Working with Loops, Conditional
Statements, and Arrays
Working with Program Loops
• Program loop
– Set of commands that executes repeatedly until a
stopping condition is met
• The For loop
– Counter variable is used to track number of times
a set of commands is run
– General structure
for (start; continue; update) {
commands
}
The For Loop
• Can be nested inside another
• Command block
– Collection of commands that is run each time
through a loop
– Distinguished by opening and closing curly braces
{}
Counter Values in the For loop
The While Loop
• A command block is run as long as a specific
condition is met
• Condition does not depend on value of a
counter variable
• General syntax
while (continue) {
commands
}
The Do/While Loop
• Test to determine whether to continue to loop
is made after the command block is run
• Structure
do {
commands
}
while (continue);
Working with Conditional
Statements
• Conditional statement
– Runs a command or command block only when
certain conditions are met
• If statement
– Most common conditional statement
The If Statement
• Syntax
if (condition) {
commands
}
• Modulus operator
– Returns the integer remainder after dividing one
integer by another
Nesting If Statements
• General structure
if (thisYear % 4 == 0) {
if statement for century years
}
The If...Else Statement
• General structure
if (condition) {
commands if true
} else {
commands if false
}
Using Multiple Else...If Statements
• General structure
if (condition 1) {
first command block
}
else if (condition 2) {
second command block
}
else if (condition 3) {
third command block
}
...
else {
default command block
}
The Switch Statement
• Syntax
switch (expression) {
case label1: commands1
break;
case label2: commands2
break;
case label3: commands3
break;
...
default: default commands
}
Managing Program Loops and
Conditional Statements
• Break command
– Used to terminate any program loop or conditional
statement
– Often used to exit a program loop
– Syntax: break;
The continue Command
• Stops processing commands in current
iteration of loop and jumps to next iteration
• Example
var total = 0;
for (var i=0; i < data.length; i++) {
if (data[i]==null) continue; // continue with the
next iteration
total += data[i];
}
Statement Labels
• Labels
– Used to identify statements in your code
– Often used with break and continue commands to
direct a program to a particular statement
• Syntax:
label: statement
• Some programmers
– Discourage use of break, continue, and label
statements
Working with Arrays
• Array
– Collection of data values organized under a single
name
– Each data value has a number or index
– General form
array[I]
Creating and Populating an Array
• To create an array
var array = new Array(length);
• Arrays without a defined length can take up
more memory
• Creating and populating an array
var array = new Array(values);
• Array literal
var array = [values];
Working with Array Length
• JavaScript arrays
– Not required to stay at a fixed size
– Definition of a value not required for every item
• Sparse arrays
– Arrays with several missing or null items
Reversing an Array
• Arrays
– Associated with a collection of methods that allow
you to change their contents, order, and size
– Syntax for applying methods
array.method()
• Methods for changing order of array items
after array is created
– reverse() and sort()
Sorting an Array
• sort() method
– Sorts array items in alphabetical order
• Compare function
– Used to sort nontextual data correctly
– Compares values of two adjacent items in the
array at a time
• Applying compare function to sort() method
array.sort(function)
Extracting and Inserting Array
Items
• Subarray
– A section of an array
• slice() method
– Extracts a part of an array
– Syntax: array.slice(start, stop)
– Can be used to insert new items into an array
array.splice(start, size, values)
Extracting and Inserting Array
Items
• Most efficient methods to insert or remove
items
–
–
–
–
push()
pop()
unshift()
shift()
Array Methods
For Loops and Arrays
• For loops
– Used to cycle through different values contained
within an array
– General structure for accessing each value in
array
for (var i=0; i < array.length; i++) {
commands involving array[i]
}
Tips for Arrays, Program Loops, and
Conditional Statements
• Save space in your program code by
– Declaring and populating each array in a single
new Array() declaration
• Use array methods like
– sort() and reverse() to quickly rearrange contents
of arrays
• Use a For loop when
– Your loop contains a counter variable
Tips for Arrays, Program Loops, and
Conditional Statements
• Use While loop for
– More general stopping conditions
• To simplify your code
– Avoid nesting too many levels of If statements
• Use Switch statement for
– Conditional statements that involve variables with
several different possible values
Tips for Arrays, Program Loops, and
Conditional Statements
• Avoid using break and continue statements to
– Cut off loops unless necessary
– Instead
• Set break conditions in the conditional expression for a
loop