Trying to write function map() getting error IndexError: list index out of range

Question

I'm trying to write function map() getting error IndexError: list index out of range

     def map1(fn, a):
         i = 0
         b = []
         while i != len(a):
             print(len(a))
             i += 1
             b.append(fn(a[i]))
         return b

with working function

    def translate(x):
        dicti = {"merry": "god", "christmas": "jul", "and": "och", "happy": "gott", "new": "nytt", "year": "år"}
        return dicti[x]

got the error

     IndexError: list index out of range

Please post a full stacktrace of the IndexError so that it is possible to see where it occurs. What are the arguments to map1() that produce the error? — dhke
– dhke, Commented Mar 10, 2015 at 13:53

mkrieger1 · Accepted Answer · 2015-03-10 13:58:20Z

1

You increment i before accessing a. In the last iteration, i is len(a)-1 at the beginning of the loop body, then it is incremented to len(a), but this is just outside the valid index range. To fix it, you have to increment after the access:

while i != len(a):
    b.append(fn(a[i]))
    i += 1

However, an improved way to do this, is to use range, which produces the correct values for i for you automatically:

for i in range(len(a)):
    b.append(fn(a[i]))

An even better way is to iterate over the entries of a directly:

for x in a:
    b.append(fn(x))

An even better way is to use a list comprehension:

b = [fn(x) for x in a]

answered Mar 10, 2015 at 13:58

mkrieger1

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2 Comments

caleb farley Over a year ago

That worked when I used it, although I'm not sure how to set it up with the last line of code you posted...

mkrieger1 Over a year ago

Instead of: def map1(fn, a): b = []; ...; return b, you do: def map1(fn, a): b = [fn(x) for x in a]; return b, or simply def map1(fn, a): return [fn(x) for x in a]

user3181422 · Accepted Answer · 2015-03-10 13:59:46Z

1

You where increasing i before using it.

def map1(fn, a):
         i = 0
         b = []
         while i != len(a):
             print(len(a))

             b.append(fn(a[i]))
             i += 1
         return b

def translate(x):
        dicti = {"merry": "god", "christmas": "jul", "and": "och", "happy": "gott", "new": "nytt", "year": "ar"}
        return dicti[x]

map1(translate, ["merry", "and"])

answered Mar 10, 2015 at 13:59

user3181422

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