将列表理解转换为 while 循环

Question

I have the following line of code:

mass_grouped3['records_to_select'] = [math.ceil(int((el * target) / freq_sum)) if el > 6 else el for el in mass_grouped3['freq']]

I want to rewrite it with while loop, but don't have much experience with while loops and find it difficult. This is what I have for now, but is not working for some reason:

trgt = 0
while trgt <= 7500:
    if trgt == 7500:
        break
    else:
        for el in mass_grouped3['freq']:
            if el > 6:
                mass_grouped3['records_to_select'] = [math.ceil(int((el * target) / freq_sum))]
                trgt += 1
            else:
                pass

I want to do the same, but when the counter hit 7500 to break.

Answer 1

One thing you could do is just wrap the list comprehension with the while loop which should help avoid length issues. I'm not sure if target is supposed to be trgt in the second block of code you posted in math.ceil(int((el * trgt) / freq_sum)) so tell me and I'll edit it as needed.

trgt = 0
while trgt < 7500:
    mass_grouped3['records_to_select'] = [math.ceil(int((el * trgt) / freq_sum)) 
                                              if el > 6 else el for el 
                                              in mass_grouped3['freq']]
    trgt += 1

Answer 2

To answer more generally: In general a list comprehension of the form

result = [foo(a) for a in as if condition]

can be written as a for loop of the form

result = []
for a in as:
    if condition:
        result.append(foo(a))

and in general for loops can be rewritten as while loops, though the exact transformation will depend on the form of the for loop and the structure you're looping over. If you're looping over a serially-indexed collection like a list, you could generally do:

result = []
ix = 0
end = len(as)
while ix < end:
    if condition:
        result.append(foo(as[ix]))

and expect to get the same result as the original comprehension.

Note: the gymnastics that I have to go through and the questions I have to ask in order to do these rewrites serve as a good argument for the use of list comprehensions when possible, and a good guide to when they're possible.