Calculating a Function

### Basic Function ###

import math
import numpy as np
import matplotlib.pyplot as plt
plt.style.use(['seaborn-whitegrid'])

### A LINEAR FUNCTION ###
 # y = ax + b
 # a: 기울기, b: y절편
 # 그래프 상에서 직선인 그래프(linear)
 
def linear_function(x):
    a = 0.5
    b = 2
    
    return a * x + b

print(linear_function(5))   # 4.5

x = np.arange(-5, 5, 0.1)
y = linear_function(x)

plt.plot(x, y)
plt.xlabel('x')
plt.ylabel('y')
plt.title('LINEAR FUNCTION')
plt.show()

### QUADRATIC FUNCTION ###

# y = ax^2 + bx + c
# 일반적으로 두 개의 실근을 가짐

def quadratic_function(x):
    a = 1
    b = -1
    c = -2
    
    return a*x**2 + b*x + c

print(quadratic_function(2))    # 0

x = np.arange(-5, 5, 0.1)
y = quadratic_function(x)

plt.plot(x, y)
plt.xlabel('x')
plt.ylabel('y')
plt.title('QUADRATIC FUNCTION')
# plt.show()

### CUBIC FUNCTION (polynomial function) ###

# y = a*x^3 + b*x^2 + c*x + d

def cubic_function(x):
    a = 4
    b = 0
    c = -1
    d = -8
    
    return a*x**3 + b*x**2 + c*x + d

print(cubic_function(3))    # 97

x = np.arange(-5, 5, 0.1)
y = cubic_function(x)

plt.plot(x, y)
plt.xlabel('x')
plt.ylabel('y')
plt.title('CUBIC FUNCTION')
plt.show()

### MIN & MAX VALUES OF A FUNCTION ###

def my_func(x):
    a = 1
    b = -3
    c = 10
    return a*x**2 + b*x + c

x = np.arange(-10, 10, 0.1)
y = my_func(x)

plt.plot(x, y)
plt.xlabel('x')
plt.ylabel('y')
plt.scatter(1.5, my_func(1.5))
plt.text(1.5-1.5, my_func(1.5)+10, 'min value of f(x)\n({}, {})'.format(1.5, my_func(1.5)), fontsize=10)
plt.title('MY FUNC')
plt.show()

min_val = min(y)
print(min_val)