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Mat 2170
Week 5
Control Statements ā Selection
Spring 2009
1
Student Responsibilities
I Reading: Textbook, Chapter 4
I Lab 5
I Attendance
I EXAM 1 Thursday evening, 7:00pm
2
Chapter Four Overview ā 4.3 ā 4.
Along with while and for, more control statements
Used to make decisions or choices
They do not cause iteration or looping
I if
I switch
3
The if Statement ā Two Forms
I The first form is used when an operation is performed only if
a particular condition is true:
if (condition) { statements to be executed if condition is true }
I The second form is used when there are two alternatives: one
for cases in which the condition is true and the other for
cases in which it is false:
if (condition) { statements to be executed if condition is true } else { statements to be executed if condition is false }
4
Choosing Between if and if/else
I No hardāandāfast rule
I Best guideline: think about the problem description (in
English) ā if it uses else or otherwise, thereās a good chance
you need to use if/else
I Example: suppose we want to change the AverageList
program so it didnāt include any zero values in the average. A
single test is needed to ensure zero scores arenāt added in or
counted.
I However, if we wish to also count the nonāzero scores, we
would need to add an else clause to increment a counter in
the case the value entered was equal to 0.
The AverageList Program - Checking For Zeroes
print("Average a list of non-negative integers.\n"); print("Enter one value per line, "+SENTINEL + " to end\n"); int total = 0; int count = 0; int value = readInt("Enter number, "+SENTINEL+" to end: "); while (value != SENTINEL) { if (value != 0) { total += value; count++; } value = readInt("Enter number, "+SENTINEL+" to end:"); }
if (count != 0) println("Average is: " + (double) total / (double) count; else println("No valid values were entered.");
// private static final int SENTINEL = -1;
The AverageList Program - Counting Zeroes
int total = 0; int count = 0; int zeroCount = 0; int value = readInt("Averaging list, "+SENTINEL+" to end:"); while (value != SENTINEL) { if (value != 0) { total += value; count++; } else zeroCount++; value = readInt("Enter number, "+SENTINEL+" to end:"); } if (count != 0) print("Average is: " + (double) total / (double) count + "\n"; else print("No valid values were entered.\n"); println("There were " + zeroCount + " 0ās in data.");
7
Common Forms of the if Statement
I Single line:
if (condition) statement;
I Multiline if statement with curly braces
if (condition) { statement
.. .more statements... }
8
The if Statement
ENTER
true
false
EXIT
Expression
Boolean (^) ACTION
if (Boolean Expression) { ... ACTION ... } ...
9
Example ā if Statement
int sum = 0;
int n = readInt("Enter an integer: ");
if (n > 0) // if n is positive,
sum += n; // add it to sum
When the ACTION is a single statement, braces are optional.
10
Example ā if Statement
int CDCollection = 0;
double AccountBalance = 58.90;
double CDPrice = readDouble("Enter cost: ");
if (AccountBalance >= CDPrice)
AccountBalance -= CDPrice;
CDCollection++;
When the ACTION is more than one statement, braces are required.
Common Forms ā if/else
if (condition) { statements to be executed if condition is true } else { statements to be executed if condition is false }
Common Forms ā Cascading if
if (condition 1 ) { statements 1 } else if (condition 2 ) { statements 2 } .. . else if (conditionnā 1 ) { statementsnā 1 } else { statementsn }
19
Cascading if Statements
BE (^1)
BE (^2)
BE (^) nā
BE (^) n
A 1
A 2
Anā
An An+
true
false
true
true
false
false
false
false
true
20
Example ā Cascading if Statements
// Summing positive and negative values separately
int posSum = 0; int negSum = 0; int n = readInt("Enter value, "+SENTINEL+" to stop: "); while (n != SENTINEL) { if (n > 0) // if n is positive, posSum += n; // add to positive sum else if (n < 0) // if n is negative negSum += n; // add to negative sum else // if n is zero, inform user println("That value was zero"); n = readInt("Enter value, "+SENTINEL+" to stop: "); }
21
Do the elseās Matter?
// Summing positive and negative values separately // and counting number of times zero is entered
int posSum = 0; int negSum = 0; int zeroCount = 0; int n = readInt("Enter value, "+SENTINEL+" to stop: "); while (n != SENTINEL) { if (n > 0) // if n is positive, posSum += n; // add to positive sum if (n < 0) // if n is negative negSum += n; // add to negative sum else // if n is zero zeroCount++; // count it n = readInt("Enter value, "+SENTINEL+" to stop: "); }
22
Which is Required, a while or an if Statement?
to determine whether x is positive or not
to determine the sum of the values entered by a user
to eat cookies as long as the cookie jar is not empty
to find whether there is sufficient money to buy that CD
to balance a checkbook over several statements
to determine whether GOval C is Cyan
to process clients until the line is empty or itās time to quit
to pick which direction to turn
to continue picking directions and moving until you arrive at
your destination
to discover whether any part of GRect R lies outside the
graphics window
The switch Statement
The switch statement provides a convenient syntax for choosing
from among a set of possible paths:
switch ( expr ) { case v 1 : statements executed if expr == v 1 break;
case v 2 : statements executed if expr == v 2 break;
...more case clauses if needed...
default: statements executed if no values match break; }
switch Statement Example
int month = readInt("Enter month (Jan=1): "); switch ( month ) { case 2: println("28 days, 29 in leap years"); break; case 4: case 6: case 9: case 11: println("30 days"); break; case 1: case 3: case 5: case 7: case 8: case 10: case 12: println("31 days"); break; default: println("Illegal month number"); break; }
