CODEWITHSUNDEEP
python
SATVIKI SHARMA
SATVIKI SHARMA

Reputation: 41

Python noob question: Polynomial multiplication question

Can someone help me out with my code, the only answer I get is [0,0] for all inputs-:

def add_poly(a,b,n):
    p=[]
    
    for i in range((n*2)-2):
        p.append(0)
    for i in range(0,n-1):
        for j in range(0,n-1):
            p[i+j]=(a[i]*b[j])

    return p


n=2
a=[1,3]
b=[0,2]

print(add_poly(a,b,n))

Upvotes: 1

Views: 79

Answers (1)

Mad Physicist
Mad Physicist

Reputation: 114300

Your fundamental problem is that range is exclusive on the high bound. To loop over n elements, you do range(n), not range(n - 1). That will give you a loop over 0, 1, 2, ..., n-1.

Another issue is that you're providing redundant information about the size of the polynomials. The degree is already encoded in their length. If you provide n, that's redundant information, and may be wrong. E.g., what if you want to multiply a second order polynomial (n=3) by a third order (n=4)?

Finally, the operation you are attempting to implement seems like multiplication rather than addition.

So here is a sample of how to use len to get the order of the polynomials, and enumerate to place the elements. You can also preallocate the output list in one line:

def mult_poly(a, b):
    p = [0] * (len(a) + len(b) - 1)

    for i, x in enumerate(a):
        for j, y in enumerate(b):
            p[i + j] += x * y

    return p

Upvotes: 2

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