Structure of Programming Language

Statements

Statements

Expression

What is an expression ?

• The notion of value is central to programming.

• Program variables get instantiated to values at run-time.

– Integer variables to integer values

– String variables to array of characters etc.

• With this perspective, we could define an expression

simply as:

An expression is a formal description of a value.

Expression Examples

• 2

• 2 * 5

• F(4) + 2*5 // Need to define function F

• A < B

• A < B \/ C = D // A,B,C,D are variables

• P(A, B) \/ Q(C, D) // P,Q are predicates

Prefix, Infix, Postfix

• Notation Position of Function Examples

• Prefix Left of argument(s) sqrt(16), f(3,4)

• Infix Between two arguments 3 f 4, 3 + 4

• Postfix Right of arguments 16 sqrt, 3 4 f

Postfix evaluation - Example

Expression Code Stack Contents

3 5 + 8 6 - * push 3 <3>

^ push 5 <3,5>

add <8>

3 5 + 8 6 - * push 8 <8,8>

^ push 6 <8,8,6>

sub <8, 2>

3 5 + 8 6 - * mul <16>

^

C, C++, and Java have over 50 operators and 17 different levels of precedence

Pascal: not, unary - *, /, div, mod, +, -

Ada: ** *, /, mod, rem unary -, not +, -, &and, or, xor

Operator Precedence

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

- Use relational operators and operands of various

types

- Evaluate to some boolean representation

- Operator symbols used vary somewhat among

languages (!=, /=, .NE., <>, #)

Relational Expressions

- Operands are boolean and the result is boolean

- Operators:

FORTRAN 77 FORTRAN 90 C Ada

.AND. and && and

.OR. or || or

.NOT. not ! not

xor

- C has no boolean type--it uses int type with 0

for false and nonzero for true

-

Boolean Expressions

Relational and Boolean Expressions: No Boolean Type in C

• C 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)

Evaluating an expression without evaluating all the

operands.

e.g. (a > b) and (c > 5)

If we know that a > b is false, then there is no need

To determine whether (c > 5) is true.

Short Circuit Evaluation

Pascal: does not use short-circuit evaluation index := 1; while (index <= length) and (LIST[index] <> value) do index := index + 1

If value is not in LIST, then ???

Short Circuit Evaluation

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)

FORTRAN 77: short circuit, but any side-affected place must be set to undefined

Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3)

Short circuit evaluation

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

else average = sum /count

Let expressions

• Example: let square(x) = x*x in square(square(2))

• Of the form: let function_definition in

sub_expression

• The function definition defines a function f in

equational form.

• The sub-expression contains function applications of f

• We assume that definition of f is non-recursive.

Let expressions

• Evaluation proceeds by replacing applications of f

in sub-expression with the definition of f

• Example: let square(x) = x*x in

square(square(2))

• square(2) * square(2)

• 2 * 2 * 2 * 2 = 16

• Let expressions allow for function definitions.

• Their evaluation is same as macro-expansion.

Statement

Assignment statement

Assignment Statements

• The general syntax

<target_var> <assign_operator> <expression>

• The assignment operator

= FORTRAN, BASIC, PL/I, C, C++, Java

:= ALGOLs, Pascal, Ada

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

Mixed-Mode Assignment

• Assignment statements can also be mixed-mode, for exampleint a, b;

float c;

c = a / b;• In Pascal, integer variables can be assigned

to real variables, but real variables cannot be assigned to integers

• In Java, only widening assignment coercions are done

• In Ada, there is no assignment coercion

Statement

Selection

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 clauseelse clause

• Design Issues:

– In C, Python, and C++, the control

expression can be arithmetic

– In languages such as Ada, Java, Ruby, and

C#, the control expression must be Boolean

Two-Way Selection: Examples

• FORTRAN: IF (boolean_expr) statement

• Problem: can select only a single statement; to select

more, a GOTO must be used, as in the following example

IF (.NOT. condition) GOTO 20

...

20 CONTINUE

• This problem was solved in FORTRAN 77

Two-Way Selection: Examples

• ALGOL 60:

if (boolean_expr)

then statement (then clause)

else statement (else clause)

• The statements could be single or compound

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

Multiple-Way Selection

• Early multiple selectors:

– FORTRAN arithmetic IF (a three-way selector)

IF (arithmetic expression) N1, N2, N3

– Segments require GOTOs

Multiple-Way Selection

• Modern multiple selectors

– C’s switch statement

switch (expression) {

case const_expr_1: stmt_1;

…

case const_expr_n: stmt_n;

[default: stmt_n+1]

}

31

Switch in C, C++, Jave

switch (x)

default:

if (prime(x))

case 2: case 3: case 5: case 7:

process_prime(x);

else

case 4: case 6: case 8:

case 9: case 10:

process_composite(x);

Multiple-Way Selection in C#

• 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)

• The control expression and the case constants can be strings

switch (value) {

case -1: Negatives++;break;

case 0: Zeros++; goto case 1;

case 1: Positives++;break;

default: Console.WriteLine(“!!!\n”); }

Multiple-Way Selection: Examples

• Design choices for C’s switch statement

1. Control expression can be only an integer type

2. Selectable segments can be statement

sequences, blocks, or compound statements

3. default clause is for unrepresented values (if

there is no default, the whole statement does

nothing)

The Ada case statement

case Next_Char is

when ‘I’ => Val := 1;

when ‘V’ => Val := 5;

when ‘X’ => Val := 10;

when ‘C’ => Val := 100;

when ‘D’ => Val := 500;

when ‘M’ => Val := 1000;

when others => raise Illegal_Numeral;

end case;

Statement

Iterative

Iterative Statements

• The repeated execution of a statement or

compound statement is accomplished either by

iteration or recursion

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: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?

Iterative Statements: Examples

• FORTRAN 90 syntax

DO label var = start, finish [, stepsize]

• Stepsize can be any value but zero

• Design choices:

1. Loop variable must be INTEGER

3. The loop variable cannot be changed in the loop;

because they are evaluated only once, it does not

affect loop control

Iterative Statements

• Pascal’s for statement

for variable := initial (to|downto) final do

statement

• Design choices:

1. Loop variable must be an ordinal type of usual scope

2. After normal termination, loop variable is undefined

3. The loop variable cannot be changed in the loop but

they are evaluated just once, so it does not affect

loop control

Iterative Statements: Examples

• Adafor var in [reverse] discrete_range loop

...

end loop

• A discrete range is a sub-range of an integer or enumeration type

• Scope of the loop variable is the range of the loop

• Loop variable is implicitly undeclared after loop termination

Iterative Statements: Examples

• C’s for statementfor ([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

• Everything can be changed in the loop

• The first expression is evaluated once, but the other two are evaluated with each iteration

Iterative Statements: Examples

• C++ differs from C in two ways:

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

• Design issues:

– Pre-test or post-test?

– Should the logically controlled loop be a special case of the counting loop statement ?

• General forms:

while (ctrl_expr) do

loop body loop body

while (ctrl_expr)

Iterative Statements: Logically-Controlled Loops: Examples

• Pascal has separate pre-test and post-test logical

loop statements (while-do and repeat-until)

• C and C++ also have both, but the control

expression for the post-test version is treated just

like in the pre-test case (while-do and do- while)

• Java is like 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 post-test

• FORTRAN 77 and 90 have neither

• Perl has two pre-test logical loops, while and

until, but no post-test logical loop

Iterative Statements: User-Located Loop Control Mechanisms break and continue

• C , C++, Java, Python, Ruby, C# : break statement

Unconditional; for any loop or switch; one level only

• Java and C# have a labeled break statement:

control transfers to the label

• An alternative: continue statement; it skips the

remainder of this iteration, but does not exit the

loop

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., Module-2 and Java)

• C# offers goto statement (can be used in switch statements)