TypeError: can only concatenate list (not "int") to list in python

Question

I tried to run this code, but it showed an error:

def shoot(aliens):
    s=[0]*1000
    s[0]=0
    s[1]=1
    num=len(aliens)
    b=[[0 for m in range(1000)] for n in range(1000)]
    for j in xrange(2,num):
        for i in xrange(0,j):                
            b[j][i]=s[i]+min(int(aliens[j]),f[j-i]) ##Error here
        s[j]=max(b)

and the error:

Traceback (most recent call last):
File "module1.py", line 67, in <module>
print shoot(line)
File "module1.py", line 26, in shoot
b[j][i]=s[i]+min(int(aliens[j]),f[j-i])
TypeError: can only concatenate list (not "int") to list

please help!

Edit: added more code. s, aliens and f are other arrays. I tried to save the result to the 2 dimentional array, but it showed that error.

well, this is a dynamic programming algorithm of shooting the maximum aliens coming to the base. — Tung Pham
– Tung Pham, Commented Jul 19, 2013 at 21:04

user2357112 · Accepted Answer · 2013-07-19 21:15:02Z

10

s[j] = max(b)

doesn't treat b as a 2-d array of integers and pick the biggest one. b is a list of lists. max(b) compares the lists and returns the one that compares highest. (List comparison is done by comparing the elements lexicographically.)

You want

s[j] = max(max(sublist) for sublist in b)

edited Jul 19, 2013 at 21:15

answered Jul 19, 2013 at 21:05

user2357112

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1 Comment

Tung Pham Over a year ago

oh. it seems to be another error if it goes through that error above. thank you to pointing it out.

Nuwan Alawatta · Accepted Answer · 2019-01-17 15:57:59Z

1

try:

b=[[0 for m in range(1000)] for n in range(1000)]
    for j in xrange(2,num):
        for i in xrange(0,j):
             b[j][i] = s[j][i] + min(int(aliens[j]),f[j-i])

It seems to me likes is a 2D list (list of a list), and thus, you can't perform the operation.

s[j] + min(int(aliens[j]),f[j-i])

edited Jan 17, 2019 at 15:57

Nuwan Alawatta

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answered Jul 19, 2013 at 20:58

nicolas.leblanc

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4 Comments

Tung Pham Over a year ago

it is working, but is there any other way to do that because I do not want to change 1D array to 2D array.

user2357112 Over a year ago

s is not a list of lists.

nicolas.leblanc Over a year ago

@user2357112, if it is not a list of list, what is it then?

user2357112 Over a year ago

s=[0]*1000 initializes s to a list of ints. The bugged s[j]=max(b) sets s[2] to a list, and then the error comes when the program tries to add s[2] to something.

questionto42 · Accepted Answer · 2021-10-06 09:09:57Z

1

I got the same error with the following python code:

retList = []
for anItem in aList:
   if anItem % 2 == 0:
       retList = retList + anItem
return retList

when I changed the "+" which I used for concatenation to an append statement:

retList = []
for anItem in aList:
    if anItem % 2 == 0:
        retList.append(anItem) 
return retList

it worked fine.

edited Oct 6, 2021 at 9:09

questionto42

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answered Feb 26, 2015 at 11:22

caytekin

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1 Comment

Seungho Lee Over a year ago

Just a small caveat.. This code will only work if anItem is not a list type since + operand is more equivalent to list.extend() than list.append(). That is to say, when anItem is [3] but not 3, the result will be different.

questionto42 · Accepted Answer · 2021-10-06 09:07:23Z

I had this error in a function as a conflict of a filled global vs. unused local variable: by mistake, I added something to a numerical local variable that was not known in the function, but known as a global variable of type string.

Cutting it down to a small and reproducible example:

def test():
    for j in range(2):
        print(j)
        print(i + 1) # error here, string type + 1!!
    return

i = ['a']
test()

which will output:

<ipython-input-281-10d295524223> in test()
      2     for j in range(2):
      3         print(j)
----> 4         print(i + 1)
      5     return
      6 

TypeError: can only concatenate list (not "int") to list

The global variable i is the string 'a'. You cannot add + 1 to a string.

Right function would be, of course:

def test():
    for i in range(2):
        print(i + 1)
    return

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