Gary Marsden Slide 1University of Cape Town

Statements & Expressions

Gary MarsdenSemester 2 – 2000

Gary Marsden Slide 2University of Cape Town

Expressions

“spare the rod and spoil the child”Expressions are comprised of operands and

operators Languages have precise precedence rules

about which operand should be evaluated first– 4 + 4 * 3 = 24 or 16

This is not universal (though almost so)– APL evaluates operators from right to left

Operators usually binary (dyadic) or unary (monadic)

Gary Marsden Slide 3University of Cape Town

Boolean

Boolean values crop up everywhereSome languages use true/false, some use

zero/non-zeroCan use short-circuit evaluation

– in a&&b, if ‘a’ is false, don’t evaluate ‘b’– in a||b, if ‘a’ is true, don’t evaluate ‘b’– side effect problems

Operators can be confusing to novices– &&, AND, and then– ||, OR, or else

Gary Marsden Slide 4University of Cape Town

Assignment statements

General form a:= b– ‘a’ is a reference and ‘b’ is a value

In C, assignment is an expression, not a statement: allows a = b = c = 5;

Type incompatibility is handled in different ways– some resolved through automatic coercion

• int -> real is OK (widening)• real -> int looses information

– Information losses Pascal not allowed, ok in C, but• float -> int is truncated• double -> float is rounded

Gary Marsden Slide 5University of Cape Town

Sequence and control

FORTRAN introduced ‘IF’– IF (C1) L1, L2, L3– really a glorified goto jumping to L1 when C1<0,

L2 if C1 = 0 and L3 if C1>0

Nested IF’s– if C1 then if C2 then S1 else S2

Dangling else solve by– else binds to inner if in Pascal and C– use ‘elsif’ in Ada and Modula 2– use switch or case

Gary Marsden Slide 6University of Cape Town

Conditional Iteration

While statement a big favourite– while C do S (S may never be executed)

Pascal introduced Repeat– repeat S until C (S executed at least once)

Less structured version – Loop– loop S end loop

• where S contains exit when C

– this allows multiple exits

Gary Marsden Slide 7University of Cape Town

Fixed iteration

The for statement– loops for a set number of times– control variable should be ordinal (Algol60 used

reals)– shouldn’t change control variable in loop body– shouldn’t force at least one execution (Algol66)– value of the loop variable at end of loop is a

problem• some languages limit loop variable scope to loop body• officially undefined in Pascal

– Languages with enumerated types also permit iterators• for x in 1..10 do

Gary Marsden Slide 8University of Cape Town

Exceptions

Introduced in PL/1, exceptions allow programs to react to exception circumstances by jumping to an exception handler

Usually there is a chunk of code (“try block” in Java, “frame” in Ada) which could throw an exception– control is passed (caught) to exception handler– if handled correctly control is returned to

• end of frame / block in Ada / Java• line after exception generator in PL/1

Gary Marsden Slide 9University of Cape Town

Sub-programs

Complexity control and stepwise refinement required that code be split into sub-programs

Some languages (Pascal,Ada) have functions and procedures

Other languages (C++) treat procedures as a special case of function– good idea

Gary Marsden Slide 10University of Cape Town

Declaring sub-programs

Usually something to denote difference between program and sub-program– Pascal: “Procedure” “Function” “Program”– Clean: “Module”

Form is similar – each allow (local) declarations

“Formal” parameters matched to “actual” during execution– FORTRAN and K&R C had no type checking on

parameters

Gary Marsden Slide 11University of Cape Town

FORTRAN’s model

Fortran supports parameter passing but also has “Common” variables

Each subroutine has a “Common” list, were values are matched on position, not name

• sub1: COMMON A,B,C• sub2: COMMON D,E,F• sub3: COMMON P,X

– this is called “overlaying”

Fortran also supports separate compilation of subroutines– promoted NAG library

Gary Marsden Slide 12University of Cape Town

Other approaches

The Algol family store code in one file to permit parameter type checking

This does little for code re-use– Modula-2 cludges round this using extra files

and EXPORT lists identical to Clean

C has always had separate compilationVariables external to a file can be inculded

using the “external” keyword– no type checking though

Gary Marsden Slide 13University of Cape Town

Parameter passing

Three basic things happen to parameters1. Value is passed in but unchanged (in)2. Value is undefined initially but a value is

placed in the parameter after subroutine completion (out)

3. Parameter passes in value which is modified by executing subroutine (in-out)

Although there are three basic concepts, each language implements these differently

Gary Marsden Slide 14University of Cape Town

Going in

Values can be passed in by “value” orby “constant-value” (Ada)

– The idea of constant-value is that the value of the parameter cannot be altered in the subroutine

– This is to stop accidental modification of parameters which were intended as “in” but end up as “in-out”

Gary Marsden Slide 15University of Cape Town

Coming out

This really only exists in Ada in its pure form

Ada uses “Call by result”This creates a instantiation of the formal

parameter for use in the subroutineAt the end of evaluation, the value of the

formal parameter is copied to the actual parameter (which must be a variable)

Gary Marsden Slide 16University of Cape Town

Updating

Two main methods– Call by value-result: Another Ada foible, like

“Call by result”• value of actual instantiated formal

– Call by reference: Most common, where a pointer is passed to the subroutine

• this is the most common form

Gary Marsden Slide 17University of Cape Town

Other thoughts

“Call-by-name” – Algol copied text of actual parameter to replace formal parameter

Default values to formal parameters– Good old Ada; a bit redundant in my opinion

Overloading – provided different formal parameters – Ada, C++, Java

Functional languages – no ‘out’ or ‘in-out’ parameters