Why Can't I Concatenate Items In This List But I Can Multiply Them?

I have a list that encases the items within apostrophes and I can use the following 'for loop' list1 = ['1','2','3'] list2 = [i * 5 for i in list1] print list2 which appears to tr

Solution 1:

In the first example you do not multiply numerically: if you multiply a string, you repeat the stringn times, like:

>>> "foo"*5'foofoofoofoofoo'

So your first code fragment results in:

>>> [i * 5for i in list1] # see, you repeat the string
['11111', '22222', '33333']

There is no support for addition between strings and ints, so that will error. Probably because the only reasonable operation would be to add the number to the end of the string, but that is again not numerical addition.

If you want to do numerical calculations, you have to convert it to an int first with int(..):

>>>[int(i) * 5for i in list1]
[5, 10, 15]
>>>[int(i) + 5for i in list1]
[6, 7, 8]

As you can see the result here is a list with ints (there are not quotation marks around the numbers).

Most sequences (lists, tuples, strings, etc.) will construct the same type of sequence with repetition when you multiply them with an integer. Like:

>>> [1,'a',2,"five"]*3[1, 'a', 2, 'five', 1, 'a', 2, 'five', 1, 'a', 2, 'five']
>>> (1,'a',2,"five")*3
(1, 'a', 2, 'five', 1, 'a', 2, 'five', 1, 'a', 2, 'five')

Solution 2:

Try this on the command line:

>>> '1' * 5'11111'>>> '1' + 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str'and'int' objects

Is that what you expected? Probably not...

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If you want to do calculations, then you will need to do the following:

>>> int('1') * 55>>> int('1') + 56

These both result in ints, not strs.

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If you want to do string concatenation, then you will need to do one of the following:

>>> '1' + '5''15'>>> '1' + str(5)
'15'

Solution 3:

'2' * 5 would return '22222' as string and not 10 as you would think.

You should cast your variable int(i) + 5, this would consider the i as integer instead of string.

Solution 4:

'1' * 5 will give you '11111', and not 5. You can multiply a string and an integer. However, you can't add a string and an integer.