My linear interpolation has some logical error, it works for certain cases but not completely working.
I have tried to use different way to write the logic for the cases of extrapolation.
def interpolate(x, y, x_test):
for i in range(len(x)):
if x[i] > x_test: #extrapolated condition: when the largest value of
x_below = i - 1 #list x is greater than x_test
x_above = i
y_below = i - 1
y_above = i
break
elif x[i] < x_test: #extrapolated condition: when the largest value of
x_below = i + 1 #list x is greater than x_test
x_above = i
y_below = i + 1
y_above = i
break
else: #interpolated condition: when x_test lies between
return y[i] #two sample points.
#a = (yabove - ybelow) / (xabove - xbelow)
a = (y[y_above] - y[y_below]) / (x[x_above] - x[x_below])
#b = ybelow - a * xbelow
b = y[y_below] - a * x[x_below]
#y’ = a * x’ + b
return a * x_test + b
interpolate([1, 3, 5], [1, 9, 25], 5.0)
I expect the output is 25, but the actual output is 17.0.
I think you are looking for something like this:
def interpolate(x, y, x_test):
for i in range(len(x)):
if x[i] > x_test: #extrapolated condition: when the largest value of
x_below = i - 1 #list x is greater than x_test
x_above = i
y_below = i - 1
y_above = i
continue # <---- I changed break to continue
elif x[i] < x_test: #extrapolated condition: when the largest value of
x_below = i + 1 #list x is greater than x_test
x_above = i
y_below = i + 1
y_above = i
continue # <---- I changed break to continue
else: #interpolated condition: when x_test lies between
return y[i] #two sample points.
#a = (yabove - ybelow) / (xabove - xbelow)
a = (y[y_above] - y[y_below]) / (x[x_above] - x[x_below])
#b = ybelow - a * xbelow
b = y[y_below] - a * x[x_below]
#y’ = a * x’ + b
return (a * x_test + b)
print(interpolate([1, 3, 5], [1, 9, 25], 5.0))
Output:
25
Notice, I changed the breaks to continues.