  definition
• Specifies that the following syntactic unit is a
  subprogram of some particular kind - using a
  special word (function, procedure, etc)
• Provides name of subprogram
• Specifies the list of formal parameters
• Fortran Subroutine Adder(parameters)
• Ada procedure Adder(parameters)

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Subprograms

• The parameter profile (signature) of a subprogram
  is the number, order, and types of its parameters
  
• The protocol of a subprogram is its parameter
  profile plus, if it is a function, its return
  type
• Subprograms can have declarations as well as
  definitions
• Subprogram declaration provides the subprograms
  protocol but do not include their bodies
• Function declarations in C/C are called
  prototypes

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Parameters

• A formal parameter is a dummy variable listed in
  the subprogram header and used in the
  subprogram
• An actual parameter represents a value or address
  used in the subprogram call statement
• void doNothing (int formal_param)
• main()
• int actual_param
• doNothing(actual_param)

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Parameters

• Actual/Formal Parameter Correspondence
• Binding of actual to formal parameters
• Positional parameters
• First actual param bound to first formal param,
  etc
• Keyword parameters
• Name of formal param to which actual param is
  bound is specified with actual param
• Ada Sumer( Length gt My_Length,
• List gt My_Array,
• Sum gt My_Sum )
• Advantage order is irrelevant
• Disadvantage user must know the formal
  parameters names

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Parameters

• Default values of formal parameters
• Allowed by C, Fortran 95, Ada and PHP
• Default value is used if no actual parameter is
  passed to the formal parameter
• Ada
• function Compute_Pay( Income Float Exemptions
  Integer 1
• Tax_Rate Float ) return Float
• pay Compute_Pay (20000.00, Tax_Rate gt
  0.15)
• C allows methods to accept variable number of
  params of the same type
• public void DisplayList(params int list )
• foreach (int nextValue in list)
• Console.WriteLine(Next value 0,
  nextValue)

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Procedures and Functions

• Procedures provide user-defined statements
• Functions provide user-defined operators
• Value produced by function is returned to the
  calling code, effectively replacing the call
  itself
• float power(float base, float exp)
• result 3.4 power(10.0, x)
• C-based languages
• have only functions (but they can behave like
  procedures)
• Can be defined to return no value if the return
  type is void

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Design Issues for Subprograms

1. What parameter passing methods are provided?
2. Are parameter types checked?
3. Are local variables static or dynamic?
4. What is the referencing environment of a passed
   subprogram?
5. Are parameter types in passed subprograms
   checked?
6. Can subprogram definitions be nested?
7. Can subprograms be overloaded?
8. Are subprograms allowed to be generic?
9. Is separate or independent compilation supported?

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Local Referencing Environments

• Local variables variables defined inside
  subprograms
• their scope is the body of subprogram in which
  they are defined
• Stack-dynamic bound to storage when subprogram
  begins execution, unbound when its execution
  terminates
• Advantages
• Support for recursion
• Storage for local variables of active subprogram
  can be shared with local variables of inactive
  subprograms
• Disadvantages
• Allocation/deallocation time
• Indirect addressing (indirectness because the
  place in stack where a particular local variable
  is stored can only be determined at run time)
• Subprograms cannot be history sensitive
• Cannot retain data values of local variables
  between calls
• Static bound to storage at compile-time

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Parameter Passing Semantic Models

• Semantic models for formal parameters
• In mode can receive data from corresponding
  actual parameters
• Actual value is either copied to caller, or an
  access path is transmitted
• Out mode can transmit data to actual parameters
• Inout mode can do both receive/transmit data

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Parameter Passing Implementation

• Pass by value (in mode)
• Value of actual parameter is used to initialize
  formal parameter, which acts as a local variable
• void foo (int a)
• a a 1
• void main()
• int b 2
• foo(b)
• Normally implemented by copying actual parameter
  to formal parameter
• Can also be implemented by transmitting access
  path to the value of actual parameter as long as
  cell is write protected
• Disadvantages
• Requires more storage (duplicated space)
• Cost of the moves (if the parameter is large)

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Parameter Passing Implementation

• Pass by result (out mode)
• Locals value is passed back to the caller
• No value transmitted to the subprogram
• Formal parameter acts as local variable, but just
  before control is transferred back to caller, its
  value is transmitted to actual parameter
• Disadvantages
• If value is copied back (as opposed to access
  paths), need extra time and space
• Pass-by-result can create parameter collision
• e.g. procedure sub1(y int,
  z int)
• ...
• sub1(x, x)
• Value of x in the caller depends on order of
  assignments at the return

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Parameter Passing Implementation

• Pass by value-result (or pass-by-copy)
• Combination of pass-by-value and pass-by-result
• Formal parameter acts as local variable in
  subprogram
• Actual parameter is copied to formal parameter at
  subprogram entry and copied back at subprogram
  termination
• Share disadvantages of pass-by-result and
  pass-by-value
• Requires multiple storage for parameters
• Requires time for copying values
• Problems with parameter collision

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Parameter Passing Implementation

• Pass by reference (or pass-by-sharing)
• transmits an access path (e.g., address) to the
  called subprogram
• Called subprogram is allowed to access actual
  parameter in the calling program unit
• Advantage
• passing process is efficient (no copying and no
  duplicated storage)
• Disadvantages
• Slower accesses to formal parameters due to
  additional level of indirect addressing
• Allows aliasing
• void fun (int first, int second)
• fun(total, total)

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Parameter Passing Implementation

• Pass-by-reference
• Collisions due to array elements can also cause
  aliases
• void fun(int first, int second)
• fun(listi, listj) / where ij /
• void fun1(int first, int a)
• fun1(listi, list)
• Collisions between formal parameters and nonlocal
  variables that are visible
• int global void sub(int param)
• void main() extern int global
• extern int global
• sub(global)

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Parameter Passing Implementation

• Pass by Name
• Another type of inout mode
• Actual parameter is textually substituted for the
  corresponding formal parameters
• Actual binding of value and address is delayed
  until formal parameter is assigned or referenced
• Advantage
• flexibility of late binding
• Disadvantage
• very expensive related to other parameter passing
• Not used in any widely used language
• Another Example
• Used at compile time by macros, and for generic
  subprograms in C

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Pass-by-value

• int m8, i5
• foo(m)
• print m prints 8
• since m is passed by-value
• ...
• proc foo (byval b)
• b i b
• b is byval so it is essentially a local
  variable
• initialized to 8 (the value of the actual
  back in
• the calling environment)
• the assignment to b cannot change the value
  of m back
• in the main program

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Pass-by-reference

• int m8, i5
• foo(m)
• print m prints 13
• since m is passed by-reference
• ...
• proc foo (byref b)
• b i b
• b is byref so it is a pointer back to the
  actual
• parameter back in the main program
  (containing 8 initially)
• the assignment to b actually changes the
  value in m back
• in the main program
• i accesses the variable in the main via
  scope rules

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Pass-by-value-result

• int m8, i5
• foo(m)
• print m prints 13
• since m is passed by-value-result
• ...
• proc foo (byvres b)
• b i b
• b is byves so it copies value of the actual
• parameter (containing 8 initially)
• new value of b is copied back to actual
  parameter
• in the main program
• i accesses the variable in the main via
  scope rules

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Pass-by-name

• array A 1..100 of int array A
  1..100 of int
• int i5 int i5
• foo(Ai,i) foo(Ai)
• print Ai prints A6 print Ai
  prints A5
• ... so prints 7 ...
  not sure what
• good example a problem
  here...
• proc foo (name B,name k) proc foo
  (name B)
• k 6 int i
  2
• B 7 B 7
• text substitution does this
• proc foo proc foo
• i 6 int i
  2
• Ai 7 Ai 7