Concepts of Programming Languages

340 Chapter 7 Expressions and Assignment Statements

There is a loss of error detection in the C design of the assignment opera-

tion that frequently leads to program errors. In particular, if we type

if (x = y) ...

instead of

if (x == y) ...

which is an easily made mistake, it is not detectable as an error by the com-

piler. Rather than testing a relational expression, the value that is assigned to

x is tested (in this case, it is the value of y that reaches this statement). This is

actually a result of two design decisions: allowing assignment to behave like an

ordinary binary operator and using two very similar operators, = and ==, to

have completely different meanings. This is another example of the safety defi-

ciencies of C and C++ programs. Note that Java and C# allow only boolean

expressions in their if statements, disallowing this problem.

7.7.6 Multiple Assignments

Several recent programming languages, including Perl, Ruby, and Lua, provide

multiple-target, multiple-source assignment statements. For example, in Perl

one can write

($first, $second, $third) = (20, 40, 60);

The semantics is that 20 is assigned to $first, 40 is assigned

to $second, and 60 is assigned to $third. If the values of two

variables must be interchanged, this can be done with a single

assignment, as with

($first, $second) = ($second, $first);

This correctly interchanges the values of $first and $second,

without the use of a temporary variable (at least one created and

managed by the programmer).

The syntax of the simplest form of Ruby’s multiple assign-

ment is similar to that of Perl, except the left and right sides

are not parenthesized. Also, Ruby includes a few more elaborate

versions of multiple assignment, which are not discussed here.

7.7.7 Assignment in Functional Programming

Languages

All of the identifiers used in pure functional languages and some

of them used in other functional languages are just names of val-

ues. As such, their values never change. For example, in ML,

History Note

The PDP-11 computer, on

which C was first implemented,

has autoincrement and auto-

decrement addressing modes,

which are hardware versions of

the increment and decrement

operators of C when they are

used as array indices. One might

guess from this that the design

of these C operators was based

on the design of the PDP-11

architecture. That guess would

be wrong, however, because

the C operators were inherited

from the B language, which

was designed before the first

PDP-11.