TypeError：int（）参数必须是字符串或数字，而不是“列表”

Question

#iterative program to find the highest odd number
m = sorted([a, b, c, d, e, f, g, h, j, k])
n = max(m)
n = int(n)
count = 10
while n%2==0:
    m=[m[:-1]]
    n = max(m)
    n = int(n)
    count =-1
if count==0:
    print 'There are no odd numbers'
else:
    print str(n), 'is the largest odd number'

I input variables that contain an odd number, and it gives me the correct answer, but when I input all even numbers to satisfy the 'count==0' condition, the following error occurs:

TypeError: int() argument must be a string or a number, not 'list'

I can't understand why this error doesn't occur when there are odd numbers input.

Answer 1

If you print out what m is inside the loop, this becomes pretty obvious. Or you might want to test it out using an interactive visualizer , or just the debugger.

Let's say your values are 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 . After sorting, you've got:

m = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
n = max(m) = 20
n = int(n) = 20

This max is useless, because by the definition of sorting is has to be the last value in the list (and you seem to be relying on that in your loop anyway).

And the int is misleading—it makes it look like your code will work even if the numbers are strings instead of numbers, but it actually won't, because sorted (and max ) will treat '10' as less than '2' , and so on.

But neither of those is your big problem. Because your first n is even, you will go into the loop, and the first thing in the loop is this:

m=[m[:-1]]

… which will do this:

m = [[2, 4, 6, 8, 10, 12, 14, 16, 18]]

So, the next two lines do this:

n = [2, 4, 6, 8, 10, 12, 14, 16, 18] # the max of a 1-element list is that element
n = int([2, 4, 6, 8, 10, 12, 14, 16, 18])

And boom, there's your exception.

If you wanted to set m to all but the last element of m , just do m = m[:-1] . Throwing those extra brackets around it sets m to a list consisting of one element, which is itself the list consisting of all but the last element of m .

Note that, despite what you say in your description, "I input variables that contain an odd number it gives me the correct answer", that isn't true. It only works if your max value is odd, so you never go into the loop in the first place.

Your code is actually still broken after you fix this, but hopefully now you know how to debug this yourself.

---

Meanwhile, the pythonic way to solve this is to try to translate your high-level English description directly into high-level Python. How do we find the highest odd number in m ?

First get the odd numbers in m :

odds = (n for n in m if n % 2)

(This might be more readable if you create an odd function—and, if you, you might prefer filter to a generator expression.)

Then, to get the maximum:

max_odd = max(odds)

Of course you need to handle the case where there are no odds. You can do that by checking if odd: . But in python, it's usually better to ask forgiveness than permission, so, here's your whole program:

m = [a, b, c, d, e, f, g, h, j, k]
odds = (n for n in m if n % 2)
try:
    print max(odds), 'is the largest odd number'
except ValueError:
    print 'There are no odd numbers'

Answer 2

Your error occurs with the m=[m[::-1]] , as @abarnert pointed out.

Here's a simple way to find the max odd number in a list:

m = sorted([int(n) for n in [a, b, c, d, e, f, g, h, j, k] if n%2==1])
# this makes a list of all ODD integers (converts them from strings)
if len(m) != 0:
    print str(max(m)), 'is the largest odd number'
else:
    print 'No odd integers inputted'

Simplified further into:

m = sorted([int(n) for n in [a, b, c, d, e, f, g, h, j, k] if n%2==1])
print (str(max(m)), 'is the largest odd number') if len(m)!=0 else 'No odd integers inputted'

Answer 3

When you have only even numbers, your while loop is generating this error when you've depopulated m completely. In that case max(m) returns None , which cannot be the argument of int . To fix this, you need to change your while loop condition to something more correct.

This is, however, not what most would consider "pythonic". Ideally for that you would use a loop more like for n in m[::-1] which traverses m in reverse order (or use the reverse=True argument of sorted and just for n in m .)