Numeric Values and Computations• Today, we extend our programming capabilities
by adding numeric values and computations– As introduced last time, we can store numeric values
in int and double variables• int x;
• double y;
– In order to use these values, we write expressions which are much like mathematical expressions like x + 5 or y * 1.1
– We store the results in other variables using assignment statement
Assignment Statements• The assignment statement assigns a variable to a value
– variable = expression• variable is some variable previously declared • expression is some arithmetic expression using variables, literals, and
arithmetic operators +, -, *, /, % (mod)
• For an assignment statement to properly work, the expression must compute the same (or a compatible) type as the type that the variable was declared as– x = 5 * y – z; // x must be a compatible type with y and z
• Compatibility – same type or a type that can be adapted into another – an int value can become a double or float– a float can become a double– a double cannot become a float or int
Assignment Statement ExamplesTo compute an employee’s pay:
pay = wages * hours;
To compute the area of a circle:area = PI * radius * radius;
Or area = PI * Math.pow(radius, 2);
To compute the average of 5 test scores:average = (t1 + t2 + t3 + t4 + t5) / 5;
Note that if t1, t2, t3, t4 and t5 are int values, the expression computes an integer value (anint numerator and an int denominator cause anint division, not a float or double division)
Math is another class,we use it to peformvarious math operationslike absolute value,or exponent
More on Assignments• Expressions may also contain messages to
various objects as long as the messages invoke operations that return the proper type of value– for instance, the Math class has a number of such
operations such as sqrt or abs• x = z * Math.sqrt(y); // what if y is negative?• x = z * Math.sqrt(Math.abs(y)); • x = Math.random( ); // x is a random integer value
– int values range from about -2 billion to +2 billion, this gives us a random one in that range
– or the String length message:• nameLength = firstName( ).length + middleName( ).length
+ lastName( ).length;
The Modulo (Mod) Operator• A common need in a program is to perform a division
but obtain the remainder instead of the quotient– For instance, is x an even number? We determine this by
dividing x by 2 and looking at the remainder• a remainder of 0 means x is even and a remainder of 1 means x is odd
– The mod operator is %• x % 2 will be 0 or 1 (the only remainder for x / 2 is a 0 or a 1)
• Assume we want to use the random number generator to give us a roll of a 6-sided die– Math.abs(Math.random( )) % 6
• divide positive int value by 6 and provide just the remainder (0 to 5)
– Math.abs(Math.random( )) % 6 + 1 • gives us a number between 1 and 6
More ExamplesVolume of a cube:
volume = length * length * length;Number of seconds in x years:
seconds = x * 365 * 24 * 60 * 60; // whats wrong with the above example?
Convert F to C:celcius = (5.0 / 9) * (fahrenheit – 32); celcius = ((double) 5) / 9 * (fahrenheit – 32);
Determining change (assume amount in cents as in 142 cents) numQuarters = amount / 25; //int division gives 5 quarters
amount = amount % 25; //remainder is 17 numDimes = amount / 10; //gives 1 dime amount = amount % 10; //remainder is 7 numNickels = amount / 5; //gives 1 nickel amount = amount % 5; //remainder is 2 pennies numPennies = amount;
Short Cuts• There are certain types of assignment statements that are
used so often that we given them special shortcuts– Recall earlier the “making change” example where we saw
amount = amount – numQuarters * 25;– In general, we might have a statement that looks like this:
• var = var op expression (where op is one of our arithmetic operators)
– We can replace this with var op= expression• amount –= numQuarters * 25;
– The statement x = x + y * z / q; • becomes x += y * z / q;
– Another very common statement is to increment or decrement a variable (add 1 or subtract 1)
• usually, this looks like: x = x + 1; or count = count – 1;
– We replace these with x++; and count--;
More on Increment/Decrement• There are two versions of each statement, a prefix
statement and a postfix statement• x = x + 1; can be done as x++; or ++x; (or x + =1;)• x = x – 1; can be done as x--; or --x; (or x -= 1;)
– x++; x--; ++x; --x;• If the incr/decr is by itself, it doesn’t matter which you
use, but these can appear inside of other expressions– x++ will use the value of x first, and then increment it
(postfix) whereas ++x will increment x first and then use the new value (prefix)
– y = 5 * x++;• this does y = 5 * x; and then does x = x + 1;• if x = 3, then when the statement is done, y = 15 and x = 4
– y = 5 * ++x; • this does x = x + 1; first and then y = 5 * x; • if x = 3, then when the statement is done, y = 20 and x = 4
Numeric Conversions• Two types of conversion processes in Java
– Implicit conversions (called coercions) occur automatically if the left-hand side variable is a different type as to the right-hand side expression
• x = y; // y is an int, x is a double
– Casts occur because you force a value to change types
• casts are performed by placing the new type in ( ) as in (int)
• average = ((double) t1 + t2 + t3 + t4 + t5) / 5;• changes t1 to a double, then the rest of the expression
computes a double value not an int value– we could force a coercion instead by doing average = (t1 + t2 + t3
+ t4 + t5) / 5.0; // 5.0 is a double, so the entire thing becomes a double
Converting Inputs• The JOptionPane.showInputDialog message allows
String input only – that is, whatever is entered by the user is treated as a String
• If you want to store the value as a number, or a char, you have to convert it– To convert an input into an int use:
• Integer.parseInt(JOptionPane.showInputDialog(…));• int age = Integer.parseInt(JOptionPane.showInputDialog(“Enter your
age”));– To convert an input into a double use:
• Double.parseDouble(JOptionPane.showInputDialog(…));• double gpa =
double.parseDouble(JOptionPane.showInputDialog(“Enter your GPA”));
– To convert an input into a char use:• JOptionPane.showInputDialog(…).charAt(0);• char sex = JOptionPane.showInputDialog(“Enter your sex”).charAt(0);
Control Statements• There are generally four types of executable instructions
in a program:– Assignment statements
– Input/output statements
– Messages (method calls)
– Control statements
• In a program, all instructions are executed linearly (sequentially)– Unless a control statement alters the flow of the program
• There are 2 types of control statements– Selection
– Repetition (also called iteration or loops)
Branching Behavior
Sequential instructionBranching instructionSequential instructionSequential instructionBranching instructionSequential instructionSequential instructionSequential instructionBranching instructionSequential instruction…
Ordinarily, the program isexecuted by following eachsequential instruction in order
A control statement causes a “branch” to a new location
Branches may be used to selectbetween instructions, to skipover instructions, or torepeat instructions
?
Selection• Selection statements are used to skip an
instruction or to choose among a set of instructions
• The selection is based on a boolean evaluation– Booleans evaluate to true or false
• if true, do the statement(s), if false, skip the statement(s)
• the boolean evaluation can be based on a boolean variable or a boolean expression
– In this way, given 1 or more statements, the boolean expression is used to determine which one to select and execute
Boolean Expressions• A Boolean expression is code that evaluates to true or false• Boolean expressions often use relational operators to test the value
of a variable or to compare two or more variables• There are 6 relational operators:
– < less than– > greater than– = = equal to
• notice it is not the same as “=” used in assignment statements
– ! = not equal to– < = less than or equal to– > = greater than or equal to
• Variables and values tested using relational operators can be of numeric types or char types, we will also see ways to compare String types
The if Statement
if ( condition ) statement;
ifif is a Java is a Javareserved wordreserved word
The condition must be a The condition must be a boolean expressionboolean expression..It must evaluate to either true or falseIt must evaluate to either true or false..
If the condition is true, the statement is executed.If the condition is true, the statement is executed.If it is false, the statement is skipped.If it is false, the statement is skipped.
• A common form of selection statement is the if statement
• In the if statement, a condition is tested, and if it evaluates to true then the statement which follows is executed, otherwise the statement which follows is skipped
conditionevaluated
falsefalse
statement
truetrue
Examplesif (total > amount) total = total + (amount + 1);
if (sex = = ‘m’) pay = pay + 500;
if (age > 21) System.out.println("Ok, you can drink");
if (sunny) System.out.println("Wear your sunglasses!");
if (rainy) System.out.println("Take an umbrella");
if (x > y) x = x * 5 + y;
sex is a character
sunny and rainy areBoolean variables
Warning: Do not confuse = and = =
A Few Comments• The statement starts with if (not IF, If or iF)• The boolean condition must be placed within ( )• The condition itself is not followed by a “;”
– For instance, the following statement would confuse the compiler into thinking that the instruction to execute for the if statement is ; and that x = y + 1; is the next instruction in the program
• if (x > y); x = y + 1;
• We may couple the if statements with an “else” clause – in case there is an action to perform if the condition is false
– this is known as an if-else statement (or if-then-else)
if-else statement• Used if there are two possibilities– if you want to calculate
someone’s pay, there are two formulas, one for normal pay and another for overtime pay
• example below to the right
• The if-else statement has two separate statements, one following the condition, one following the word else– neither the condition nor else
have a “;” after it, the “;” follows the statement
• we refer to the statements as they “if clause” and the “else clause”
conditionevaluated
statement1
truetrue falsefalse
statement2
if (hours <= 40) pay = hours * wages;else pay = 40 * wages + (hours – 40) * wages * 1.5;
More Examplesif (score > 60) grade = ‘P’;else grade = ‘F’;
if (number >= 0) sqt = Math.sqrt(number);else System.out.println("Can’t take the square root of a negative number!");
if (number = = 0) System.out.println("Can’t take reciprocal of 0");else rec = 1.0 / (float) number;
if (age >= 21) canGamble = true;else canGamble = false;
Use if-else to decide what value should be assigned a variable
Use if-else to decide if an operationcould/should be performed
String Comparisons• Comparing Strings is not like comparing other values• If a and b are Strings, then this does not work: if(a = = b) …;
– This compares to see if a and b are the same thing in memory, not just two equal Strings
• To see if two String variables are storing the same values, use equals– if(a.equals(b))…; – if(a.equalsIngoreCase(b)) …; // if we don’t care about upper vs lower case
• We may want to see what the user has entered to see if they followed directions– for instance, we want the user to enter their name or quit to exit the
program• if(value.equal(“quit”)) System.exit(0);
– this only works if they type in exactly “quit”, what if they typed “Quit”?• if(value.equalIgnoreCase(“quit”)) System.exit(0);
– what if they typed “q” or “Q”?• if(value.toLowerCase( ).charAt(0) = = ‘q’) System.exit(0);
Another String Comparison• When it comes to sorting Strings, we will
want to know if one String is < or > than another– We use compareTo for this– compareTo returns an int value
• negative if the first String is less than the second
• 0 if the first String is equal to the second
• positive if the first String is greater than the second
if (a.compareTo(b) > 0)System.out.println(a + “ is greater than ” + b);
else if (a.compareTo(b) < 0) System.out.println(b + “ is greater than ” + a);
else System.out.println(a + “ is the same as ” + b);
Block Statements• Notice that in all of our previous examples, the
statement in the if clause and the else clause consisted of a single instruction
• What if we needed to accomplish more than one instruction in an if or else clause?– we use block statements– a block is a sequence of statements that is treated like a
single statement when placed in an if or else clause
• Blocks are denoted by { } symbols– { is the beginning of a block, } is the ending of a block
• Blocks will be used often in our Java programs
Example
if (temperature > 80) { System.out.print("Wear shorts and t-shirt "); effectiveTemp = temperature + HEATINDEX * humidity; System.out.println("because it is " + effectiveTemp + " degrees today"); }else System.out.println("It is not hot today, the temperature is " + temperature);
The if-clause has 3 instructions, so we place them in a block
If we didn’t use a block, then we would get a syntax error because the compiler would get confused when it reached the “else” since it did not immediately follow the if-clause
