Count different numbers in list python

Question

Hey guys so this is my first year programming and I started with python. I am understanding the programming fairly well but I need help with this homework question.

I have to use a list as my parameter and then return the number of different values in the list. The example list in the question is [1, 4, 1, 7, 6, 1, 4, 3] and therefore the returned value should be 5.

Now I know my method of solving it is probably not concise or elegant but if someone could help me and tell me what to change so it works I would greatly appreciate it.

def count(mylist):
    newlist = []
    newlist.append(mylist[0])
    stor = False
    for i in mylist:
        stor = False
        for j in newlist:
            if j == i:
                stor == True
        if not stor:
            newlist.append(i)
    return newlist

Answer 1

Use a set() instead:

def count(myList):
    return len(set(myList))

A set will only hold one copy of each value, so converting your list to a set has the handy side-effect of removing all the duplicates. The length of the resulting set is the answer you are looking for.

Using a set is the most efficient method; alternatively you could use a dict() too:

def count(myList):
     return len(dict.fromkeys(myList))

which is ever so slightly less efficient because it'll reserve space for the values associated with the keys.

If all you want to use is a list, (least efficient), use the not in negative membership test:

def count(myList):
    unique = []
    for item in myList:
        if item not in unique:
             unique.append(item)
    return len(unique)

Answer 2

you can use sets here:

In [1]: lis=[1, 4, 1, 7, 6, 1, 4, 3]

In [2]: len(set(lis))
Out[2]: 5

help on set :

set(iterable) -> new set object
Build an unordered collection of unique elements.

using a for-loop:

In [6]: def count(lis):
   ...:     mylis=[]
   ...:     for elem in lis:
   ...:         if elem not in mylis:  # append the element to
                                       # mylis only if it is not already present
   ...:             mylis.append(x)
   ...:     return len(mylis)        
   ...: 

In [7]: count(lis)
Out[7]: 5

Also have a look at collections.Counter() , it returns a sub-class of dict , which contains the number of times an element was repeated:

In [10]: from collections import Counter

In [11]: c=Counter(lis)

In [12]: c
Out[12]: Counter({1: 3, 4: 2, 3: 1, 6: 1, 7: 1})

In [13]: len(c)
Out[13]: 5

Answer 3

stor == True

您实际上不是在这里将stor设置为True 。

Answer 4

If you cannot use set , try

def count(mylist):
    mylist.sort()
    total = 0
    for k in range(1, len(mylist) -1 ):
        if mylist[k] != mylist[k + 1]:
            total += 1
    return total

This sorts the list and then increments the counter each time an element is unequal to the next one.

---

If you can't use sorting, you'd normally keep track of counted values. That is too obvious though, so here is a funny way to do it without keeping a list of values you already counted:

def count(mylist):
    total = 0
    for k, value in enumerate(mylist):
        total += 1 / mylist.count(value)
    return total

So for [1, 4, 1, 7, 6, 1, 4, 3] , the weights are [1/3, 1/2, 1/3, 1, 1, 1/3, 1/2, 1] which adds up to 5 as it should.

This is a much more awesome way than what your teacher is looking for (despite the inefficiency of this method).

Answer 5

If you are supposed to use a loop, then this is the way(or one of them). :)

the_list = [1, 4, 1, 7, 6, 1, 4, 3]

def count(the_list):
    unique_list = []
    for item in the_list:
        if item not in unique_list:
            unique_list.append(item)
    return len(unique_list)

Answer 6

At first, a fixed version of your program

def count(mylist):
    newlist = []
    newlist.append(mylist[0])
    stor = False
    for i in mylist:
        stor = False
        for j in newlist:
            if j == i:
                stor = True # stor == True test for equality
        if not stor:
            newlist.append(i)
    return len(newlist) # you do not want the list itself but its length

And here some suggestions:

• you do not need to initialize stor outside the outer loop. This is done two lines later again.
• consider a break in the inner loop - this speeds things up a bit (no unneeded comparison)
• newlist can be initialized to an empty list without appending the first item. The algorithm stays valid (the inner loop have zero iterations the first time)

here as code-example:

def count(mylist):
    newlist = []
    for i in mylist:
        stor = False
        for j in newlist:
            if j == i:
                stor = True
                break
        if not stor:
            newlist.append(i)
    return len(newlist)

And more elegant (and pythonic): use the in -syntax ;)

def count(mylist):
    newlist = []
    for i in mylist:
        if i not in newlist:
            newlist.append(i)
    return len(newlist)

Basically item in something_iterable returns true, if item can be found in something_iterable . Most collection of items is iterable (eg lists, sets, strings ... 'a' in 'abc' returns true )

and the most pythonic way but without for/while-loops:

def count(mylist):
    return len(set(mylist))

Please look into other answers for an explanation.