import numpy as np
import matplotlib.pyplot as plt

PI = np.pi

f_s1 = 1
f_s2 = 2.5
f_s3 = 3
f_s4 = PI

t1 = np.arange(0,40,1/f_s1)
t2 = np.arange(0,40,1/f_s2)
t3 = np.arange(0,40,1/f_s3)
t4 = np.arange(0,40,1/f_s4)

x1 = np.cos(2*PI*t1/7)
x2 = np.cos(2*PI*t2/7)
x3 = np.cos(2*PI*t3/7)
x4 = np.cos(2*PI*t4/7)

plt.subplot(2,2,1)
plt.stem(t1,x1)
plt.title("1[Hz] Sampling")

plt.subplot(2,2,2)
plt.stem(t2,x2)
plt.title("2.5[Hz] Sampling")

plt.subplot(2,2,3)
plt.stem(t3,x3)
plt.title("3[Hz] Sampling")

plt.subplot(2,2,4)
plt.stem(t4,x4) 
plt.title("$\pi$[Hz] Sampling")

plt.show()