用带有 python 列表的 for 语句替换 while 循环

Question

I have to create a list of cars, a function to display the cars but replace the fourth car with a phrase, and call the function, for an assignment. One of the parts of the grading scheme is to use a for statement to iterate, not a while loop. It also says that it can be done in under 10 lines of code.

I have created a while loop that does prints the proper list, but I cannot manage to change it to a for statement successfully.

def my_Assign3_challenge(cars):
  i=0
  while i<len(cars):
    print(cars[i])
    i=i+1
    if i==3:
      print("I would never buy that!")
      i=i+1
cars=["toyota","mitsubishi","dodge","ford","mini"]
my_Assign3_challenge(cars)

This for statement just prints the list, without the change:

def my_Assign3_challenge(cars):
  for x in cars:
    if x!= 3:
      print(x)
    else:
      print("I would never buy that!")
cars=["toyota","mitsubishi","dodge","ford","mini"]
my_Assign3_challenge(cars)

I also managed a couple times to have just the first list item repeat indefinitely, I can't find the code that did that now, though.

I would greatly appreciate if anyone could take a look at this and tell me where I went wrong. I'm sure it's just a simple mistake, as I am very new to coding, but I would love your help!

Answer 1

Your while loop iterates through the list indices...

while i<len(cars):
  print(cars[i])

... but you changed your referencing with the for loop: you now reference the elements themselves, the brand names:

for x in cars:

but you still treat x as if it were an integer index. Since x is now a sequence of brand names, it will never equal 3 . You need to make up your mind which way you're going to refer to the list elements, and make your second set of code use that way consistently.

Answer 2

The for loop in the function:

def my_Assign3_challenge(cars):
  for x in cars:
    if x!= 3:
      print(x)
    else:
      print("I would never buy that!")
cars=["toyota","mitsubishi","dodge","ford","mini"]
my_Assign3_challenge(cars)

Outputs

toyota
mitsubishi
dodge
ford
mini

You need to get the index using .index()

The code would be:

def my_Assign3_challenge(cars):
  for x in cars:
    if cars.index(x) != 3:
      print(x)
    else:
      print("I would never buy that!")
cars=["toyota","mitsubishi","dodge","ford","mini"]
my_Assign3_challenge(cars)

Answer 3

I can do it in 3 lines:

challenge = lambda x : [v if i !=3 else "I would never buy that!" for i, v in enumerate(x)]
cars=["toyota","mitsubishi","dodge","ford","mini"]
challenge(cars)

output:

Out[54]: ['toyota', 'mitsubishi', 'dodge', 'I would never buy that!', 'mini']