Parameter passing mechanism: pass-by-value

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Parameter passing mechanism pass-by-value
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Introduction

• In the last webpage, we discussed how to pass
  information to a method
• I have kept it (deliberately) simple by using
  constant values as parameters
• In this webpage, we will discuss how to pass
  information stored in variables to a method.
• Specifically, we will study the pass-by-value
  mechanism

r ToolBox.min( 1.0, 4.0 )
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Example passing information stored in variables

• Consider the following program

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Example passing information stored in variables
(cont.)

• Question to ponder
• There are quite a few ways to allow (enable) you
  to
• accomplish this "passing"
• The possible answers ranges from simple to pretty
  weird

• How can we pass (give) the information stored
  inside the variables x and y to the method
  ToolBox.min

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Parameter passing mechanisms

• Definition

• Parameter passing mechanism agreement between
  the calling method and the called method on how a
  parameter is passed between them

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Parameter passing mechanisms (cont.)

• Important note

• Both the calling method and the called method
  must agree to use the same passing mechanism (or
  else, the information will be passed incorrectly)

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Parameter passing mechanisms (cont.)

• Most commonly used parameter passing mechanisms

Pass-by-value
The calling method passes the information stored
inside a variable by passing ( copying) the
value contained inside a variable into the
parameter variable.
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Parameter passing mechanisms (cont.)
This is the most obvious way to pass
information... Example

• if you want to give you phone number of your
  home to someone, you make a copy of the
  information (in the parameter variable)

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Parameter passing mechanisms (cont.)
Pass-by-reference

• The calling method passes the information stored
  inside a variable by passing ( copying) the
  address (location) of a variable into the
  parameter variable.

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Parameter passing mechanisms (cont.)
This is a less obvious but more powerful way to
pass information... Example

• if you want to give you phone number of your
  home to someone, you make a copy of the address
  of your home (in the parameter variable)
• He/she can find the phone number by visiting that
  address !!!

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Parameter passing mechanisms (cont.)

• Terminology

• A reference in Computer Science is the location
  (or address) (of a variable or a method)

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Terminology formal parameters and actual
parameters

• Definitions

• Formal parameter a parameter variable
• Actual parameter a variable whose value is to
  be passed to some formal parameter

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Terminology formal parameters and actual
parameters (cont.)

• Illustrated example

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Terminology formal parameters and actual
parameters (cont.)

• Explanation

• The parameter variables a and b in the
  definition of the ToolBox.min method are formal
  parameters
• The variables x and y used in the method
  invocation ToolBox.min(x, y) are actual
  parameters

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The Pass-by-value mechanism - the agreement

• Recall

• Parameter passing mechanism agreement between
  the calling method and the called method on how a
  parameter is passed between them

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The Pass-by-value mechanism - the agreement
(cont.)

• The agreement used in the Pass-by-value
  mechanism

• For the calling method

• The calling method creates the parameter
  variables for the called method, .... and
• The calling method copies the value of the
  actual parameter into the formal parameter

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The Pass-by-value mechanism - the agreement
(cont.)

• For the called method

• The called method obtains the information
  directly from the parameter variables

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The Pass-by-value mechanism - an example

• Example program

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The Pass-by-value mechanism - an example (cont.)

• When main starts running, it will first create
  its local variables

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The Pass-by-value mechanism - an example (cont.)

• When execution reaches the method call
  ToolBox.min(x,y), the Pass-by-value mechanism
  first creates the parameter variables

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The Pass-by-value mechanism - an example (cont.)

• Then the Pass-by-value mechanism copies the value
  of the actual parameter to the corresponding
  formal parameter

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The Pass-by-value mechanism - an example (cont.)

• The method invocation mechanism is completed as
  usually with the following steps

• Save the return address on the stack

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The Pass-by-value mechanism - an example (cont.)

• Jump to the called method

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The Pass-by-value mechanism - an example (cont.)

• When the min method executes, it will create its
  local variable m

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The Pass-by-value mechanism - an example (cont.)

• Notice how the called method uses the parameter
  variables

• When the called method uses a parameter
  variable, the information is obtained directly
  from the parameter variable

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A quiz on the Pass-by-value mechanism

• Consider the following program

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A quiz on the Pass-by-value mechanism (cont.)

• Questions

• What value is printed by the statement
  System.out.println(x) ?
• What value is printed by the statement
  System.out.println(y) ?
• What value is printed by the statement
  System.out.println(r) ?

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A quiz on the Pass-by-value mechanism (cont.)

• Example Program (Demo above code)        
• Prog file
• http//www.mathcs.emory.edu/cheung/Courses/170/S
  yllabus/08/Progs/pass-by-value/quiz/MyProgram.java
  
• Prog file http//www.mathcs.emory.edu/cheung/Cou
  rses/170/Syllabus/08/Progs/pass-by-value/quiz/Tool
  Box.java
• How to run the program

• Right click on links and save in a scratch
  directory
• To compile   javac MyProgram.java
• To run          java MyProgram

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A quiz on the Pass-by-value mechanism (cont.)

• Output of the program
• Did you understand why the update statements "a
  a 1" and "b b 2" did not update the
  actual parameters x and y ???

1.0 (the value in x is UNCHANGEDD !) 4.0 (the
value in y is UNCHANGEDD !) 8.0 ( 2.0 6.0)
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The quiz explained

• Notice the similarities between the ToolBox.min
  and the ToolBox.fun methods

public static double min ( double a,
double b ) double m 0
if ( a lt b ) m a
else m b
return(m)
public static double fun ( double a,
double b ) double m 0 a
a 1 b b 2 m a b
return(m)
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The quiz explained (cont.)

• Both methods have 2 parameter variables and 1
  local variable
• I have constructed the quiz in such a way that I
  can re-use the diagrams from the Pass-by-value
  example above.

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The quiz explained (cont.)

• So according to the Pass-by-value example above,
  when the ToolBox.min method starts running, the
  following variables have been created on the
  System Stack

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The quiz explained (cont.)

• Notice that
• are different variables (they occupy different
  memory cells !)

• The local variables x and y in the main method
       and      
• The parameter variables a and b in the fun
  method

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The quiz explained (cont.)

• The assignment statements
• will change the values of the parameter
  variables

a a 1 b b 2
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The quiz explained (cont.)
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The quiz explained (cont.)

• Notice that

• The values in the actual parameters (x and y)
  are unchanged !!!

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The quiz explained (cont.)

• That's why the statements

System.out.println(x) ---gt prints 1.0
System.out.println(y) ---gt prints 4.0
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The quiz with an additional twist...

• Now, consider the following program

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The quiz with an additional twist... (cont.)

• We use the same names for actual and formal
  parameters !!!
• Questions

• What value is printed by the statement
  System.out.println(a) ?
• What value is printed by the statement
  System.out.println(b) ?
• What value is printed by the statement
  System.out.println(r) ?

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The quiz with an additional twist... (cont.)

• Example Program (Demo above code)        
• Prog file
• http//www.mathcs.emory.edu/cheung/Courses/170/S
  yllabus/08/Progs/pass-by-value/quiz2/MyProgram.jav
  a
• Prog file http//www.mathcs.emory.edu/cheung/Cou
  rses/170/Syllabus/08/Progs/pass-by-value/quiz2/Too
  lBox.java
• How to run the program

• Right click on links and save in a scratch
  directory
• To compile   javac MyProgram.java
• To run          java MyProgram

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The quiz with an additional twist... (cont.)

• Output of the program
• Did you understand why the update statements "a
  a 1" and "b b 2" (that updates the formal
  parameters) did not update the actual parameters
  a and b ???

1.0 (the value in x is UNCHANGEDD !) 4.0 (the
value in y is UNCHANGEDD !) 8.0 ( 2.0 6.0)
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The quiz explained

• Recall that

• Different method scopes are always disjoint
  scopes
• You can define different variables with the same
  name in disjoint scopes (See http//www.mathcs.em
  ory.edu/cheung/Courses/170/Syllabus/08/scope.html
  disjoint)

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The quiz explained (cont.)

• In other words
• are different variables

• The local variables named a and b defined inside
  the main method      and      
• The parameter variables named a and b defined
  inside the fun method

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The quiz explained (cont.)

• The following diagram shows the fact that there
  are 2 different variables with the same name
  created created on the System Stack

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The quiz explained (cont.)

• Notice that
• are different variables --- it's possible
  because of the scopes are non-overlapping
  (furthermore, they use different memory cells !)

• The blue colored variables named a and b are
  inside the scope of the main method      and
       
• The magenta colored variables named a and b are
  inside the scope of the fun method

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The quiz explained (cont.)

• The assignment statements
• will change the values of the parameter
  variables

a a 1 b b 2
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The quiz explained (cont.)
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The quiz explained (cont.)

• Notice that

• The values in the actual parameters (a and b)
  are unchanged !!!

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The quiz explained (cont.)

• That's why the statements

System.out.println(a) ---gt prints 1.0
System.out.println(b) ---gt prints 4.0