sorting interview questions
Top sorting frequently asked interview questions
What I want to do seems pretty simple, but I can't find any answers on the web. I have an NSMutableArray of objects, let's say they are 'Person' objects. I want to sort the NSMutableArray by Person.birthDate which is an NSDate.
I think it has something to do with this method:
NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(???)];
In Java I would make my object implement Comparable, or use Collections.sort with an inline custom comparator...how on earth do you do this in Objective-C?
Source: (StackOverflow)
I often have to sort a dictionary, consisting of keys & values, by value. For example, I have a hash of words and respective frequencies, that I want to order by frequency.
There is a SortedList which is good for a single value (say frequency), that I want to map it back to the word.
SortedDictionary orders by key, not value. Some resort to a custom class, but is there a cleaner way?
Source: (StackOverflow)
If I have a JavaScript object such as:
var list = {"you": 100, "me": 75, "foo": 116, "bar": 15};
is there a way to sort the properties based on value? So that I end up with
list = {"bar": 15, "me": 75, "you": 100, "foo": 116};
I'm having a real brain-dead moment regarding this.
Source: (StackOverflow)
I got a list of dictionaries and want that to be sorted by a value of that dictionary.
This
[{'name':'Homer', 'age':39}, {'name':'Bart', 'age':10}]
sorted by name, should become
[{'name':'Bart', 'age':10}, {'name':'Homer', 'age':39}]
Source: (StackOverflow)
I've got a list of Python objects that I'd like to sort by an attribute of the objects themselves. The list looks like:
>>> ut
[<Tag: 128>, <Tag: 2008>, <Tag: <>, <Tag: actionscript>, <Tag: addresses>, <Tag: aes>, <Tag: ajax> ...]
Each object has a count:
>>> ut[1].count
1L
I need to sort the list by number of counts descending.
I've seen several methods for this, but I'm looking for best practice in Python.
Source: (StackOverflow)
I have an array of JavaScript objects:
var objs = [
{ first_nom: 'Lazslo', last_nom: 'Jamf' },
{ first_nom: 'Pig', last_nom: 'Bodine' },
{ first_nom: 'Pirate', last_nom: 'Prentice' }
];
How can I sort them by the value of last_nom in JavaScript?
I know about sort(a,b), but that only seems to work on strings and numbers. Do I need to add a toString method to my objects?
Source: (StackOverflow)
I'm using C# on Framework 3.5. I'm looking to quickly sort a Generic List<>. For the sake of this example lets say I have a List of a Person type with a property of lastname. How would I sort this List using a lambda expression?
List<Person> people = PopulateList();
people.OrderBy(???? => ?????)
Source: (StackOverflow)
I have some data either in list contains lists, or list contains tuples.
data = [[1,2,3], [4,5,6], [7,8,9]]
data = [(1,2,3), (4,5,6), (7,8,9)]
And I want to sort by the 2nd element in the subset. Meaning, sorting by 2,5,8 where 2 is from (1,2,3), 5 is from (4,5,6). What is the common way to do this? Should I store tuples or lists in my list? Since tuples are more inflexible. Thanks.
Source: (StackOverflow)
I have a dictionary of values read from two fields in a database: a string field and a numeric field. The string field is unique, so that is the key of the dictionary.
I can sort on the keys, but how can I sort based on the values?
Note: I have read Stack Overflow question How do I sort a list of dictionaries by values of the dictionary in Python? and probably could change my code to have a list of dictionaries, but since I do not really need a list of dictionaries I wanted to know if there is a simpler solution.
Source: (StackOverflow)
I was implementing an algorithm in Swift and noticed that the performance was very poor. After digging deeper I realised that one of the bottlenecks was something as simple as sorting arrays. The relevant part is here:
let n = 1000000
let x = Int[](count: n, repeatedValue: 0)
for i in 0..n {
x[i] = random()
}
// start clock here
let y = sort(x)
// stop clock here
In C++, a similar operation takes 0.06 s on my computer.
In Python it takes 0.6 s (no tricks, just y = sorted(x) for a list of integers).
In Swift it takes 6 s if I compile it with the following command:
xcrun swift -O3 -sdk `xcrun --show-sdk-path --sdk macosx`
And it takes as much as 88 s if I compile it with the following command:
xcrun swift -O0 -sdk `xcrun --show-sdk-path --sdk macosx`
Timings in Xcode with "Release" vs. "Debug" builds are similar.
What is wrong here? I could understand some performance loss in comparison with C++, but not a 10-fold slowdown in comparison with pure Python.
Edit: mweathers noticed that changing -O3 to -Ofast makes this code run almost as fast as the C++ version! However, -Ofast changes the semantics of the language a lot — in my testing, it disabled the checks for integer overflows and array indexing overflows. For example, with -Ofast the following Swift code runs silently without crashing (and prints out some garbage):
let n = 10000000
println(n*n*n*n*n)
let x = Int[](count: n, repeatedValue: 10)
println(x[n])
So -Ofast is not what we want; the whole point of Swift is that we have the safety nets in place. Of course the safety nets have some impact on the performance, but they should not make the programs 100 times slower. Remember that Java already checks for array bounds, and in typical cases the slowdown is by a factor much less than 2. And in Clang and GCC we have got -ftrapv for checking (signed) integer overflows, and it is not that slow, either.
Hence the question: how can we get a reasonable performance in Swift without losing the safety nets?
Edit 2: I did some more benchmarking, with very simple loops along the lines of
for i in 0..n {
x[i] = x[i] ^ 12345678
}
(Here the xor operation is there just so that I can more easily find the relevant loop in the assembly code. I tried to pick an operation that is easy to spot but also "harmless" in the sense that it should not require any checks related to integer overflows.)
Again, there was a huge difference in the performance between -O3 and -Ofast. So I had a look at the assembly code:
With -Ofast I get pretty much what I would expect. The relevant part is a loop with 5 machine language instructions.
With -O3 I get something that was beyond my wildest imagination. The inner loop spans 88 lines of assembly code. I did not try to understand all of it, but the most suspicious parts are 13 invocations of "callq _swift_retain" and another 13 invocations of "callq _swift_release". That is, 26 subroutine calls in the inner loop!
Edit 3: In comments, Ferruccio asked for benchmarks that are fair in the sense that they do not rely on built-in functions (e.g. sort). I think the following program is a fairly good example:
let n = 10000
let x = Int[](count: n, repeatedValue: 1)
for i in 0..n {
for j in 0..n {
x[i] = x[j]
}
}
There is no arithmetic, so we do not need to worry about integer overflows. The only thing that we do is just lots of array references. And the results are here—Swift -O3 loses by factor almost 500 in comparison with -Ofast:
- C++ -O3: 0.05 s
- C++ -O0: 0.4 s
- Java: 0.2 s
- Python with PyPy: 0.5 s
- Python: 12 s
- Swift -Ofast: 0.05 s
- Swift -O3: 23 s
- Swift -O0: 443 s
(If you are concerned that the compiler might optimise out the pointless loops entirely, you can change it to e.g. x[i] ^= x[j], and add a print statement that outputs x[0]. This does not change anything; the timings will be very similar.)
And yes, here the Python implementation was a stupid pure Python implementation with a list of ints and nested for loops. It should be much slower than unoptimised Swift. Something seems to be seriously broken with Swift and array indexing.
Edit 4: These issues (as well as some other performance issues) seems to have been fixed in Xcode 6 beta 5.
For sorting, I now have the following timings:
- clang++ -O3: 0.06 s
- swiftc -Ofast: 0.1 s
- swiftc -O: 0.1 s
- swiftc: 4 s
For nested loops:
- clang++ -O3: 0.06 s
- swiftc -Ofast: 0.3 s
- swiftc -O: 0.4 s
- swiftc: 540 s
It seems that there is no reason anymore to use the unsafe -Ofast (a.k.a. -Ounchecked); plain -O produces equally good code.
Source: (StackOverflow)
I read the following objects using Ajax and stored them in an array:
var homes = [
{
"h_id": "3",
"city": "Dallas",
"state": "TX",
"zip": "75201",
"price": "162500"
}, {
"h_id": "4",
"city": "Bevery Hills",
"state": "CA",
"zip": "90210",
"price": "319250"
}, {
"h_id": "5",
"city": "New York",
"state": "NY",
"zip": "00010",
"price": "962500"
}
];
How do I create a function to sort the objects by the price property in ascending and descending order using only JavaScript?
Source: (StackOverflow)
I am relatively new to Java, and often find that I need to sort a Map<Key, Value> on the values. Since the values are not unique, I find myself converting the keySet into an array, and sorting that array through array sort with a custom comparator that sorts on the value associated with the key. Is there an easier way?
Source: (StackOverflow)
I have a class called Order which has properties such as OrderId, OrderDate, Quantity, and Total. I have a list of this Order class:
List<Order> objListOrder = new List<Order>();
GetOrderList(objListOrder); // fill list of orders
Now I want to sort the list based on one property of the Order object, for example I need to sort it by the order date or order id.
How can i do this in C#?
Source: (StackOverflow)
How can I sort this array of objects by one of its fields, like name or count ?
Array
(
[0] => stdClass Object
(
[ID] => 1
[name] => Mary Jane
[count] => 420
)
[1] => stdClass Object
(
[ID] => 2
[name] => Johnny
[count] => 234
)
[2] => stdClass Object
(
[ID] => 3
[name] => Kathy
[count] => 4354
)
....
Source: (StackOverflow)
Let's say you've got an airplane, and it is low on fuel. Unless the plane drops 3000 pounds of passenger weight, it will not be able to reach the next airport. To save the maximum number of lives, we would like to throw the heaviest people off of the plane first.
And oh yeah, there are millions of people on the airplane, and we would like an optimal algorithm to find the heaviest passengers, without necessarily sorting the entire list.
This is a proxy problem for something I'm trying to code in C++. I would like to do a "partial_sort" on the passenger manifest by weight, but I don't know how many elements I'm going to need. I could implement my own "partial_sort" algorithm ("partial_sort_accumulate_until"), but I'm wondering if there's any easier way to do this using standard STL.
Source: (StackOverflow)