Transcript The assignment expre..

The assignment expressions
The assignment operator in an
assignment statement
• We have seen the assignment statement:
var = expr;
• Effect:
• Stores the value of expression expr in the
variable named var
The assignment expression
• Consider the following arithmetic expression:
int a = 3;
int b = 4;
Arithmetic expression: a + b
Returns the result: 7
The assignment expression (cont.)
• Definition: assignment expression:
var = expr
Result returned by an assignment expression:
Result of var = expr is equal to the value of
expr
The assignment expression (cont.)
• Note:
• The assignment operator will (still) update the value
stored inside the variable var
• In addition to updating the variable. the assignment
operator also returns a value
The assignment expression (cont.)
• Example:
public class AssignExpr01
{
public static void main(String[] args)
{
int a;
a = 0;
System.out.println(a);
System.out.println(a = 2); // Prints 2
System.out.println(a); // Prints 2
}
}
The assignment expression (cont.)
• Explanation:
• System.out.println(a = 2) will print the value between
its brackets
• The expression between the brackets is:
a=2
which is an assignment expression
• The assignment expression returns the value 2
Therefore, System.out.println will print the value 2
The assignment expression (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
AssignExpr01.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac AssignExpr01.java
• To run:
java AssignExpr01
The result returned by the assignment
operators
• We have seen many different assignment operators:
•=
• +=
• -=
• *=
• /=
• %=
The result returned by the assignment
operators (cont.)
• Result returned by the assignment operators:
Operator
Example
Returned value
=
a=2
2
+=
a += 2
a+2
-=
a -= 2
a-2
*=
a *= 2
a *2
/=
a /= 2
a/2
%=
a %= 2
a%2
The result returned by the assignment
operators (cont.)
• Example:
public class AssignExpr02
{
public static void main(String[] args)
{
int a;
a = 5;
System.out.println(a = 2);
a = 5;
System.out.println(a += 2);
a = 5;
System.out.println(a -= 2);
a = 5;
System.out.println(a *= 2);
a = 5;
System.out.println(a /= 2);
a = 5;
System.out.println(a %= 2);
}
}
The result returned by the assignment
operators (cont.)
• Output:
2
7
3
10
2
1
The result returned by the assignment
operators (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
AssignExpr02.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac AssignExpr02.java
• To run:
java AssignExpr02
Priority and associativity of the assignment
operators
• Priority of the assignment operators:
• All of the assignment operators have the lowest
priority among all operators in Java
Priority and associativity of the assignment
operators
• Associativity of the assignment operators:
• All of the assignment operators are right associative
• Meaning:
• When there are multiple assignment
operators in an expression, the
expression is evaluate from right to left
Priority and associativity of the assignment
operators (cont.)
• Example: cascaded assignment
public class AssignExpr03
{
public static void main(String[] args)
{
int a, b, c;
c = b = a = 4 + 2;
// Cascade assignment
System.out.println(a); // Prints 6
System.out.println(b); // Prints 6
System.out.println(c); // Prints 6
}
}
Priority and associativity of the assignment
operators (cont.)
• Explanation:
c = b = a = 4 + 2; is evaluated as follows:
c = b = a = 4 + 2; higher priority
^^^^^
Reduces to:
c = b = a = 6; from right to left
^^^^^
assigns 6 to variable a and returns 6
Reduces to:
c = b = 6;
^^^^^
from right to left
assigns 6 to variable b and returns 6
Reduces to:
c = 6;
^^^^^
assigns 6 to variable c (and returns 6)
Priority and associativity of the assignment
operators (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
AssignExpr03.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac AssignExpr03.java
• To run:
java AssignExpr03
Exercise
• What will the following Java program print:
public class AssignExpr04
{
public static void main(String[] args)
{
int a, b, c;
a = 1;
b = 1;
c = 2;
c *= b -= a += 1 + 2;
System.out.println(a);
System.out.println(b);
System.out.println(c);
}
}
Exercise (cont.)
• Answer:
4
-3
-6
Priority and associativity of the assignment
operators (cont.)
• Example Program: (Verify for yourself...)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/
AssignExpr04.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac AssignExpr04.java
• To run:
java AssignExpr04