Python - Can only concatenate list not float to list

Question

I understand that I can only concatenate things of similar types, but I'm really confused as to why the following are of different types.

n = 100
table = [[0]*n for x in range(n)]
array1 = [[0] for i in range(n)]
mini = array1[1] + table[1][1]

I am trying to make mini store the integer that is the result of array1[1] and a table[1][1]'s value. But I get this error:

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

There must be something simple I'm missing. When I print just table[1][1] I get 0, so why is table[1][1] not treated as just 0 (I.e. 0 + 0)?

`print array1` - it's an array of arrays. is that what you expected? the problem is not with `table1`, but with `array1` — andrew cooke, Mar 15 '12 at 00:43
@andrewcooke a `list` of `list`s. `array` means something quite different in Python. — Karl Knechtel, Mar 15 '12 at 00:46

score 6 · Accepted Answer · answered Mar 15 '12 at 00:42

table[1] is indeed [0]*1 which is [0], and table[1][1] is indeed 0.
but array[1] is [0], which is a list
thus your attempt to do array1[1] + table[1][1] is actually [0] + 0

To debug such things in the future, print each part of the expression the interpreter is complaining about:

print(array1[1])
print(table[1][1])

Karl Knechtel · Answer 2 · 2022-11-04T21:09:28.713

so why is table[1][1] not treated as just 0

It is - but 0 cannot be added to array1[1] with +. It's exactly as the error message says. + is used to add two lists together:

[1, 2, 3] + 4 # WRONG: can't concatenate a non-list with a list
[1, 2, 3] + [4] # RIGHT: concatenates the two lists, producing [1, 2, 3, 4]

If array1[1] is intended to be a float rather than a list, then array1 should store numbers rather than lists. The code array1 = [[0] for i in range(n)] makes each element in array1 be [0], i.e. a list.

That said, please remember that array indexes start with 0.

score 0 · Answer 3 · answered Mar 15 '12 at 00:43

0

Look at what array1 contains:

>>> array1[1]
[0]

It's not a list filled with zeroes, it's a list filled with a bunch of lists, each of them containing a single zero.

you probably want this line:

array1 = [[0] for i in range(n)]

to be this:

array1 = [0 for i in range(n)]

answered Mar 15 '12 at 00:43

bgporter

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Thanks. This is my first Python program so I knew I'd be missing something. – user16647 Mar 15 '12 at 00:55

score 0 · Answer 4 · answered Mar 15 '12 at 00:44

Both table and array1 are lists of lists of integers (0 in this case). But

array1[1]

is a list of integers while

table[1][1]

is an integer. You could solve the problem using a slice, like this:

mini = array1[1] + table[1][1:2]

which results in mini being equal to

[0, 0]

The slice table[1][1:2] is itself a list containing a single element (table[1][1]), and since concatenating lists is allowed, it works.

score 0 · Answer 5 · answered Mar 15 '12 at 00:44

Your variable array1 is a list containing lists that each only have a single element, 0, for i in range(n). That is, array1[1] = [0], not array1[0] = 0. That's why the types mismatch.

Also, don't forget that Python is 0-based, not 1-based. It doesn't cause an error here, but seems to betray your thinking of [1] as the first slot in the list, when [0] is.

score 0 · Answer 6 · answered Mar 15 '12 at 00:45

When you print each operand, you will get the following output:

>>> array1[1]
[0]
>>> table[1][1]
0

So the first is a list, and the second is a single integer. As such it obviously fails.

So you either need to use array1[1][0] to add the integer values, or use table[1] to concatenate lists.

Python - Can only concatenate list not float to list

6 Answers6