So, I was doing this problem from MIT OCW's Introduction to Computer Science and Programming:
Problem #2
Implement the compute_deriv function. This function computes the derivative
of a polynomial function. It takes in a tuple of numbers poly and returns
the derivative, which is also a polynomial represented by a tuple.
def compute_deriv(poly):
"""
Computes and returns the derivative of a polynomial function. If the
derivative is 0, returns (0.0,).
Example:
>>> poly = (-13.39, 0.0, 17.5, 3.0, 1.0) # x4 + 3.0x3 + 17.5x2 - 13.39
>>> print compute_deriv(poly) # 4.0x3 + 9.0x2 + 35.0x
(0.0, 35.0, 9.0, 4.0)
poly: tuple of numbers, length > 0
returns: tuple of numbers
"""
# TO DO ...
And this is my program (it works):
def compute_deriv(poly):
derivatives=()
for i in poly:
if list(poly).index(i)==0: #my version is 2.5.4, so I cannot use
continue #tuple.index(i) and since I didn't
else: #find another way, I converted the
deriv=i*list(poly).index(i) #tuple to a list
derivatives=derivatives+(deriv,)
return derivatives
polyx=(-13.39, 0.0, 17.5, 3.0, 1.0)
print compute_deriv(polyx)
polyxx=(1.3, 7.0, 4.0, 2.5, 0.0, -8.0)
print compute_deriv(polyxx)
The first thing is that I wanted for the program to ask me to input the polynomial instead of writing it inside of it:
...
polyx=tuple(raw_input("Enter your polynomial tuple here:"))
print compute_deriv(polyx)
but that didn't work:
Enter your tuple here:-13.39, 0.0, 17.5, 3.0, 1.0
('1', '33', '...', '33', '99999', ',,,,,,', ' ', '00000000', '...',
'00000000', ',,,,,,', ' ', '1', '77777777777777', '...',
'5555555555555555', ',,,,,,', ' ', '33', '...', '00000000', ',,,,,,',
' ', '1', '...', '00000000')
Why? And the other problem is with the second tuple (-8x^5+2.5x^3+4x^2+7x+1.3) - when its members are respectively (1.3, 7.0, 4.0, 2.5, 0.0, -8.0), it returns as expected - (7.0, 8.0, 7.5, 0.0, -40.0), but if the first member is 0.0 (as in -8x^5+2.5x^3+4x^2+7x), that changes - (7.0, 8.0, 7.5, -40.0). The second 0.0 is omitted and that's a problem as it implies that the power of the -40.0 is 3 when it is 4. Again, why?
Thank you for your time!
raw_input will convert your input to a string. Thus, entering -13.39, 0.0, 17.5, 3.0, 1.0 will result in a string.
Then you convert it to a tuple. By default, when given a string, tuple will split each character in the string and form a tuple with all of them. Example:
tuple("Hello World")
Output: ('H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd')
That's why you get those results. Possible solution: input the tuple separated by commas ',' and then split it:
polyx = tuple(raw_input("Enter your polynomial tuple here: ").split(","))
Example:
Enter your polynomial tuple here: -13.39,0.0,17.5,3.0,1.0
Output: (u'-13.39', u'0.0', u'17.5', u'3.0', u'1.0')
Notice though, that this tuple will contain strings, not floats. You need to convert them:
polyx = tuple(float(x) for x in raw_input("Enter your polynomial tuple here: ").split(","))
Example:
Enter your polynomial tuple here: -13.39, 0.0, 17.5, 3.0, 1.0
Output: (-13.39, 0.0, 17.5, 3.0, 1.0)