FUNDAMENTAL ALGORITHMS
EXCHANGING THE VALUES OF TWO
VARIABLES
Algorithm Development
The problem of
interchanging the values associated two variables involves a very fundamental
mechanism that occurs in many sorting and data manipulation algorithms.
Algorithms Description
1.
Save the original value of a in t.
2.
Assign to a the original value of b.
3.
Assign to b the original value of a that is stored
in t.
Pascal Implementation
Procedure exchange (var
a,b : integer);
Var t : integer;
begin {save the original value of a then exchange a and b}
{assert : a=a0^b=b0}
t:=a;
a:=b;
b:=t;
{assert : a=a0^b=a0}
end
Notes of
Design
1.
The use of an intermediate temorary variable
allows the exchange of two variables to proceed correctly.
2.
This example emphasizes that at any stage in a
computation a variable always assumes the value dictated by the most recent
assigment made to that variable.
3.
Working through the mechanism with a particular
example can be a useful way of detecting design faults.
4.
A more common application of exchange involves two
array elements.
Applications
Sorting algoritms.
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