def second_derivative_convergence_figure(fun, point, real_value):
# making different intervals and empty array for y coordinates
dx = np.linspace(1, 10e-3, 100)
y = np.zeros((len(dx)))
# calculating difference to analytical value for all intervals
for i in range(len(dx)):
diff = abs(real_value - second_derivative(fun, point, dx[i]))
y[i] = diff
# plotting figure
my_plot(dx, y, "second_derivative_error_figure.pdf")
def conv_plot(fun, number, correct, fun_str):
# fun : function to be tested with
# number : number for names and titles
# correct : correct answer
# fun_str : function as a string
# Takes a function and plots its convergence
# to a figure and saves it with name and title
# specified in funciton parameters
# logarithmic values to check convergence
dx = 10**(-np.linspace(1, 5, 21))
value1 = np.zeros(21)
value2 = np.zeros(21)
value3 = np.zeros(21)
# values for certain intervals
for n in range(0, 21):
value1[n] = first_derivative(fun, 5, dx[n]) - correct[0]
value2[n] = second_derivative(fun, 5, dx[n]) - correct[1]
value3[n] = num_simpson(fun, 0, 5, int(
2 * round(1 / dx[n] / 2))) - correct[2]
fig = plt.figure()
ax = fig.add_subplot(111)
# specify logaritmhic scale
plt.yscale('log')
plt.xscale('log')
ax.plot(dx, abs(value1), label='first derivative', marker='o')
ax.plot(dx, abs(value2), label='second derivative', marker='o')
ax.plot(dx, abs(value3), label='simpson integral', marker='o')
plt.legend(loc='upper right', ncol=1, borderaxespad=0.)
# x axis limit and axis labels
ax.set_xlim(dx.max(), dx.min())
ax.set_xlabel('interval length')
ax.set_ylabel('convergence')
title = 'Convergence figure: ' + fun_str
fig.suptitle(title)
name = 'convergence' + str(number) + '.pdf'
# save and show figures
fig.savefig(name, dpi=200)
plt.show()
return
def my_plot1():
fig = plt.figure()
ax = fig.add_subplot(111)
# calculate numerical derivatives for sine function at x = pi/2 with different spacings
# correct first derivative is 0
# correct second derivative is -1
x = np.pi / 2
f = np.sin(x)
correct_der1 = 0
correct_der2 = -1
dx_spacing = np.linspace(0.001, 1, 100)
first_derivative_abs_err = []
second_derivative_abs_err = []
# save absolute error for each spacing into array first_derivative_abs_err
for i in range(dx_spacing.shape[0]):
err = abs(first_derivative(np.sin, x, dx_spacing[i]) - correct_der1)
first_derivative_abs_err.append(err)
# same for second derivative
for i in range(dx_spacing.shape[0]):
err = abs(second_derivative(np.sin, x, dx_spacing[i]) - correct_der2)
second_derivative_abs_err.append(err)
# plot and add label if legend desired
ax.plot(dx_spacing,
first_derivative_abs_err,
label=r'first_derivative abs error')
ax.plot(dx_spacing,
second_derivative_abs_err,
label=r'second_derivative abs error')
ax.legend(loc=0)
# set axes labels and limits
ax.set_xlabel(r'x spacing')
ax.set_ylabel(r'Absolute error')
ax.set_xlim(dx_spacing.min(), dx_spacing.max())
ax.set_ylim(bottom=0)
plt.title(
'Absolute error as a function of x spacing. \nEvaluated derivative of function sin(x) at x=pi/2'
)
fig.tight_layout(pad=1)
# save figure as pdf with 200dpi resolution
fig.savefig('derivative_error.pdf', dpi=200)
plt.show()
def calculate_derivative_errors():
# Calculates the numerical first and second order derivatives
# and compares them to analytical values
f = lambda x: np.sin(x)
f_dot = lambda x: np.cos(x)
f_dot2 = lambda x: -np.sin(x)
x = 1.5
errors1 = []
errors2 = []
dxs = np.linspace(1e-6, 1e-1, 1000)
for dx in dxs:
errors1.append(abs(first_derivative(f, x, dx) - f_dot(x)))
errors2.append(abs(second_derivative(f, x, dx) - f_dot2(x)))
plot_derivative_errors(dxs, errors1, errors2)
def compute_derivatives(test_func: Callable, x_point: float, grid_spacing: list) -> tuple:
"""
Compute the first and second derivatives of the given 1D function at the given
point, using different values of grid spacing.
:param test_func: Function for which derivatives are approximated
:param x_point: Point in x-axis where derivative is approximated
:param num_grids: List of grid spacing values to use
:returns: Tuple of first derivate approximations and second derivative
approximations in respective indices of the tuple.
"""
# With each grid spacing value, get the approximation of first and second
# derivatives
first_derivatives = [num_calculus.first_derivative(test_func, x_point, dx)
for dx in grid_spacing]
second_derivatives = [num_calculus.second_derivative(test_func, x_point, dx)
for dx in grid_spacing]
return first_derivatives, second_derivatives
# Analytical solutions for each function
analytical_fd = der_test_function(x)
analytical_sd = sec_der_test_function(x)
analytical_rs = int_test_function(xmin, xmax)
errors_fd = [] # errors for first derivative
errors_sd = [] # errors for second derivative
errors_rs = [] # errors for riemann_sum integration
# Calculate errors for derivatives
for dx in dxs:
numerical_fd = first_derivative(test_function, x, dx)
error_fd = np.abs(numerical_fd - analytical_fd)
numerical_sd = second_derivative(test_function, x, dx)
error_sd = np.abs(numerical_sd - analytical_sd)
errors_fd.append(error_fd)
errors_sd.append(error_sd)
# Calculate errors for integration
for intervals in range(2, 50):
x_range = np.linspace(xmin, xmax, num=intervals)
numerical_rs = riemann_sum(x_range, test_function)
error_rs = np.abs(numerical_rs - analytical_rs)
errors_rs.append(error_rs)
# Plot first derivative
fig_fd = plt.figure(1)
ax = fig_fd.add_subplot(111)