python中的这个简单功能无法正常工作

Question

I have some problems with these lines of program. I'm writing a function in python that takes a list of lists and a string as input and returns "the name is here" if the second element of a list in the list of lists is equal to the string given. In this case the list of list I have is this

railway = [["Milan","Zurich"],["Zurich","Bern"],["Bern","Berlin"],["Berlin","Copenaghen"]] 

my function is:

def travel( list , stringdestination):
        i = 0
    for elemento in range(len(list)):
      if list[i][1] == stringdestination:
          print "target reached"   

when I run:

travel(railway, "Bern") 

it should display: "target reached" but it doesn't, it doesn't show anything, why?

Answer 1

You are never incrementing i. Your loop should be:

for pair in list:
    if pair[1] == stringdestination:
        print "target reached"

Answer 2

As it's been answered, you're not incrementing the loop variable. But that's what's wrong trivially, more importantly, you're coming at this screw like a man with only a hammer in his toolbox. This is pretty much the exact reason the data structure called dictionary was made. It's a built in.

Read up on this, it's much easier and nicer. http://www.tutorialspoint.com/python/python_dictionary.htm

Answer 3

A few points:

1. Don't use list as a variable name. list is a built in name
2. Iterate lists directly for i in mylist
3. It is possible to 'unpack' the pairs in your list by assigning to 2 variables in the iteration

For example:

def travel(places, destination):
    for start, dest in places:
        if destination == dest:
            print "target reached"
            break

It is likely you want to stop iterating when you find the destination. Do so either by returning immedately from the function or breaking and not returning anything (if no return in a function it implicitly returns None ).

Answer 4

I would wrap it in a function , so I stop when a founded the right string. It prevents from looping through the whole list:

def arrived(s, raileway):
    for r in railway:
        if r[1] == s:
            return True
    return False

if arrived("Bern", railway):
    print("target reached")