Tugas Alpro. Ringkasan "Exchanging the values of two variables"

In this chapter, we will learn how to exchange values of two variable on a fundamental algorithms.
Lets begin the summary!

Problem

•  Given two variables a and b, exchange the values assigned to them.

Algorithm development

Consider that the variables a and b are assigned values as outlined below. That is:

Starting Configuration

Configuration above means that on variable a contains value 721, and variable b contains value 463, Now our task is find a way how to make variable a contains 463 and variable b contains 721. Like this:

Target Configuration

To exchange the value of a variables, we can use the assignment operator:

Assignment operator

":=" is the assignment operator. ":=" in (1) means value of variable b copied and change the original value of variable a .
Lets practice!

This is the started configuration:

then after execution of the assignment a:=b; we have:

After execution a:=b

Assignment (1) change the value of variable a with copy value of variable b. and value of variable b still same just like the starting configuration. and when we execution assignment "b:=a" it will end up like this:

After execution b:=a

Still same.. Why? Because after executed assignment "a:=b" value of variable a is changed. Already changed. Result is after execute assignment "b:=a" original value of variable a lost. And it doesn't represent solution we are seeking.

To solve this problem, we need to find a way of not lost the original value of variable a when we make assignment "a:=b". That way is, we must introduce a new variable, or we can call, a temporary variable. Name is variable t. this variable needed to copy the original value of variable a, for temporary, before executing assignment "a:=b". The Step to do that is:

Assignment for variable t

after two steps above, result is:

After execution "t:=a" and "a:=b"

than last time, we doesn't lost value of variable a, because that value saved on variable t. Now, we just need to execute assignment :

Assignment to target configuration

After execution "b:=t" assignment, we have our target configuration:

Target configuration

We can see that a and b have now been interchanged as required. Now we can outlined exchange procedure.

Algorithm description

• Save original value of a into t 
• Assign original value of b into a
• Assign to b original value of a that is saved in t

Exchange mechanism as a programming tool is most usefully implemented as a procedure that accepts two variables and return their exchanged values.

Pascal Implementation

Notes on design

1. The use of an intermediate temporary variable allows the exchange of two variable proceed correctly.
2. Any stage in a computation variable always assumes the value dictated by the most recent assignment made to that variable.
3. Working through the mechanism with a particular example can be a useful way of detecting design faults.