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Horner’s Method
summary
This subchapter looks at using Horner’s Method to reduce the computational overhead for evaluating a polynomial function.
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This subchapter looks at using Horner’s Method to reduce the computational overhead for evaluating a polynomial function.
free computer programming text book projecttable of contents
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This subchapter looks at using Horner’s Method to reduce the computational overhead for evaluating a polynomial function.
Consider the polynomail function:
ƒ(x) = 8x5 + 5x4 - 3x3 + 9x2 + 2x -7
Assuming that the exponentiation is performed by repeated multiplication (that is, 3x3 is the same as 3 · x · x · x, which is four multiplications), then there are a total of 6 + 4 + 3 + 2 +1 or 15 multiplications and five (5) additions.
We can use Horner’s Method to successively factor out x as follows:
ƒx) = 8x5 + 5x4 - 3x3 + 9x2 + 2x -7
= (8x4 + 5x3 - 3x2 + 9x + 2)x -7
= ((8x3 + 5x2 - 3x + 9)x + 2)x -7
= (((8x2 + 5x - 3)x + 9)x + 2)x -7
= ((((8x + 5)x - 3)x + 9)x + 2)x -7
The new form of the polynomial only requires five multiplcations and five additions,.
This is an improvement from N(N +1)/2 multiplications and N additions to N multiplications and N additions.
We can use a simple loop to do the steps in this process.
…
{numbers will be entered in the order x, -7, 2, 9, -3, 5, 8}
writeln('enter x: ');
read(x);
lower := 1;
upper := 5;
subtotal := 0;
writeln('enter coefficients right to left:');
for counter := lower to higher
do begin
writeln('enter next coefficient ');
read(n);
subtotal := subtotal * x + n;
end; {for}
writeln('the answer is',subtotal);
…
Note that we can slightly increase efficiency by moving the first coefficient out of the loop (because we don’t really need to multiply by zero):
…
{numbers will be entered in the order x, -7, 2, 9, -3, 5, 8}
writeln('enter x: ');
read(x);
lower := 2;
upper := 5;
writeln('enter coefficients right to left:');
writeln('enter first coefficient ');
read(subtotal);
for counter := lower to higher
do begin
writeln('enter next coefficient ');
read(n);
subtotal := subtotal * x + n;
end; {for}
writeln('the answer is',subtotal);
…
It should be easy to write a short program that can evaluate a polynomial of any degree. And that is left as a problem for the student.
The next refinement to our sample code is to get the information from arrays rather than from user input.
…
{array has the numbers in the order x, -7, 2, 9, -3, 5, 8}
{x is set to the value to be evaluated}
lower := 2;
upper := 5;
subtotal := n[1];
for counter := lower to higher
do begin
writeln('enter next coefficient ');
subtotal := subtotal * x + n[counter];
end; {for}
writeln('the answer is',subtotal);
…
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Created: November 23, 2010
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