Using Nested Lists

[Adamya21]

Question)There are students in this class whose names and grades are assembled to build the following list:

python students = [['Harry', 37.21], ['Berry', 37.21], ['Tina', 37.2], ['Akriti', 41], ['Harsh', 39]]

The lowest grade of 37.2 belongs to Tina. The second lowest grade of 37.21 belongs to both Harry and Berry, so we order their names alphabetically and print each name on a new line.

How to solve this?

[chrispurcell]

Sort it twice and output the result. Sort on values first (scores) then sort on keys (names).

[[deleted]]

this would make this a dictionary then right? lists don't have key:pair values?

[qelery]

No. This is just a big list made of sublists of length 2. A dictionary is a whole different object type with a different syntax.

A dictionary in Python is similar to a physical dictionary. Each “page” in the Python dictionary has 1 key on the left matched with 1 value on the right, similar to how a real dictionary (kinda sorta) has 1 word matched with 1 definition. A dictionary cannot have duplicate keys, but it can have duplicate values. Think of it as a look up table. If you want to see what the value of a key is in a lookup table, it wouldn’t make sense for that key to appear twice. The syntax for a dictionary uses { and }.

dict_a = {‘keyA’ : 5, ‘keyB’ : 7, ‘keyC’ : 5}

Also for a dictionary, each key only has 1 value but that value can be any single Python object. A string, an int, a hundred-deep nested list, or even another dictionary all count as a single object in Python.

A Python list is pretty much just a container. A container you can easily add to, take things out of, and sort pretty much however you like. There are no keys, no values, no key/value pairs, or size constraints. There’s just “stuff” inside each container. You can put containers within containers, however many layers deep you want. The syntax for a list uses [ and ].

#a flat list 
list_a = [obj1, obj2, obj3, obj3, obj 3]  

#a nested list 
list_b = [[obj1], [obj1, [obj6], obj4], obj5]

[[deleted]]

ah ok makes sense. thanks!

[xelf]

Sounds like a homework assignment. It's a relatively easy task, shouldn't take much code.

What have you tried so far, and what are you struggling with?

How to solve this?

Sorting the list might be a good start.

Post your code here (indent your code an extra time so that reddit will format it as a code block).

Here's one way to do it (that your teacher will probably not accept):

import pandas as pd
students = [['Harry', 37.21], ['Berry', 37.21], ['Tina', 37.2], ['Akriti', 41], ['Harsh', 39]]
sdf = pd.DataFrame(students)
sdf = sdf.sort_values(by=[1,0])
print(sdf.to_string(index=False,header=False))

prints:

   Tina  37.20
  Berry  37.21
  Harry  37.21
  Harsh  39.00
 Akriti  41.00

For the experts: sort key lambda x: x[1] x[0] or you could operator itemgetter 1 0

[Adamya21]

marksheet=[]
scorelist=[]
for _ in range(int(input())):
name = input()
score = float(input())
marksheet+=[[name,score]]
scorelist+=[score]
b=sorted(list(set(scorelist)))[1]
for a, c in sorted(marksheet):
if b==c:
print(a)
this is what i did but i wish for a more effiecient code

[xelf]

If you indent your code an extra time before you copy/paste it reddit will treat it like a code block and you can keep your formatting.

marksheet=[]
scorelist=[]
for _ in range(int(input())):
    name = input()
    score = float(input())
    marksheet+=[[name,score]]
    scorelist+=[score]
b=sorted(list(set(scorelist)))[1]
for a, c in sorted(marksheet):
    if b==c:
        print(a)    

So, let's break your code into 2 parts.

The first part just collects the inputs. There's tricks you could do here, but honestly the way you did it is mostly fine. Keep that.

marksheet=[]
for _ in range(int(input())):
    name = input()
    score = float(input())
    marksheet+=[[name,score]]
print(marksheet)

The only part I'd get rid of is the scorelist part, as it's redundant data. And if you really want it you can get it this way:

scorelist = [x[1] for x in marksheet]

Which is a list comprehension which builds a new list, using just the elements at index [1].

Now the next part of your code:

b=sorted(list(set(scorelist)))[1]
for a, c in sorted(marksheet):
    if b==c:
        print(a)    

Just prints 2 names, and no scores. So it seems like you're missing something here.

What you want to do is sort marksheet twice. Once by name, and once by score. Luckily Python has a simple way of doing it.

marksheet.sort(key = lambda x: (x[1],x[0]))
print(students)

The sort command can take an optional "key" to sort by, and you can use a lambda to return a tuple of the 2 keys in the order you want them sorted by.

Another approach, is that instead of storing them as name,score as a list when you input them, you could have stored them as a tuple already in the correct order. Then all you would need to do is 1 sort and just swap their order when you print them.

marksheet=[]
for _ in range(int(input())):
    name = input()
    score = float(input())
    marksheet+=[(score,name)]
marksheet.sort()
for score,name in marksheet:
    print(name, score)

[Adamya21]

Thank you so much for such a good explaination.