Chapter 1

Recall the grade-school approximation to the number π.

p = 22/7
print(p)

print(pi)

print(f"absolute accuracy: {abs(p - pi)}")

rel_acc = abs(p - pi) / pi
print("relative accuracy: {rel_acc:.4e}")

print(f"accurate digits: {-log10(rel_acc):.1f}")

Python has native int and float types.

print(f"The type of {1} is {type(1)}")
print(f"The type of {float(1)} is {type(1.0)}")

The numpy package has its own float types:

one = float64(1)
print(f"The type of {one} is {type(one)}")

Both float and float64 are double precision, using 64 binary bits per value. Although it is not normally necessary to do so, we can deconstruct a float into its significand and exponent:

x = 3.14
mantissa, exponent = frexp(x)
print(f"significand: {mantissa * 2}, exponent: {exponent - 1}")

mantissa, exponent = frexp(x / 8)
print(f"significand: {mantissa * 2}, exponent: {exponent - 1}")

The spacing between floating-point values in $[2^n,2^{n+1})$ is $2^n \epsilon_\text{mach}$, where $\epsilon_\text{mach}$ is machine epsilon, given here for double precision:

mach_eps = finfo(float).eps
print(f"machine epsilon is {mach_eps:.4e}")

Because double precision allocates 52 bits to the significand, the default value of machine epsilon is 2^-52.

print(f"machine epsilon is 2 to the power {log2(mach_eps)}")

A common mistake is to think that $\epsilon_\text{mach}$ is the smallest floating-point number. It’s only the smallest relative to 1. The correct perspective is that the scaling of values is limited by the exponent, not the significand. The actual range of positive values in double precision is

finf = finfo(float)
print(f"range of positive values: [{finf.tiny}, {finf.max}]")

For the most part you can mix integers and floating-point values and get what you expect.

1/7

37.3 + 1

2**(-4)

You can convert a floating value to an integer by wrapping it in int.

int(3.14)

There is no double precision number between 1 and $1+\varepsilon_\text{mach}$. Thus, the following difference is zero despite its appearance.

eps = finfo(float).eps
e = eps/2
print((1.0 + e) - 1.0)

However, $1-\varepsilon_\text{mach}/2$ is a double precision number, so it and its negative are represented exactly:

print(1.0 + (e - 1.0))

This is now the “correct” result. But we have found a rather shocking breakdown of the associative law of addition!

ep = 1e-6   
a, b, c = 1/3, (-2 - ep) / 3, (1 + ep) / 3   # coefficients of p

Here are the roots as computed by the quadratic formula.

d = sqrt(b**2 - 4*a*c)
r1 = (-b - d) / (2*a)
r2 = (-b + d) / (2*a)
print(r1, r2)

The display of r2 suggests that the last five digits or so are inaccurate. The relative error in the value is

print(abs(r1 - 1) / abs(1))
print(abs(r2 - (1 + ep)) / abs(1 + ep))

The condition number of each root is

Thus, relative error in the data at the level of roundoff can grow in the result to be roughly

print(finfo(float).eps / ep)

This matches the observation pretty well.

Here we show how to use horner to evaluate a polynomial. First, we have to ensure that the FNC package is imported.

import FNC

Here is the help string for the function:

help(FNC.horner)

We now define a vector of the coefficients of $p(x)=(x−1)^3=x^3−3x^2+3x−1$, in descending degree order. Note that the textbook’s functions are all in a namespace called FNC, to help distinguish them from other Python commands and modules.

c = array([1, -3, 3, -1])
print(FNC.horner(c, 1.6))

The above is the value of $p(1.6)$, up to a rounding error.

a = 1;  b = -(1e6 + 1e-6);  c = 1;
x1 = (-b + sqrt(b**2 - 4*a*c)) / 2*a
x2 = (-b - sqrt(b**2 - 4*a*c)) / 2*a
print(x1, x2)

The first value is correct to all stored digits, but the second has fewer than six accurate digits:

error = abs(1e-6 - x2) / 1e-6 
print(f"There are {-log10(error):.2f} accurate digits.")

The instability is easily explained. Since $a=c=1$, we treat them as exact numbers. First, we compute the condition numbers with respect to $b$ for each elementary step in finding the “good” root:

Using (1.2.9), the chain rule for condition numbers, the conditioning of the entire chain is the product of the individual steps, so there is essentially no growth of relative error here. However, if we use the quadratic formula for the “bad” root, the next-to-last step becomes $u_4=(-u_3) - b$, and now $\kappa=|u_3|/|u_4|\approx 5\times 10^{11}$. So we can expect to lose 11 digits of accuracy, which is what we observed. The key issue is the subtractive cancellation in this one step.

We repeat the rootfinding experiment of Demo 1.4.1 with an alternative algorithm.

a = 1;  b = -(1e6 + 1e-6);  c = 1;

First, we find the “good” root using the quadratic formula.

x1 = (-b + sqrt(b**2 - 4*a*c)) / 2*a

Then we use the identity $x_1x_2=\frac{c}{a}$ to compute the smaller root:

x2 = c / (a * x1)
print(x1, x2)

To be sure we have an accurate result, we compute its relative error.

print(abs(x2 - 1e-6) / 1e-6)

Our first step is to construct a polynomial with six known roots.

r = [-2, -1, 1, 1, 3, 6]
p = poly(r)
print(p)

Now we use a standard numerical method for finding those roots, pretending that we don’t know them already. This corresponds to $\tilde{y}$ in Definition 1.4.1.

r_computed = sort(roots(p))
print(r_computed)

Here are the relative errors in each of the computed roots.

print(abs(r - r_computed) / r)

It seems that the forward error is acceptably close to machine epsilon for double precision in all cases except the double root at $x=1$. This is not a surprise, though, given the poor conditioning at such roots.

Let’s consider the backward error. The data in the rootfinding problem is the polynomial coefficients. We can apply poly to find the coefficients of the polynomial (that is, the data) whose roots were actually computed by the numerical algorithm. This corresponds to $\tilde{x}$ in Definition 1.4.1.

p_computed = poly(r_computed)
print(p_computed)

We find that in a relative sense, these coefficients are very close to those of the original, exact polynomial:

print(abs(p - p_computed) / p)

In summary, even though there are some computed roots relatively far from their correct values, they are nevertheless the roots of a polynomial that is very close to the original.