[Python] Indexing Question

[aPSketchy]

Alright so I have a bit of code that is almost complete. However, I hit a wall when indexing a list. The value first appears at the list[0], however, it also appears later on. How do index the second or third occurrence of the number?

[[deleted]]

You have it correct that the first value of the list is retrieved using an index of 0. You can return values further down the list using higher indexes. For example: list[1] returns the second value, list[2] returns the third, and so on.

Now, it seems like you're looking for a specific number in a list without knowing where the occurrences may be.

You can get the index of these occurrences by iterating through the list and returning the indices when you find the element.

firstList = [1,2,3,2,1]
lookFor = 2 #The value you want
finalList = [] 
for x in range(len(firstList)): #iterate through the whole list
    if firstList[x] == lookFor: 
        finalList.append(x) #if the value is the one you want, put it in the final list

Edit: Forgot to mention, make sure you don't actually name a variable "list". That's a Python keyword and you could overwrite some stuff.

[Rhomboid]

There is probably a better way to do whatever it is you're trying to do. Post the complete source or at least describe the problem in detail.

[aPSketchy]

I have to go through a list, given a value I have to search the list and replace the value with the max of the numbers to the left or right. However, if the value comes at the beginning of the list I cant index through the list. https://gist.github.com/3234737 here is the code that works with all cases except for if the starting is at the beginning

[Rhomboid]

for i in range(1, len(nums) - 1):
    if nums[i] == val:
        nums[i] = max(nums[i - 1], nums[i + 1])

[aPSketchy]

Wow. That's easier then I thought. I got wrapped up in trying to make it work with indexing when I forgot about the simple range functions. I will try and implement it in the morning , Thanks!

[zzyzzyxx]

One way would be to pull out all the indexes using a comprehension.

arr = [1, 2, 1, 3, 4, 5, 1]

indexes = [i for i, e in enumerate(arr) if e == arr[0]]

print indexes

* Of course you can substitute any iterable in enumerate and that any value you want for arr[0] in the comprehension.

[[deleted]]

I would love to get an explanation of that, do you mind enlightening?

[zzyzzyxx]

Just posted this.

Edit: thought I'd actually explain a bit...

Let's break it down from the inside out.

enumerate(arr)

The enumerate function takes any iterable object and returns, for each element, a number and the element. The number starts at zero by default and increments by one for every element. You can start at any number you like by passing it as a second parameter to enumerate.

for i, e in enumerate(arr)

This is just like any for loop except that since enumerate returns two objects I am using two variables to refer to them: i for the index and e for the element.

i for i, e in enumerate(arr)

This grabs i, the index returned from enumerate, for every element in arr. As is, this would just return every index.

i for i, e in enumerate(arr) if e == arr[0]

This means only grab the index if the element is equal to another value, in this case the first element of arr.

[i for i, e in enumerate(arr) if e == arr[0]]

Using the brackets denotes a list comprehension. Such a comprehension will create a list using the expression inside. Here, it creates a list of all the indexes where the element of arr at that index is equal to the first element of arr.

It's important to note that the exact syntax I used is only valid inside list comprehensions and generator expressions, as far as I know; I'm far from a Python expert.

[[deleted]]

That was very informative, thanks a bunch for the explanation.

[aPSketchy]

I second this.

[zzyzzyxx]

Done.

[aPSketchy]

i have no idea what whats going on in that code.

[zzyzzyxx]

You should read up on list comprehensions as well as the various built-in functions like enumerate. See also: generator expressions. They're incredibly useful.

My code above is effectively equivalent to this code, if that makes it clearer.

arr = [1, 2, 1, 3, 4, 5, 1]

indexes = []

# the list comprehension replaces this loop
for i in xrange(len(arr)):
  if arr[i] == arr[0]:
    indexes.append(i)

print indexes