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CSCI 100
Think Like Computers
Lecture 20
Fall 2008
Last Time …
•Coding style
•Conditional execution
•logical operators and logical connectives
•if statement
if branch
else branch
•nested if statement
•cascading if statement
Conditional Repetition
•an if statement is known as a control
statement
it is used to control the execution of other
JavaScript statements
provides for conditional execution
is useful for solving problems that involve
choices
either do this or don't, based on some condition (if)
either do this or do that, based on some condition
(if-else)
Conditional Repetition
•closely related to the concept of conditional
execution is conditional repetition
many problems involve repeating some task
over and over until a specific condition is met
e.g., rolling dice until a 7 is obtained
e.g., repeatedly prompting the user for a valid
input
in JavaScript, while loops provide for
conditional repetition
While Loops
•a while loop resembles an if statement in
that its behavior is dependent on a
boolean condition.
however, the statements inside a while loop’s
curly braces (a.k.a. the loop body) are
executed repeatedly as long as the condition
remains true
general form:
While Loops
•when the browser encounters a while loop,
it first evaluates the boolean test
if the test succeeds, then the statements inside
the loop are executed in order, just like an if
statement
once all the statements have been executed,
program control returns to the beginning of the
loop
the loop test is evaluated again, and if it
succeeds, the loop body statements are
executed again
this process repeats until the boolean test fails
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