# Define funtion interpolate
def interpolate(x,y,x_test):

    # Define variables needed for equation
    # Search for the lowest closest matching value in the x array against the inputted x_test
    lower_x = min(x, key=lambda z:abs(z-x_test))

    # Test if the closest value is higher than the given value to test
    if lower_x > x_test:

        # Make sure array wont go out of range
        if (x[x.index(lower_x)] > 0):

            # Redefine value with shifted index down by one
            lower_x = x[x.index(lower_x)-1]

    # Get value 1 index higher than that
    if x[x.index(lower_x)] < (len(x)-1):
        higher_x = x[x.index(lower_x) + 1]

    # Else
    else:
        # Higher x is maximum
        higher_x = x[x.index(lower_x)]

        # Lower x is one below
        lower_x = x[x.index(higher_x)-1]

    # Get corresponding lower and higher values from y array
    lower_y = y[x.index(lower_x)]
    higher_y = y[x.index(higher_x)]

    # Perform given equation a*x'+b
    a = (higher_y - lower_y) / (higher_x - lower_x)
    b = lower_y - a * lower_x

    yPrime = a * x_test + b


    # Return
    return yPrime