4

CHAPTER

in our code have always executed in the same or-
der in which we write them. The first statement executes, then the sec-
ond, and so on. One variation on this ordering is the method call, which
executes a separate sequence of statements. But what if we want the
computer to execute the statements in an order other than sequentially?
For example, suppose we want to check the validity of input data and then
perform a calculation ordisplay an error message, but not both. To do so,
we must be able to ask a question and then, based on the answer, choose
one or another course of action.
The ifstatement allows us to execute statements in an order that
differs from their physical order. We can ask a question with this kind of
statement and do one thing if the answer is yes (true) or another if the
answer is no (false). In the first part of this chapter, we deal with asking
questions; in the second part, we examine the ifstatement itself. We
then use the ifstatement to handle multiple event sources, and apply
what we’ve seen in the Case Study.

So far, the statements

1947

The world’s first

transistor is

developed at Bell

Labs

1948

Researchers at the

University of

Manchester develop

the Manchester

Mark I, a computer

with all the main

components of

today’s computers

that can store data

and user programs

in electronic

memory

1949

Jay Forrester refines

magnetic core

memory in his work

on the Whirlwind

computer

1949

John Mauchly

creates the first

high-level

programming

language: Short

Order Code

1950

Alan Turing asks

“Can Machines

Think?” and

outlines criteria for

the Turing Test of

machine intelligence

1951

The UNIVAC

(Universal

Automatic

Computer) I is

completed