The Assignment Operator and Statement The Most Common

The Assignment Operator and Statement The Most Common Statement you will use Copyright © 2004 -2007 Curt Hill

Changing variables • What can change in a variable? – Only the value – The type and name are set only at compile time • What statements can change the value? – Declaration might or might not initialize – An assignment – Some types of method calls – Input statements are a type of method call Copyright © 2004 -2007 Curt Hill

Assignment Statement • Simple form: variable = expression ; • Simple meaning: the value computed from the expression is stored in the variable • Similar to the assignment in very many languages Copyright © 2004 -2007 Curt Hill

What is an expression? • • • A legal combination of the following: Constants Variables Operators The legal combinations are usually intuitive Copyright © 2004 -2007 Curt Hill

Legal Examples • Suppose the following declarations: int a, b, c; double x, y, z; • The following are legal: a = 5; // constant x = y; // variable y = 2*z – x/y; // expression Copyright © 2004 -2007 Curt Hill

General rules • Item on left must be a variable • The two sides should be of the same type with a few exceptions – A weaker type may be converted to a stronger type – A float is weaker than a double – A double is stronger than an int • The right hand side items are not changed • The old value of the left hand side is lost when a new value is stored Copyright © 2004 -2007 Curt Hill

More Examples • Suppose the following declarations: int a, b, c; double x, y, z; • The following are not legal: 5 = -c; // cannot change a = 5. 8; // loses precision x = false; // incompatible x = y // no semicolon x = y + * z; // malformed expr • The following are legal: x = a; // Stronger type Copyright © 2004 -2007 Curt Hill

Expression Values • What is the value of: 2 + 3*2 + 1 • Two reasonable candidates: • 11 = ((2+3) * 2) + 1 – Left to right evaluation – Some calculators do it this way • 9 = 2 + (3*2) + 1 – The multiplication precedes addition – Algebra uses this way • We must choose one of these Copyright © 2004 -2007 Curt Hill

Precedence • Clearly we cannot have both 11 and 9 correct or we never know what a computation produces • Java like most (but not all) programming languages follows Algebra • Multiplication precedes addition • This gives rise to a precedence chart – Shows the level of importance for operators Copyright © 2004 -2007 Curt Hill

Precedence Chart • Highest to lowest –() – - + (Unary sign change) –*/% – + - (Binary arithmetic) • There are several others which we will encounter soon enough Copyright © 2004 -2007 Curt Hill

Algebra and the Assignment • There is often confusion about the assignment because it looks like an equation • An equation is a true or false statement • An assignment is a command to perform some action Copyright © 2004 -2007 Curt Hill

Equations and Assignments • The equation: x=y+5 – States that the value of x is five larger than y’s value – This may be true or false depending on the value of x and y – Usually our job is to find an x and y that makes it true – this is called a solution • The assignment: x = y + 5; – Commands that the value of x is computed to be five plus y’s value Copyright © 2004 -2007 Curt Hill

Another Example • The equation: x=x+1 has no solution – It can never be true – No value may equal itself plus one • The assignment: x = x + 1; increments x – It is a command will be carried out Copyright © 2004 -2007 Curt Hill

Another Example • Suppose: int a=2, b=5, c=-7; • What happens to the variable values when: c = a*b - b%c+1; b = a-c/(1+a*2); is executed? • The value of c becomes 6 and the value of b becomes 1 Copyright © 2004 -2007 Curt Hill

How does this work? • int c = • b = • b = • b = a=2, b=5, c=-7; a*b - b%c+1; 2*5 – 5%-7+1 10 – 5 +1 6 a – c/(1+a*2); 2 – 6/(1+2*2) 2 – 6/(1+4) 2 – 6/5 2 – 1 1 Copyright © 2004 -2007 Curt Hill

The assignment operator • Some languages have an assignment statement – Pascal – BASIC, Visual BASIC • The C family (C, C++, Java) treats = as a side-effect operator • What is the difference? Copyright © 2004 -2007 Curt Hill

= as operator • The following is legal: a = b = 5; • The 5 is first assigned to b • Next b is assigned to a • This is known as a multiple assignment and some other languages support this • There is more Copyright © 2004 -2007 Curt Hill

= really is an operator • The following statement is also legal: a = (b = 2*b) - (c=2); • It is equivalent to the following sequence: c=2; b = 2*b; a = b – c; Copyright © 2004 -2007 Curt Hill

Side-Effect Operator • Any operator that changes one of its operands is called a side-effect operator • Most operators produce a value but do not change their operands • For now the assignment operator is the only one, but there are more • Method calls may also have side-effects • An input statement must have a side-effect while an output will usually not Copyright © 2004 -2007 Curt Hill

Another Property of Operators • What is the value: 8– 4– 3 • Is it? (8 – 4) – 3 = 1 8 – (4 – 3) = 7 • How about exponentiation: 2 3 2 • Is it (2^3)^2 = 64 or 2^(3^2)=512? Copyright © 2004 -2007 Curt Hill

Associativity • The concept of associativity is the property that determines the order of execution when the same operator is used twice in a row • Subtraction is left to right • Exponentiation is right to left • Most operators in Java are left to right • The exception is the assignment operator Copyright © 2004 -2007 Curt Hill

Assignment Operator Properties • The associativity of = is right to left • Thus a = b = c; is the same as a = (b = c); • The assignment operator also has low precedence • If it did not, then it would not allow all computations on the right to complete • This is why this needs parentheses: a = (b = 2*b) - (c=2); Copyright © 2004 -2007 Curt Hill

Precedence Chart Again • Highest to lowest –() – - + (Unary – sign change) –*/% – + - (Binary arithmetic) –= • Yes, there are quite a few more! Copyright © 2004 -2007 Curt Hill