Expression and control structure

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Expression and control structure

• Kun-Yuan Hsieh
• kyshieh_at_pllab.cs.nthu.edu.tw
• Programming Language Lab., NTHU

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Expressions

• Expressions are the fundamental means of
  specifying computations in a programming language
• Expression evaluation depends on the orders of
  operator and operand evaluation
• Issues
• Precedence
• Associativity
• Order of operand evaluation
• Side effects

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Operators

• A unary operator has one operande.g.
• A binary operator has two operandse.g. , -, ,
  /
• A ternary operator has three operandse.g. ?

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Operator precedence

• Precedence rules define the order in which
  adjacent operators of different precedence
  levels are evaluated.
• Ex
• a 10 b 3 c 4
• d a b c
• d (in C) 22

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Operator associativity

• Associativity rules define the order in which
  adjacent operators with the same precedence level
  are evaluated
• Associativity in most PL is left-to-right, except
  for exponentiation op (right-to-left)
• Ex
• a 10 b 3 c 4
• d a b c
• d (in C) 7

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Parentheses

• Precedence and associativity rules can be
  overriden with parentheses
• a 10 b 3 c 4
• d (a b) c
• d (in C) 52
• Use parentheses when not clear in your code to
  increase readability
• Ex
• a 10 b 3 c 4
• d (in C) abc a(bc) 1081

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Functional side effects

• Side effect of a function occurs when a function
  changes one of its parameter or global variable
• Ex

void main() fun2() a (in C) 8
int a 5 int fun1() a 17 return 3
int fun2() a a fun1()
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Operator overloading

• Use of an operator for more than one purpose
• Increase readability but also decrease
  readability
• EX
• Matrix operation
• A B C, instead of MatrixAdd( MatrixMult( A, B
  ), C )
• A B is actually implemented as (A AND B)

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Type Conversions

• Narrowing conversion converts an object to a type
  that cannot include all of the values of the
  original type e.g., float to int
• Widening conversion convets an object to a type
  that can include at least approximations to all
  of the values of the original type
  e.g., int to float
• Coercion is an implicit type conversion

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Relational and Boolean Expressions

• Compares the values of its operands
• Evaluate to some Boolean representation
• C has no Boolean type--it uses int type with 0
  for false and nonzero for true
• Ex
• a 3, b 4, c a 1 gt 2 b
• c 0

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Short Circuit Evaluation

• The result of an expression is determined without
  evaluating all the operands and/or operators
• EX
• while ((index lt length) (listindex ! key))
• I ndex index 1

Table lookup
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Assignment Statements

• The operator symbol
• FORTRAN, BASIC, PL/I, C, C, Java
• ALGOLs, Pascal, Ada
• Unary assignment ops
• count (count count 1)
• Undetected error in C/C
• if (x y) is different from if (x y)
• Java only allows Boolean expressions in if
  statements

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Control structure

• Control structure is a control statement and the
  statements whose execution it controls
• Selection statement provides the means of
  choosing between two or more paths of execution
• Two-Way Selection Statements
• Multi-Way Selection Statements

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Two-Way Selection Statements

• if-then-else
• matching else problem
• rules else matches the most recent then
• used in Java and most other imperative PL (C,
  Pascal)
• ALGOL 60 use compound statement for nesting if
• Ada use end if reserve word

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Multiple selection constructs

• FORTRAN IF (exp) N1, N2, N3
• FORTRAN GO TO (lb1, lb2, , lbN), exp
• Pascal case (exp) of end
• C, C switch (index) case
  break default

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Iterative Statements

• repeated execution of a statement or compound
  statement is accomplished either by iteration or
  recursion
• Counter-Controlled Loops
• Design Issues
• 1. What are the type and scope of the loop
  variable?
• 2. What is the value of the loop variable at loop
  termination?
• 3. 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?
• 4. Should the loop parameters be evaluated only
  once, or once for every iteration?

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examples

• Iterative statements
• FORTRAN 90 DO lavel var init, final ,step
• ALGOL 60 for count 1 step 1 until 10 do
• for index 1, 4, 13, 41 step 2 until 47,
• 3 index while index lt 1000,
• 34, 2, -24, do
• ? 1, 4,13, 41,43, 45, 47, 147, 441, 34, 2, -24
• Pascal for var init (to downto) final do
• Ada for k in 1..10 loop
• C, C, Java for (k 0 k lt 10 k)
• Iteration on data list
• Perl _at_names Bob, John, Tom
• foreach name (_at_names) print name

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Unconditional Branching

• Problem readability
• Some languages do not have them e.g., Java
• Loop exit statements are restricted and somewhat
  camouflaged gotos

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Guarded statements

• Guarded commands
• suggested by Dijkstra (1975)
• all boolean exps are evaluated
• one of the true-stmts is nondeterministically
  chose for execution
• if all exps are false, run-time error occurs
• if i 0 -gt sum sum i
• i gt j -gt sum sum j
• i lt j -gt sum sum i
• fi

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Guarded statements

• do ltboolean expgt -gt ltstmtgt
• ltboolean expgt -gt ltstmtgt
• ltboolean expgt -gt ltstmtgt
• od
• all boolean exps are evaluated on each iteration
• one of the true stmts are nondeterministically
  executed, and then the loop is evaluated again
• when all exps are false, the loop terminates
• sort q1 to q4 such that q1 lt q2 lt q3 lt q4
• do q1 gt q2 -gt temp q1 q1 q2 q2 temp
• q2 gt q3 -gt temp q2 q2 q3 q3 temp
• q3 gt q4 -gt temp q3 q3 q4 q4 temp
• od