Principles of Programming Languages


Expressions and Statements

  • Topics
    * Arithmetic Expressions
    * Overloaded Operators
    * Type Conversions
    * Relational and Boolean Expressions
    * Short-Circuit Evaluation
    * Assignment Statements
    * Mixed-Mode Assignment
    * Control Structures
    * Introduction
    * Selection Statements
    * Iterative Statements
    * Unconditional Branching
    * Guarded Commands
    * Conclusions
    * Expressions are the fundamental means of specifying computations in a programming language
    * To understand expression evaluation, need to be familiar with the orders of operator and operand evaluation
    * Essence of imperative languages is dominant role of assignment statements Arithmetic Expressions
    * Arithmetic evaluation was one of the motivations for the development of the first programming languages
    * Arithmetic expressions consist of operators, operands, parentheses, and function calls
    Arithmetic Expressions: Design Issues
    * Design issues for arithmetic expressions
    > Operator precedence rules?
    > Operator associativity rules?
    > Order of operand evaluation?
    > Operand evaluation side effects?
    > Operator overloading?
    > Type mixing in expressions?
    Arithmetic Expressions: Operators
    * A unary operator has one operand
    * A binary operator has two operands
    * A ternary operator has three operands
    Arithmetic Expressions: Operator Precedence Rules
    * The operator precedence rules for expression evaluation define the order in which ―adjacent|| operators of different precedence levels are evaluated
    * Typical precedence levels
    > parentheses
    > unary operators
    > ** (if the language supports it)
    > *, /
    > +, -
    Arithmetic Expressions: Operator Associativity Rule
    * The operator associativity rules for expression evaluation define the order in which adjacent operators with the same precedence level are evaluated
    * Typical associativity rules
    > Left to right, except **, which is right to left
    > Sometimes unary operators associate right to left (e.g., in FORTRAN)
    * APL is different; all operators have equal precedence and all operators associate right to left
    * Precedence and associativity rules can be overriden with parentheses
    Arithmetic Expressions: Conditional Expressions
    * Conditional Expressions
    > C-based languages (e.g., C, C++)
    > An example:
    average = (count == 0)? 0 : sum / count
    > Evaluates as if written like
    if (count == 0)
    average = 0
    average = sum /count

    Arithmetic Expressions: Operand Evaluation Order
    * Operand evaluation order
    * Variables: fetch the value from memory
    * Constants: sometimes a fetch from memory; sometimes the constant is in the machine language instruction
    * Parenthesized expressions: evaluate all operands and operators first
    * The most interesting case is when an operand is a function call
    Arithmetic Expressions: Potentials for Side Effects
    * Functional side effects: when a function changes a two-way parameter or a non-local variable
    * Problem with functional side effects:
    > When a function referenced in an expression alters another operand of the expression; e.g., for a parameter change:
    a = 10;
    /* assume that fun changes its parameter */
    b = a + fun(a);
    Functional Side Effects
    * Two possible solutions to the problem
    * Write the language definition to disallow functional side effects
    * No two-way parameters in functions
    * No non-local references in functions
    * Advantage: it works!
    * Disadvantage: inflexibility of one-way parameters and lack of non-local references
    * Write the language definition to demand that operand evaluation order be fixed
    * Disadvantage: limits some compiler optimizations
    * Java requires that operands appear to be evaluated in left-to-right order
    Overloaded Operators
    * Use of an operator for more than one purpose is called operator overloading
    * Some are common (e.g., + for int and float)
    * Some are potential trouble (e.g., * in C and C++)
    > Loss of compiler error detection (omission of an operand should be a detectable error)
    > Some loss of readability
    > Can be avoided by introduction of new symbols (e.g., Pascal‘s div for integer division)
    Overloaded Operators (continued)
    * C++, Ada, Fortran 95, and C# allow user-defined overloaded operators
    * Potential problems:
    > Users can define nonsense operations
    > Readability may suffer, even when the operators make sense
    Type Conversions
    * A narrowing conversion is one that converts an object to a type that cannot include all of the values of the original type e.g., float to int
    * A widening conversion is one in which an object is converted to a type that can include at least approximations to all of the values of the original type e.g., int to float
    Type Conversions: Mixed Mode
    * A mixed-mode expression is one that has operands of different types
    * A coercion is an implicit type conversion
    * Disadvantage of coercions:
    > They decrease in the type error detection ability of the compiler
    * In most languages, all numeric types are coerced in expressions, using widening conversions
    * In Ada, there are virtually no coercions in expressions
    Explicit Type Conversions
    * Called casting in C-based languages
    * Examples
    > C: (int)angle
    > Ada: Float (Sum)
    Note that Ada’s syntax is similar to that of function calls
    Type Conversions: Errors in Expressions
    * Causes
    > Inherent limitations of arithmetic e.g., division by zero
    > Limitations of computer arithmetic e.g. overflow
    * Often ignored by the run-time system
    Relational and Boolean Expressions
    * Relational Expressions
    > Use relational operators and operands of various types
    > Evaluate to some Boolean representation
    > Operator symbols used vary somewhat among languages (!=, /=, .NE., <>, #)
    * JavaScript and PHP have two additional relational operator, === and !==
    > Similar to their cousins, == and !=, except that they do not coerce their operands
    * Boolean Expressions
    > Operands are Boolean and the result is Boolean
    > Example operators
    * AND. and && and
    * OR. or || or
    * NOT. not ! not
    Relational and Boolean Expressions: No Boolean Type in C
    * C89 has no Boolean type--it uses int type with 0 for false and nonzero for true
    * One odd characteristic of C‘s expressions: a < b < c is a legal expression, but the result is not what you might expect:
    > Left operator is evaluated, producing 0 or 1
    > The evaluation result is then compared with the third operand (i.e., c)
    Short Circuit Evaluation
    * An expression in which the result is determined without evaluating all of the operands and/or operators
    * Example: (13*a) * (b/13–1)
    If a is zero, there is no need to evaluate (b/13-1)
    * Problem with non-short-circuit evaluation
    index = 1;
    while (index <= length) && (LIST[index] != value)

    > When index=length, LIST [index] will cause an indexing problem (assuming LIST has length -1 elements)
    Short Circuit Evaluation (continued)
    * C, C++, and Java: use short-circuit evaluation for the usual Boolean operators (&& and ||), but also provide bitwise Boolean operators that are not short circuit (& and |)
    * Ada: programmer can specify either (short-circuit is specified with and then and or else)
    * Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3)
    Assignment Statements1
    * The general syntax
    <target_var> <assign_operator> <expression>
    * The assignment operator
    = FORTRAN, BASIC, the C-based languages
    := ALGOLs, Pascal, Ada
    * = can be bad when it is overloaded for the relational operator for equality (that‘s why the Cbased languages use == as the relational operator)
    Assignment Statements: Conditional Targets
    * Conditional targets (Perl) ($flag ? $total : $subtotal) = 0
    Which is equivalent to
    if ($flag){
    $total = 0
    } else {
    $subtotal = 0
    Assignment Statements: Compound Operators
    * A shorthand method of specifying a commonly needed form of assignment
    * Introduced in ALGOL; adopted by C
    * Example
    a = a + b
    is written as
    a += b
    Assignment Statements: Unary Assignment Operators
    * Unary assignment operators in C-based languages combine increment and decrement operations with assignment
    * Examples
    sum = ++count (count incremented, added to sum)
    sum = count++ (count incremented, added to sum)
    count++ (count incremented)
    > count++ (count incremented then negated)
    Assignment as an Expression
    * In C, C++, and Java, the assignment statement produces a result and can be used as operands
    * An example:
    while ((ch = getchar())!= EOF){…}
    ch = getchar() is carried out; the result (assigned to ch) is used as a conditional value for the while statement
    List Assignments
    * Perl and Ruby support list assignments
    ($first, $second, $third) = (20, 30, 40);
    Mixed-Mode Assignment
    * Assignment statements can also be mixed-mode, for example
    int a, b;
    float c;
    c = a / b;
    * In Fortran, C, and C++, any numeric type value can be assigned to any numeric type variable
    * In Java, only widening assignment coercions are done
    * In Ada, there is no assignment coercion
    Levels of Control Flow
    > Within expressions
    > Among program units
    > Among program statements
    Control Statements: Evolution
    * FORTRAN I control statements were based directly on IBM 704 hardware
    * Much research and argument in the 1960s about the issue
    > One important result: It was proven that all algorithms represented by flowcharts can be coded with only two-way selection and pretest logical loops
    Control Structure
    * A control structure is a control statement and the statements whose execution it controls
    * Design question
    > Should a control structure have multiple entries?
    Selection Statements
    * A selection statement provides the means of choosing between two or more paths of execution
    * Two general categories:
    > Two-way selectors
    > Multiple-way selectors
    Two-Way Selection Statements
    * General form:
    if control_expression
    then clause
    else clause
    * Design Issues:
    > What is the form and type of the control expression?
    > How are the then and else clauses specified?
    > How should the meaning of nested selectors be specified?
    The Control Expression
    * If the then reserved word or some other syntactic marker is not used to introduce the then clause, the control expression is placed in parentheses
    * In C89, C99, Python, and C++, the control expression can be arithmetic
    * In languages such as Ada, Java, Ruby, and C#, the control expression must be Boolean
    Clause Form
    * In many contemporary languages, the then and else clauses can be single statements or compound statements
    * In Perl, all clauses must be delimited by braces (they must be compound)
    * In Fortran 95, Ada, and Ruby, clauses are statement sequences
    * Python uses indentation to define clauses
    if x > y :
    x = y
    print "case 1"
    Nesting Selectors
    * Java example
    if (sum == 0)
    if (count == 0)
    result = 0;
    else result = 1;
    * Which if gets the else?
    * Java's static semantics rule: else matches with the nearest if
    Nesting Selectors (continued)
    * To force an alternative semantics, compound statements may be used:
    if (sum == 0) {
    if (count == 0)
    result = 0;
    else result = 1;
    * The above solution is used in C, C++, and C#
    * Perl requires that all then and else clauses to be compound
    * Statement sequences as clauses: Ruby
    if sum == 0 then
    if count == 0 then
    result = 0
    result = 1
    * Python
    if sum == 0 :
    if count == 0 :
    result = 0
    else :
    result = 1
    Multiple-Way Selection Statements
    * Allow the selection of one of any number of statements or statement groups
    * Design Issues:
    * What is the form and type of the control expression?
    * How are the selectable segments specified?
    * Is execution flow through the structure restricted to include just a single selectable segment?
    * How are case values specified?
    * What is done about unrepresented expression values?
    Multiple-Way Selection: Examples
    * C, C++, and Java
    switch (expression) {
    case const_expr_1: stmt_1;

    case const_expr_n: stmt_n;
    [default: stmt_n+1]
    * Design choices for C‘s switch statement
    * Control expression can be only an integer type
    * Selectable segments can be statement sequences, blocks, or compound statements
    * Any number of segments can be executed in one execution of the construct (there is no implicit branch at the end of selectable segments)
    * default clause is for unrepresented values (if there is no default, the whole statement does nothing)
    Multiple-Way Selection: Examples
    * C#
    > Differs from C in that it has a static semantics rule that disallows the implicit execution of more than one segment
    > Each selectable segment must end with an unconditional branch (goto or break)
    * Ada
    case expression is
    when choice list => stmt_sequence;

    when choice list => stmt_sequence;
    when others => stmt_sequence;]
    end case;
    * More reliable than C‘s switch (once a stmt_sequence execution is completed, control is passed to the first statement after the case statement
    * Ada design choices:
    1. Expression can be any ordinal type
    2. Segments can be single or compound
    3. Only one segment can be executed per execution of the construct
    4. Unrepresented values are not allowed
    * Constant List Forms:
    1. A list of constants
    2. Can include:
    > Subranges
    > Boolean OR operators (|)
    Multiple-Way Selection Using if
    * Multiple Selectors can appear as direct extensions to two-way selectors, using else-if clauses, for example in Python:
    if count < 10 :
    bag1 = True
    elsif count < 100 :
    bag2 = True
    elif count < 1000 :
    bag3 = True
    Iterative Statements
    * The repeated execution of a statement or compound statement is accomplished either by iteration or recursion
    * General design issues for iteration control statements:
    1. How is iteration controlled?
    2. Where is the control mechanism in the loop?
    Counter-Controlled Loops
    * A counting iterative statement has a loop variable, and a means of specifying the initial and terminal, and stepsize values
    * 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?
    Iterative Statements: Examples
    * FORTRAN 95 syntax
    DO label var = start, finish [, stepsize]
    * Stepsize can be any value but zero
    * Parameters can be expressions
    * Design choices:
    1. Loop variable must be INTEGER
    2. Loop variable always has its last value
    3. The loop variable cannot be changed in the loop, but the parameters can; because they are evaluated only once, it does not affect loop control
    4. Loop parameters are evaluated only once
    * FORTRAN 95 : a second form:
    [name:] Do variable = initial, terminal [,stepsize]

    End Do [name]
    > Cannot branch into either of Fortran‘s Do statements
    * Ada
    for var in [reverse] discrete_range loop ...
    end loop
    * Design choices:
    > Type of the loop variable is that of the discrete range (A discrete range is a sub-range of an
    integer or enumeration type).
    > Loop variable does not exist outside the loop
    > The loop variable cannot be changed in the loop, but the discrete range can; it does not affect loop control
    > The discrete range is evaluated just once
    * Cannot branch into the loop body
    * C-based languages
    for ([expr_1] ; [expr_2] ; [expr_3]) statement
    > The 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
    > If the second expression is absent, it is an infinite loop
    * Design choices:
    > There is no explicit loop variable
    > Everything can be changed in the loop
    > The first expression is evaluated once, but the other two are evaluated with each iteration
    * 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)
    * Java and C#
    > Differs from C++ in that the control expression must be Boolean
    Iterative Statements: Logically-Controlled Loops
    * Repetition control is based on a Boolean expression
    * Design issues:
    > Pretest or posttest?
    > Should the logically controlled loop be a special case of the counting loop statement or a separate statement?
    Iterative Statements: Logically-Controlled Loops: Examples
    * C and C++ have both pretest and posttest forms, in which the control expression can be
    while (ctrl_expr) do
    loop body loop body
    while (ctrl_expr)
    * Java is like C and C++, except the control expression must be Boolean (and the body can only be entered at the beginning -- Java has no goto
    Iterative Statements: Logically-Controlled Loops: Examples
    * Ada has a pretest version, but no posttest
    * FORTRAN 95 has neither
    * Perl and Ruby have two pretest logical loops, while and until. Perl also has two posttest loops
    Iterative Statements: User-Located Loop Control Mechanisms
    * Sometimes it is convenient for the programmers to decide a location for loop control (other than top or bottom of the loop)
    * Simple design for single loops (e.g., break)
    * Design issues for nested loops
    * Should the conditional be part of the exit?
    * Should control be transferable out of more than one loop?
    Iterative Statements: User-Located Loop Control Mechanisms break and continue
    * C , C++, Python, Ruby, and C# have unconditional unlabeled exits (break)
    * Java and Perl have unconditional labeled exits (break in Java, last in Perl)
    * C, C++, and Python have an unlabeled control statement, continue, that skips the remainder of the current iteration, but does not exit the loop
    * Java and Perl have labeled versions of continue
    Iterative Statements: Iteration Based on Data Structures
    * Number of elements of in a data structure control loop iteration
    * Control mechanism is a call to an iterator function that returns the next element in some chosen order, if there is one; else loop is terminate
    * C's for can be used to build a user-defined iterator:
    for (p=root; p==NULL; traverse(p)){
    * C#‘s foreach statement iterates on the elements of arrays and other collections:
    Strings[] = strList = {"Bob", "Carol", "Ted"};
    foreach (Strings name in strList)
    Console.WriteLine ("Name: {0}", name);
    > The notation {0} indicates the position in the string to be displayed
    * Perl has a built-in iterator for arrays and hashes, foreach
    Unconditional Branching
    * Transfers execution control to a specified place in the program
    * Represented one of the most heated debates in 1960‘s and 1970‘s
    * Well-known mechanism: goto statement
    * Major concern: Readability
    * Some languages do not support goto statement (e.g., Java)
    * C# offers goto statement (can be used in switch statements)
    * Loop exit statements are restricted and somewhat camouflaged goto‘s
    Guarded Commands
    * Designed by Dijkstra
    * Purpose: to support a new programming methodology that supported verification (correctness) during development
    * Basis for two linguistic mechanisms for concurrent programming (in CSP and Ada)
    * Basic Idea: if the order of evaluation is not important, the program should not specify one
    Selection Guarded Command
    * Form
    if <Boolean exp> -> <statement>
    [] <Boolean exp> -> <statement>
    [] <Boolean exp> -> <statement>
    * Semantics: when construct is reached,
    > Evaluate all Boolean expressions
    > If more than one are true, choose one non-deterministically
    > If none are true, it is a runtime error
    Selection Guarded Command: Illustrated
    Loop Guarded Command
    * Form
    do <Boolean> -> <statement>
    [] <Boolean> -> <statement>
    [] <Boolean> -> <statement>
    * Semantics: for each iteration
    > Evaluate all Boolean expressions
    > If more than one are true, choose one non-deterministically; then start loop again
    > If none are true, exit loop
    Guarded Commands: Rationale
    * Connection between control statements and program verification is intimate
    * Verification is impossible with goto statements
    * Verification is possible with only selection and logical pretest loops
    * Verification is relatively simple with only guarded commands
    * Expressions
    * Operator precedence and associativity
    * Operator overloading
    * Mixed-type expressions
    * Various forms of assignment
    * Variety of statement-level structures
    * Choice of control statements beyond selection and logical pretest loops is a trade-off between language size and writability
    * Functional and logic programming languages are quite different control structures
    1) What is the significance of precedence and associativity? Give examples
    2) Explain conditional expressions of C language.
    3) Discuss precedence and associativity rules of different programming languages
    4) Explain the side effect related to evaluation of expression.
    5) What is selection statement? Explain different types of selection statements.
    6) Explain with examples user located loop control mechanisms provided by various anguages.
    7) Explain in detail counter-controlled loops.
    8) Explain Dijkstra’s selection construction and loop structure.

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