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Plotting a function

Pyplot tutorial

Plot a straight line using a function(python).

Here function is a concept in python which is a block of organized, reusable code that is used to perform a single, related action. Functions provide better modularity for your application and a high degree of code reusing.

In the text below, not to confuse with function in coding, function sometimes means a math function when we say a parabola function, an exponential function or a sine function.

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import numpy as np # A generic library for arrays and matrices
import matplotlib.pyplot as plt # library for plotting
# to enable plotting in line in notebook
%matplotlib inline

dx = 0.5 # step length. Set a smaller value to get more points in the range and higher resolution.
x = np.arange(-5,5,dx) # create an array of x from -5 to 5 with step 0.5
print(x)

# define a function for straight line, given x, slope m, y-intersept b.
# Input arguments x,m,b here are all internal variables of the function not used by the main program.
def line(x,m,b):
    y = m*x+b
    return y
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k = -0.5 # assign slope
y_intercept = 2 # assign y-intercept
# Here m,b are are global variables, not the same as those which define the line function. Don't have to use the same name as input arguments.
y = line(x,k,y_intercept) # call the line function with specificed input. 
print(y)

plt.figure(figsize=(6,4))
plt.plot(x,y)
plt.grid()
plt.show()

Plot a parabola

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# define a function for parabola function, given x, in vertex form
def parabola(x,xv,yv,a):
    y = a*(x-xv)**2+yv
    return y

xv = 0
yv = -2
a = 0.5
y = parabola(x,xv,yv,a)

plt.figure(figsize=(6,4))
plt.plot(x,y)
plt.grid()
plt.show()

Approximate the tangent line and the slope at some point

Animated illustration: the tangent line (derivative) as the limit of secants

Animated illustration: the tangent line (derivative) as the limit of secants

Youtube - Formal and alternate form of the derivative | Differential Calculus | Khan Academy

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from IPython.display import HTML,YouTubeVideo
# Use either line below to play the youtube video
HTML('<iframe width="640" height="360" src="https://www.youtube.com/embed/Df2escG-Vu0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>')
# YouTubeVideo('Df2escG-Vu0')
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idx1 = 10 # index of a point in x. Choose an index less than the length of the array x.
d = 5 # increment of index. Decrease this value to approach Q toward P.

idx2 = idx1+d # index of a second point in x
x1,x2 = x[idx1],x[idx2]
print([x1,x2])

P,Q = [x1,parabola(x1,xv,yv,a)],[x2,parabola(x2,xv,yv,a)] # two points on the parabola
print([P,Q])

# define a function for a straight line determined by two points
def line2pts(x,P0,P1):
    slope = (P1[1]-P0[1])/(P1[0]-P0[0])
    y = (x-P0[0])*slope+P0[1]
    return (y,slope) # output 2 variables

tangent_y,k = line2pts(x,P,Q) # output function value to y, and slope to k
tangent_y = line2pts(x,P,Q)[0] # put an index 0 if you want only the first output
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plt.figure(figsize=(6,4))
plt.plot(x,y)
plt.plot((P[0],Q[0]),(P[1],Q[1]),'r.')
plt.plot(x,tangent_y,'g')
plt.grid()
plt.show()

print('The approximate tangent line slope at x=%.2f is %.2f' % (P[0],k)) # format printing

Assingment

Like the example above, write codes to plot the curves of exponetial function and sine function as indicated below. And define a function to calculate the curve and approximate a tangent line for each.

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# Simplest case
y = np.exp(x) # exp is a numpy function. So you need np to refer to it
y = np.sin(x) # sin is also a numpy function


# For more variability of the functions, you can define them with some parameters like below.

A,B = # A - y-intercept, B - steepness
y = A*np.exp(B*x)

Amp,phi = # Amp - amplitude, phi - initial phase
y = Amp*np.sin(x+phi)

Since line2pts for getting a line given two points has been defined in the previous cell, you can directly call the function later. You need to define it only once.

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