List exercise Python

Ive been doing a lot of algorithm questions lately and discovered I dont understand lists as well as I should

This is my compilation of 60 list questions Ive written to evaluate my own knowledge.

I hope youll find it as useful as writing it has been for me.

Questions 110:

1. Check if a list contains an element

The in operator will return True if a specific element is in a list.

2. How to iterate over 2+ lists at the same time

You can zip[] lists and then iterate over the zip object. A zip object is an iterator of tuples.

Below we iterate over 3 lists simultaneously and interpolate the values into a string.

3. When would you use a list vs dictionary?

Lists and dictionary generally have slightly different use cases but there is some overlap.

The general rule of algorithm questions Ive come to is that if you can use both, use a dictionary because lookups are faster.

List

Use a list if you need to store the order of something.

Ie: ids of database records in the order theyll be displayed.

While both lists and dictionaries are ordered as of python 3.7, a list allows duplicate values while a dictionary doesnt allow duplicate keys.

Dictionary

Use a dictionary if you want to count occurrences of something. Like the number of pets in a home.

Each key can only exist once in a dictionary. Note that keys can also be other immutable data structures like tuples. Ie: {['a',1]:1, ['b',2]:1}.

4. Is a list mutable?

Yes. Notice in the code below how the value associated with the same identifier in memory has not changed.

5. Does a list need to be homogeneous?

No. Different types of object can be mixed together in a list.

6. What is the difference between append and extend?

.append[] adds an object to the end of a list.

This also means appending a list adds that whole list as a single element, rather than appending each of its values.

.extend[] adds each value from a 2nd list as its own element. So extending a list with another list combines their values.

7. Do python lists store values or pointers?

Python lists dont store values themselves. They store pointers to values stored elsewhere in memory. This allows lists to be mutable.

Here we initialize values 1 and 2, then create a list including the values 1 and 2.

Notice how the list has its own memory address. But 1 and 2 in the list point to the same place in memory as the 1 and 2 we previously defined.

8. What does del do?

del removes an item from a list given its index.

Here well remove the value at index 1.

Notice how del does not return the removed element.

9. What is the difference between remove and pop?

.remove[] removes the first instance of a matching object. Below we remove the first b.

.pop[] removes an object by its index.

The difference between pop and del is that pop returns the popped element. This allows using a list like a stack.

By default, pop removes the last element from a list if an index isnt specified.

10. Remove duplicates from a list

If youre not concerned about maintaining the order of a list, then converting to a set and back to a list will achieve this.

Questions 1120:

11. Find the index of the 1st matching element

For example, you want to find the first apple in a list of fruit. Use the .index[] method.

12. Remove all elements from a list

Rather than creating a new empty list, we can clear the elements from an existing list with .clear[].

Or with del.

13. Iterate over both the values in a list and their indices

enumerate[] adds a counter to the list passed as an argument.

Below we iterate over the list and pass both value and index into string interpolation.

14. How to concatenate two lists

The + operator will concatenate 2 lists.

15. How to manipulate every element in a list with list comprehension

Below we return a new list with 1 added to every element.

16. Count the occurrence of a specific object in a list

The count[] method returns the number of occurrences of a specific object. Below we return the number of times the string, fish exists in a list called pets.

17. How to shallow copy a list?

.copy[] can be used to shallow copy a list.

Below we create a shallow copy of round1, assign it to a new name, round2, and then remove the string sonny chiba.

18. Why create a shallow copy of a list?

Building off the previous example, modifying round2 will modify round1 if we dont create a shallow copy.

Without a shallow copy, round1 and round2 are just names pointing to the same list in memory. Thats why it appears that changing the value of one changes the value of the other.

19. How to deep copy a list?

For this we need to import the copy module, then call copy.deepcopy[].

Below we create a deep copy of a list, round1 called round2, update a value in round2, then print both. In this case, round1 isnt affected.

Above we can see that changing the nested array in round2 did not update round1.

20. What is the difference between a deep copy and a shallow copy?

Building off the previous example, creating a shallow copy and then modifying it would have affected the original list..

Why does this happen?

Creating a shallow copy does create a new object in memory, but its filled with the same references to existing objects that the previous list has.

Creating a deep copy creates copies of the original objects and points to these new versions. So the new list is completely unaffected by changes to the old list and vice versa.

Questions 21-30:

21. What is the difference between a list and a tuple.

Tuples cannot be updated after creation. Adding/removing/updating an existing tuple requires creating a new tuple.

Lists can be modified after creation.

Tuples often represent an object like a record loaded from a database where elements are of different datatypes.

Lists are generally used to store an ordered sequence of a specific type of object [but not always].

Both are sequences and allow duplicate values.

22. Return the length of a list

len[] can returns the length of a list.

But note it counts top level objects, so a nested list of several integers will only be counted as a single object. Below, li has a length of 2, not 5.

23. What is the difference between a list and a set?

While a list is ordered, a set is not. Thats why using set to find unique values in a list, like list[ set[[3, 3, 2, 1]] ] loses the order.

While lists are often used to track order, sets are often used to track existence.

Lists allow duplicates, but all values in a set are unique by definition.

24. How to check if an element is not in a list?

For this we use the in operator, but prefix it with not.

25. Multiply every element in a list by 5 with the map function

.map[] allows iterating over a sequence and updating each value with another function.

map[] returns a map object but Ive wrapped it with a list comprehension so we can see the updated values.

26. Combine 2 lists into a list of tuples with the zip function

zip[] combines multiple sequences into an iterator of tuples, where values at the same sequence index are combined in the same tuple.

27. Insert a value at a specific index in an existing list

The insert[] method takes an object to insert and the index to insert it at.

Note that the element previously at the specified index is shifted to the right, not overwritten.

28. Subtract values in a list from the first element with the reduce function

reduce[] needs to be imported from functools.

Given a function, reduce iterates over a sequence and calls the function on every element. The output from the previous element is passed as an argument when calling the function on the next element.

Above we subtracted 10, 5, 1, 2, 7 and 5 from 100.

29. Remove negative values from a list with the filter function

Given a function, filter[] will remove any elements from a sequence on which the function doesnt return True.

Below we remove elements less than zero.

30. Convert a list into a dictionary where list elements are keys

For this we can use a dictionary comprehension.

Questions 31-40:

31. Modify an existing list with a lambda function

Lets take the previous map function we wrote and turn it into a one-liner with a lambda.

I could have left it as a map object until I needed to iterate over it but I converted to a list to show the elements inside.

32. Remove elements in a list after a specific index

Using the slice syntax, we can return a new list with only the elements up to a specific index.

33. Remove elements in a list before a specific index

The slice syntax can also return a new list with the values after a specified index.

34. Remove elements in a list between 2 indices

Or between two indices.

35. Return every 2nd element in a list between 2 indices

Or before/after/between indices at a specific interval.

Here we return every 2nd value between the indices 10 and 16 using the slice syntax.

36. Sort a list of integers in ascending order

The sort[] method mutates a list into ascending order.

37. Sort a list of integers in descending order

Its also possible to sort in descending order with sort[] by adding the argument reverse=True.

38. Filter even values out of a list with list comprehension

You can add conditional logic inside a list comprehension to filter out values following a given pattern.

Here we filter out values divisible by 2.

39. Count occurrences of each value in a list

One option is to iterate over a list and add counts to a dictionary. But the easiest option is to import the Counter class from collections and pass the list to it.

40. Get the first element from each nested list in a list

A list comprehension is well suited for iterating over a list of other objects and grabbing an element from each nested object.

Questions 4150:

41. What is the time complexity of insert, find and delete for a list?

Insert is O[n]. If an element is inserted at the beginning, all other elements must be shifted right.

Find by index is O[1]. But find by value is O[n] because elements need to be iterated over until the value is found.

Delete is O[n]. If an element is deleted at the beginning, all other elements must to be shifted left.

42. Combine elements in a list into a single string.

This can be done with the join[] function.

43. Whats the affect of multiplying a list by an integer?

Multiplying a list by an integer is called multiple concatenation and has the same affect as concatenating a list to itself n-times.

Below we multiple a list by 5.

This is the same as.

44. Use the any function to return True if any value in a list is divisible by 2

We can combine any[] with a list comprehension to return True if any values in the returned list evaluate to True.

Below the 1st list comprehension returns True because the list has a 2 in it, which is divisible by 2.

45. Use the all function to return True if all values in a list are negative

Similar to the any[] function, all[] can also be used with a list comprehension to return True only if all values in the returned list are True.

46. Can you sort a list with None in it?

You cannot sort a list with None in it because comparison operators [used by sort[]] cant compare an integer with None.