I was doing some work by my own for upcoming test and found this question that I could not understand.
int[] div = new int[]{2,3,5};
IEnumerable<int>seq = new int[]{10,15,20,25,30};
for(int i = 0; i<div.Length;i++){
int y = div[i];
seq = seq.Where(s => s%y == 0)
}
seq = seq.ToList();
I thought the resulting sequence would be 10 15 20 25 30 but the actual answer was 30.
I tried to run the code by my self and I found that when the code runs the for-loop, the sequence does not resets to the original but it keeps the sequence created by the previous div[i].
For example, when i = 0, div[0] is 2 so the seq picks the 10, 20 and 30.
As the code proceeds to i = 1 and div[1] = 3, the sequence it uses for the calculation part is still {10,20,30} not {10,15,20,25,30}.
Can someone explain why?
When I move int y to the outside of the for-loop like this,
int[] div = new int[]{2,3,5};
IEnumerable<int>seq = new int[]{10,15,20,25,30};
int y;
for(int i = 0; i<div.Length;i++){
y = div[i];
seq = seq.Where(s => s%y == 0)
}
seq = seq.ToList();
it gives the answer I was expecting.
Any help will be really helpful.
Thank you.
This is the resulting query you end up with:
IEnumerable<int> seq = new int[]{10,15,20,25,30};
seq = seq
.Where(s => s%2 == 0)
.Where(s => s%3 == 0)
.Where(s => s%5 == 0);
So, every number that is divisible by 2, 3 and 5:
10 is not divisible by 3
15 is not divisible by 2
20 is not divisible by 3
25 is not divisible by 2
30 is divisible by 2 (15), 3 (10), and 5 (6)
So 30 is the only result.
Now, if you instead want this query: "I want every number in seq that is divisible by 2, 3 or 5", then you need to do it different.
The code you had will filter every step of the way, only letting through to the next step any numbers that fit, so each will filter, but you want the opposite, if any one of them would say that it was OK, then you want the number.
To get the or expression instead, this would be the code:
seq.Where(s => div.Any(y => s % y == 0))
This gives this result:
10
15
20
25
30
Basically this query says:
Any number s in seq, which for any number y in div, where s is divisible by y.
As for your last part, why did it change after you moved the y outside the loop?
Well, let's see what actually happens.
When you have the y inside the loop, as in the first example, for each iteration through the loop, you can think of y as a "new y just for this loop iteration".
As such, you can think of the code that is being executed producing this query:
int y1 = 2;
int y2 = 3;
int y3 = 5;
seq = seq
.Where(s => s%y1 == 0)
.Where(s => s%y2 == 0)
.Where(s => s%y3 == 0);
But, if you move the y variable outside the loop you only ever have one y, which you change, so the final result is this:
int y = 5; // the last number in the loop
seq = seq
.Where(s => s%y == 0)
.Where(s => s%y == 0)
.Where(s => s%y == 0);
Which basically checks that every number is divisible by 5, 3 times.
So while you say that this gives you the result you want, it doesn't do what you want. Or rather, I assume it doesn't do what you want since you haven't actually described what you wanted the code to do.
Here are some links that explain the differences:
Anonymous Methods (MSDN)
Captured variable in a loop in C#
Detailed Explanation of Variable Capture in Closures
You code can be simplified by adding some more LINQness.
If what you are trying to get is the values of seq that can be divided by any of the values of div, you can simply do:
seq.Where(s => div.Any(d => s%d == 0));
If you want the values of seq that can be divided by all of the values of div, replace the .Any with .All.
Related
I'm trying to understand arrays better and I've found this piece of code on this website that fills an array with a load of random numbers. I was wondering how you would go about, say, extracting a range of numbers. So if i wanted to find out how many of these random numbers inside of the array, were between 25 and 50, how would i go about doing this? I've heard about the Array.FindAll<> however i have no clue on how to use it.
Thank you in advance.
Random r = new Random();
int count = 100;
// Create an array with count elements.
int[] numbers = new int[count];
// Loop over each index
for (int i = 0; i < count; i++)
{
// Generate and store a random number at current index
numbers[i] = r.Next(1, 100);
}
Using LINQ you can try the following:
var numbersInRange = numbers.Where(number => number > 25 && number < 60)
.ToArray();
The above will filter out all the numbers in the range (25,60). If you want to get also the numbers that are equal to either 25 or 60, you just have to use >= and <= repsectively.
If you don't want to use LINQ (which I couldn't explain), you could try the following:
var numbersInRange = new List<int>();
foreach(var number in numbers)
{
if(number > 25 && number < 60)
{
numbersInRange.Add(number);
}
}
So at the end you would have a list that would contains your numbers.
Comparing this to the LINQ version, I think that LINQ is the winner in simplicity and readability.
Using Linq it is a single line
int[] inRange = numbers.Where(x => x >= 25 && x <= 50).ToArray();
The Enumerable methods are a collection of methods that operates on sequences of values and apply a predicate expression (x => 25 && x <= 50) to each member (x) of the sequence (numbers). In your particular case the Where method filters the sequence based on the result of the boolean expression (the return includes the members where the expression returns true).
We can use Linq. First have using System.Linq in your namespaces so the extension methods are available.
To find the numbers:
numbers.Where(x => x >= 25 & x <= 50)
This takes a lambda expression (in this case x => x >= 25 & x <= 50) that takes on object of the type in the sequence (in this case int) returns bool and then evaluates for that expression for all contained items, filtering out those that do not.
You can use ToArray() on the result to get it back into an array, or just work on the results directly.
To get which numbers, that is to say, which indices the matches where at, you can use:
numbers.Select((x, i) => new {El = x, Idx = i}).Where(x => x.EL >= 25 & x.El <= 50).Select(x => x.Idx)
This first creates a new anonymous object for each item where El is the value held and Idx is the index it was at. Then it does the same Where as before, but on the El property of the object we created in the first step. Finally it extracts out the Idx property of the survivors so we have all of the matching indices.
Although "Linq" can do the job as shown by other answers ; there is also, as you heard about, Array.FindAll which behaves the same but already returns an array (and so doesn't need a call to ToArray)
int[] matchedNumbers = Array.FindAll (currentNumber => 25 <= currentNumber && currentNumber <= 50);
As for the "strange" arrow notation it's a lambda expression
Panagiotis Kanavos introduced the following clever solution to produce LetterNumberNumber pattern in this SOF question: For loop when the values of the loop variable is a string of the pattern LetterNumberNumber?
var maxLetters=3; // Take 26 for all letters
var maxNumbers=3; // Take 99 for all the required numbers
var values=from char c in Enumerable.Range('A',maxLetters).Select(c=>(char)c)
from int i in Enumerable.Range(1,maxNumbers)
select String.Format("{0}{1:d2}",(char)c,i);
foreach(var value in values)
{
Console.WriteLine(value);
}
A01
A02
A03
B01
B02
B03
C01
C02
C03
D01
D02
D03
Is there way to instruct irregular course in Enumerable stuff? "Enumerable.Range(1, maxNumbers)" leads 01, 02, ...,99 (for maxNumbers 99).
Restriction Examples:
1. Restrict (01,02,...,99) only to (01,03,05,07,09,11,13)
2. Restrict (01,02,...,99) only to (02,04,06,08,10)
3. Restrict (01,02,...,99) only to (01,04,09,10)
What I did:
I worked "Enumarable", tried its methods like: Enumerable.Contains(1,3,5,7,9,13) gave big error, and I could not achieve to reach:
A01, A03, A05, ....,Z09, Z11, Z13.
If Enumarable is not suitable for this type of job, what do you offer to handle the problem?
This isn't a direct feature in C#, while it is in F#.
F# example:
[1..2..10]
will produce a list of [1,3,5,7,9].
You first example, "Restrict (01,02,...,99) only to (01,03,05,07,09,11,13)" can be achieved with
Enumerable.Range(1,99).Where(x => x % 2 == 1).Take(7);
The second example, "Restrict (01,02,...,99) only to (02,04,06,08,10)" can be achieved with
Enumerable.Range(1,99).Where(x => x % 2 == 0).Take(5);
And your third example, "Restrict (01,02,...,99) only to (01,04,09,10)" seems odd. I'm not sure what the pattern here is. If the last element isn't a typo, then starting at one and incrementing by 3, then 5, then 1 seems unclear, but here's a method that can accomplish it.
public static IEnumerable<int> GetOddMutation(int start, int max, int count, List<int> increments) {
int counter = 0;
int reset = increments.Count - 1;
int index = 0;
int incremented = start;
while(counter < count) {
var previous = incremented;
incremented += increments[index];
index = index == reset ? 0 : index + 1;
counter++;
if(previous != incremented) //Avoid duplicates if 0 is part of the incrementation strategy. Alternatively, call .Distinct() on the method.
yield return incremented;
}
}
called with
GetOddMutation(1,99,4, new List<int> {0,3,5,1})
will result in [1,4,9,10]
It sounds to me like you want the Enumerable.Range(1, maxNumbers) to be restricted by a certain condition rather than having all of the integers. Since Enumerable.Range() produces an IEnumerable<int>, you can chain any LINQ filtering method calls, namely the Enumerable.Where() method. For example, Enumerable.Range(1, 99).Where(x => x % 3 == 0) would yield (3,6,9,...99).
If you only wanted the specific case you specified where the list only contains (1,3,5,7,9,13), you could simply make a list with the desired numbers: new List<int> {1,3,5,7,9,13}; you could also use Enumerable.Range(1, maxNumbers).Where(x => x % 2 == 1) and maxNumbers = 13.
Use this, Rx generates a series of random numbers between 0 and 99.
var R = new Random();
var ints = Observable.Interval(TimeSpan.FromSeconds(1));
var RandomNos = ints.Select(i=> R.Next(100)); // was new Random().Next(100)
RandomNos.Subscribe(r=> Console.Write(r+ ","));
1,75,49,23,97,71,45,19,93,66,40,14,88,62,36,10,84
I want to capture/detect when I get 6 more-than-50 numbers in a row. Can Rx do it?
RandomNos.?????()
.Subscribe(l=> Console.WriteLine ("You got 6 more-than-50 numbers in a row"));
One way to do this is with the Buffer method.
var random = new Random();
var result = Observable.Interval(TimeSpan.FromSeconds(1))
.Select(i => random.Next(100))
.Buffer(6, 1)
.Where(buffer => buffer.All(n => n > 50))
If instead of 6-in-a-row you were trying to detect K-in-a-row, where K was really really huge, then you'd probably want to do something using Window instead, but since K = 6 here it's easiest to just do what I suggested.
Also, be aware that the probability of a number drawn uniformly from {0, 1, ..., 99} being greater than 50 is 49/100, not 1/2.
Just for fun, here's one that only uses a single counter for any "n" in a row - it keeps a running count of numbers over 50 - the Take(1) completes the stream at the first occurrence.
RandomNos.Scan(0, (a,x) => x > 50 ? ++a : 0)
.Where(x => x == 6)
.Take(1)
.Subscribe(_ => Console.WriteLine("You got 6 more-than-50 numbers in a row"));
I recently took a Codility test and the question was to find the first unique number in a numeric sequence. Although I get the correct result using LINQ, it is apparently too expensive computationally and not scalable enough.
How would I improve my solution?
var a = new int[] {1, 2, -3, 4, 5, -6, 0, 8, 9, 1, 2};
const int expected = -3;
var retVal = -1;
var y = a.GroupBy(z => z).Where(z => z.Count() == 1).Select(z => z.Key).ToList();
if (y.Count > 0) retVal = y[0];
Console.Write(retVal==expected);
How about this:
var result = a.ToLookup(i => i).First(i => i.Count() == 1).Key;
This should give -3.
It builds a Lookup object with a key created using each number in the list, and a value of the same number.
In the case of duplicates, then multiple entries are created under each key. The first unique value will be the first group in the Lookup with one entry.
(You could just as easily use GroupBy, instead of ToLookup. The end result will be the same.)
try this
var result = a.GroupBy(g => g).Where(w => w.Count() == 1).Select(s => s.Key).FirstOrDefault();
I decided to test the various answers posted above and count the number of ticks using the StopWatch class.
Changing y.Count > 0 to y.Any() resulted in the second lowest tick count, but the largest reduction belongs to the group by comment from Baldrick
To get the average each test was run 50 times.
Ticks
Min Max Avg Actual
2 14502 584 -3 Original
2 919 40 -3 if (y.Any()) actual = y[0];
2 1423 60 -3 a.GroupBy(g => g).Where(w => w.Count() == 1)
.Select(s => s.Key)
.FirstOrDefault();
2 1553 65 -3 a.ToLookup(i => i).First(i => i.Count() == 1).Key;
2 317 15 -3 a.GroupBy(i => i).First(i => i.Count() == 1).Key;
Given two arrays, I need to extract values from arrayB based on where the range(actual values) falls in arrayA.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12
-------------------------------------------------------------
ArrayA = {0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6}
ArrayB = {1, 0.2, 3, 4, 5, 6,5.5, 8, 9,11.1, 11, 12, 3}
Given the following ranges, I need to extract the following results
RangeToExtract* IndexInArrayA Expected Values To Extract
-------------- ------------- --------------------------
0 -> 1 [0,2] 1,0.2,3
1 -> 3 [3,6] 4,5,6,5.5
3 -> 5 [7,10] 5.5,8,9,11.1,11
1 -> 5 [3,10] 4,5,6,5.5,8,9,11.1,11
3 -> 10 [7,12] 8,9,11.1,11,12,3
* Refers to the actual values in ArrayA
Note: Given the RangeToExtract (0->1), determine the indexes in ArrayA where these values are, the result being (0->1) maps to [0,2] (The value 1 is in position 2 in ArrayA)
I only figured that the following special cases exists (not sure if there are more)
the lower limit is equal to zero and
when the upper limit does not exist in ArrayA
Further info:
Both arrays will be the same size
ArrayA will always be sorted
Code:
private double[] GetRange(double lower, double upper)
{
var myList = new double[ArrayA.Length];
var lowerIndex = Array.IndexOf(ArrayA, lower);
var upperIndex = Array.IndexOf(ArrayA, upper);
// special case 1
if (lowerIndex != 0)
{
lowerIndex = lowerIndex + 1;
}
// special case 2
if (upperIndex == -1)
{
upperIndex = ArrayA.Length-1;
}
for (int i = lowerIndex; i <= upperIndex; i++)
{
myList[i] = ArrayB[i];
}
return myList;
}
Given the above code, have all the special cases been taken into account? Is there a better way to write the above code?
Yap! There is a quite better way, that comes with lovely LINQ. I put here in two forms. First looks complicated but not at ALL! Believe me ;)
At the first step you have to take out those A'indexes that their values fall into your range (I call it min...max), based on your example I got that your range is closed from the lower boundary and closed on upper side, I means when you mentioned 3 -> 5 actually It is [3, 5)! It does not contain 5. Anyway that is not the matter.
This can be done by following LINQ
int[] selectedIndexes = a.Select((value, index) =>
new { Value = value, Index = index }).
Where(aToken => aToken.Value > min && aToken.Value <= max).
Select(t => t.Index).ToArray<int>();
The first select, generates a collection of [Value, Index] pairs that the first one is the array element and the second one is the index of the element within the array. I think this is the main trick for your question. So It provides you with this ability to work with the indexes same as usual values.
Finally in the second Select I just wrap whole indexes into an integer array. Hence after this you have the whole indexes that their value fall in the given range.
Now second step!
When you got those indexes, you have to select whole elements within the B under the selected Indexes from the A. The same thing should be done over the B. It means again we select B element into a collection of [Value, Index] pairs and then we select those guys that their indexes exist within the selected indexes from the A. This can be done as follow:
double[] selectedValues = b.Select((item, index) =>
new { Item = item, Index = index }).
Where(bToken => selectedIndexes.Contains(bToken.Index)).
Select(d => d.Item).ToArray<double>();
Ok, so first select is the one I talked about it in the fist part and then look at the where section that check whether the index of the bToken which is an element of B exists in the selectedIndexes (from A) or not!
Finally I wrap both codes into one as below:
double[] answers = b.Select((item, index) =>
new { Item = item, Index = index }).
Where(bTokent =>
a.Select((value, index) =>
new { Value = value, Index = index }).
Where(aToken => aToken.Value > min && aToken.Value <= max).
Select(t => t.Index).
Contains(bTokent.Index)).Select(d => d.Item).ToArray<double>();
Buy a beer for me, if it would be useful :)
I don't know if you're still interested, but I saw this one and I liked the challenge. If you use .Net 4 (having the Enumberable.Zip method) there is a very concise way to do this (given the conditions under futher info):
arrayA.Zip(arrayB, (a,b) => new {a,b})
.Where(x => x.a > lower && x.a < upper)
.Select (x => x.b)
You may want to use >= and <= to make the range comparisons inclusive.