Python concatenate (multiple) strings to number-CodePudding

I have a python list ['a1', 'b1', 'a2', 'b2','a3', 'b3']. Set m=3 and I want get this list using loops, because here m=3 could be a larger number such as m=100.

Since we can have

m = 3

['a'   str(i) for i in np.arange(1,m 1)]
# ['a1', 'a2', 'a3']

['b'   str(i) for i in np.arange(1,m 1)]
# ['b1', 'b2', 'b3']

then I try to get ['a1', 'b1', 'a2', 'b2','a3', 'b3'] using

[ ['a','b']   str(i) for i in np.arange(1,m 1)]

and have TypeError: can only concatenate list (not "str") to list

Then I try

[ np.array(['a','b'])   str(i) for i in np.arange(1,m 1)]

and I still get errors as UFuncTypeError: ufunc 'add' did not contain a loop with signature matching types (dtype('<U1'), dtype('<U1')) -> None.

How can I fix the problem? And even more, how to get something like ['a1', 'b1', 'c1', 'a2', 'b2','c2','a3', 'b3', 'c3'] through similar ways?

CodePudding user response：

You can have more than one for in a list comprehension:

prefixes = ['a', 'b', 'c']
m = 3

output = [f"{prefix}{num}" for num in range(1, m 1) for prefix in prefixes]
print(output) # ['a1', 'b1', 'c1', 'a2', 'b2', 'c2', 'a3', 'b3', 'c3']

If you have multiple fors, those will be nested, as in

for num in range(1, m 1):
    for prefix in prefixes:
        ...

CodePudding user response：

You could use itertools.product to get all combinations of the the letters and the m range and then join them in an f-string (rather than using join as one element is an integer so would require converting to a string):

[f'{t[1]}{t[0]}' for t in itertools.product(range(1, m 1), ['a', 'b'])]

Output:

['a1', 'b1', 'a2', 'b2', 'a3', 'b3']

CodePudding user response：

You need to iterate on both the range of numbers and the list of strings

In [106]: [s str(i) for i in range(1,4) for s in ['a','b']]
Out[106]: ['a1', 'b1', 'a2', 'b2', 'a3', 'b3']