CODEWITHSUNDEEP
pythonappend
levi
levi

Reputation: 3

Appending negative numbers (Python)

I am creating a program that would get all the factors of a number(including its negative counterpart) for rational root theorem purposes.

I am successful in getting the positive roots but I can't seem to append the negative roots into my list and it just keeps requiring an input. Here's the code.

a = input("Number:")
c = []

for b in (1,a+1):
    if a%b == 0:
       c.append(b)

for d in c:
    e = (-1)*d
    c.append(d)

print c

So I tried deleting the "c.append(d)" part and replaced it with "print e". And it printed the negative factors.

Upvotes: 0

Views: 3749

Answers (4)

A.J. Uppal
A.J. Uppal

Reputation: 19264

1st, never modify the list as you're looping over it, there is always a high chance something bad will happen.

2nd, you call for b in (1,a+1), you need a range before the (1, a+1), as such: for b in range(1,a+1).

3rd, if print e gives the correct info, then e must contain what you want to add. Thus, you can just add c.append(e) under c.append(d).

4th, your use of input() leads me to think you are using . However, if this is the case, consider using int(raw_input()) instead of input(). If you are not using , and using then cast int() to your input() to make int(input()).

a = input("Number:")
c = []

for b in range(1,a+1):
    if a%b == 0:
       c.append(b)

temp = []

for d in c:
    e = (-1)*d
    temp.append(d)
    temp.append(e)

for i in temp:
    c.append(i)

print c

Upvotes: 1

John La Rooy
John La Rooy

Reputation: 304393

Here you are chasing your tail forever. Since you are appending one new item for each iteration. ie. you are looping over the "live" c, not a snapshot of what it was when the loop started.

for d in c:
    e = (-1)*d
    c.append(d)

Simpler to replace it with a list comprehension

c += [-d for d in c]

Here is a snippet that shows the problem of extending c on the fly

>>> for d in c:
...     print -d
...     c.append(-d)
... 
-1
-2
-3
-4
-5
1
2
3
4
5
-1
-2
-3
-4
-5
1
2
3
4
5
-1
-2
-3
-4
-5
1
2
3
4
5
-1
-2
-3
-4
-5

Upvotes: 0

FuzzyWuzzy
FuzzyWuzzy

Reputation: 805

This may be simpler to read

a = input("Number:")

positives = []
for i in range(1, a+1):
    if a % i == 0:
        positives.append(i)

negatives = [-x for x in positives]

print 'positives:', positives
print 'negatives:', negatives

Upvotes: 0

MattDMo
MattDMo

Reputation: 102902

You're appending the wrong variable. Try this:

a = int(input("Number: "))
c = []

for b in range(1, a+1):
    if a % b == 0:
        c.append(b)

for d in c:
    e = -1 * d
    c.append(e) # append the negated number instead of the original...

And yes, modifying the list you're iterating over is a bad idea. Try using a temp list instead.

Upvotes: 0

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