Loop Structures and Strings
Chapter 6 Review
The while/do-while Statement
• The while statement and do-while statement
are loop structures that iterate a set of
statements repeatedly based on a condition.
• The difference between the loops is when the
condition is evaluated.
• The while statement evaluates the condition
before any iterations are performed.
• The do-while does not evaluate the condition
until after the first iteration.
while/do-while syntax
while (<condition>) {
<statement>;
}
do {
<statement>;
} while (<condition>);
while examples
int num = 0;
while (num < 5) {
num += 1>;
}
do-while examples
do {
System.out.print("Enter a # less than 4:");
playerNum = input.nextInt();
} while (playerNum >= 4);
Infinite Loops
• Some syntax errors and logic errors can lead to
an infinite loop, which executes forever
causing the application to just hang.
• A logic error can also cause an overflow, which
occurs when there are not enough bits to
store a number.
Infinite Loop example
int num = -1;
while (num < 0) {
num = -1>;
}
Counters & Accumulators
• Counters are used for keeping track of loop
iterations and are used in applications that
keep track of the number of guesses or the
number of values entered.
• An accumulator is increased by varying
amounts.
• Accumulators are often used to sum values.
• Both counters and accumulators should be
initialized when they are declared.
flag, sentinel
• A flag, also called a sentinel, is used to signify
that a loop should stop iterating.
• Flags are usually a constant that is declared in
the application,but may also be determined
by prompting the user for a value.
The for Statement
• The for statement is a loop structure that
executes a set of statements a fixed # of times.
• A loop control variable is used to determine
loop iterations and can be declared in the for
statement itself.
for (<initialization>; <condition>; increment>) {
<statements>;
}
The for Statement example
for (int i=1; i <=10; i++) {
System.out.println(i);
}
• i is the loop control variable
• i<10 is the condition
• i++ is the increment statement
scope/programming style
• Note that the counter is declared in the
initialization of the for statement (int i = 1).
• With a declaration in this location, the scope of
the counter is from the initialization to the
closing curly brace of the for statement.
• The application will not recognize the variable
outside of that statement.
• Declaring variables so that their scope is limited
to where they are needed is good programming
style because it produces cleaner code and
helps eliminate the possibility of errors.
Debugging Techniques
• Debugging is the process of getting an application to
work correctly.
• Debugging techniques include using a debugger, often
included with a compiler, and a variable trace, which is
a manual technique.
• Debuggers have the advantage of being able to display
the actual value of a variable as it changes.
• Other techniques include adding println statements
before and after variable assignment.
• Commenting out statements can also locate an error
through process of elimination.
The String Class
• The String class is used to declare string variables.
• The string class is included in the java.lang package
• It contains methods for determining the length of a
string, converting a string to lower/uppercase,
extracting substrings, & comparing.
• A string is immutable, which means it cannot be
changed from its original value. However, a String
object can be assigned a new string in memory.
• The characters of a string have an index value, with
the first character at index 0.
Useful String Class Methods
Method
Description
length()
returns an integer corresponding to the number of
characters in the string.
substring(int start, int end)
returns a substring of the string, which starts at start
position and ends one character before the end position.
substring(int start)
returns a substring of the string, which starts at start
position and extends to the end of the string.
toLowerCase()
returns a copy of the string with all lowercase letters.
toUpperCase()
returns a copy of the string with all uppercase letters.
trim()
returns a copy of the string with all leading and
trailing spaces removed.
Useful String Class Methods Cont’d
Method
Description
replaceFirst(String str,
String str2)
returns a string with the first occurrence of str replaced by
str2.
replaceAll(String str, String
str2)
returns a string with all occurrences of str replaced by
str2.
Comparing Strings
• Strings are compared when determining
equality or alphabetical order.
• Some of the String class methods for
comparing strings include:
Method
Description
equals(String str)
returns true when the string is the same as str.
Returns false otherwise.
equalsIgnoreCase(String str)
same as equals() except that uppercase and
lowercase differences between the strings are
ignored.
Comparing Strings Cont’d
Method
Description
compareTo(String str)
returns 0 when str is the same as the string, a negative
integer is returned when str comes alphabetically after the
string, and a positive integer is returned when str comes
alphabetically before the string. Note that uppercase and
lowercase letters are considered different.
compareToIgnoreCase(
String str)
same as compareTo() except that uppercase and lowercase
differences between the strings are ignored.
indexOf(String str)
returns the integer corresponding to the location of the first
occurrence of str in the string. Otherwise –1 is returned.
lastIndexOf(String str)
returns the integer corresponding to the location of the last
occurrence of str in the string. Otherwise –1 is returned.
startsWith(String str)
returns true when the string begins with str. Returns false
otherwise.
endsWith(String str)
returns true when the string ends with str.
Returns false otherwise.