For example I have a list like:
List<double> Nums = new List<double> {12.32, 5.12, 7.64, 9.77, 4.98, 8};
How can I find the index of the second and third highest double in the list?
The result would be giving 3 (for 9.77) and 5 (for 8).
It's important that the order of the values don't change. The rest of my code depends on the order of the list to not change.
Thanks!
You can do like this :
var numbers = Nums.OrderByDescending(x => x).Skip(1).Take(2); // this will get collection of second and third highest element
var secondHighestIndex = Nums.FindIndex(x => x == numbers.First()); // will get second highest index
var thirdHighestIndex = Nums.FindIndex(x => x == numbers.Last());// will get third highest index
You can also do this:
List<double> Nums = new List<double> { 12.32, 5.12, 7.64, 9.77, 4.98, 8 };
var lst = Nums.Select((a, b) => new KeyValuePair<int, double>(b, a))
.OrderByDescending(c => c.Value).Skip(1).Take(2).ToList();
I created keyvalue pairs, using its value as value, and index as key. Then order it by its value, then skip the first result then take 2 results.
The resulting pair would be:
[0] {[3, 9.77]}
[1] {[5, 8]}
You can access it via its index:
lst[0].Key; //For Index - 3
lst[0].Value; //For Value - 9.77
You can use linq, as described in #Keyur PATEL's comment. A slight modification could get you both numbers in one statement: Nums.OrderByDescending(n => n).Skip(1).Take(2);
See #Akash KC's answer.
Another solution would be to traverse the array, comparing each element to the highest found, and saving the indexes of the 3 highest numbers as a limited-length stack, and simply ignore the highest after that's done:
var highest = Nums[0];
var topIndexes = new int[3];
for(int i=1; i<Nums.length; i++){
if(Nums[i]>highest){
topIndexes [2] = topIndexes [1]; //Record index of 3rd highest
topIndexes [1] = topIndexes [0]; //Record index of 2nd highest
topIndexes [0] = i; //Record index of highest
highest = Nums[i];
}
}
//Do something with topIndexes[1] (2nd highest number's index)
//and topIndexes[2] (3rd highest number's index)
The code above can easily be modified into a function/method useful for finding or taking the top n numbers or n indexes out of collections dynamically.
In step by step solution
List<double> Nums = new List<double> { 12.32, 5.12, 7.64, 9.77, 4.98, 8 };
var SMax = Nums.OrderByDescending(z => z).Skip(1).First();
var Tmax = Nums.OrderByDescending(z => z).Skip(2).First();
var SIndex = Nums.IndexOf(SMax);
var TIndex = Nums.IndexOf(Tmax);
This question already has answers here:
How to find the Mode in Array C#? [duplicate]
(4 answers)
Closed 7 years ago.
How can I find the mode of a list of numbers? I know the logic of it (I think) but I don't know how to implement that logic or convert what my brain thinks into workable code.
This is what I know:
I need to have a loop that goes through the list one time to see how many times a number is repeated and an array to save the times a number is repeated. I also need to tell my program to discard the lesser amount once a larger one is found.
A linq approach, more concise but almost certainly less efficient than Yeldar Kurmangaliyev's:
int FindMode(IEnumerable<int> data)
{
return data
.GroupBy(n => n)
.Select(x => new { x.Key, Count = x.Count() })
.OrderByDescending(a => a.Count)
.First()
.Key;
}
This does not handle the case where data is empty, nor where there are two or more data points with the same frequency in the data set.
Yes, you are right:
Let we have a list of numbers:
List<int> myValues = new List<int>(new int[] { 1, 3, 3, 3, 7, 7 } );
You need to have a loop that goes through the list one time:
foreach (var val in myValues)
{
}
to see how many times a number is repeated in array to save the times a number is repeated:
Dictionary<int, int> repetitions = new Dictionary<int, int>();
foreach (var val in myValues)
{
if (repetitions.ContainsKey(val))
repetitions[val]++; // Met it one more time
else
repetitions.Add(val, 1); // Met it once, because it is not in dict.
}
Now, your dictionary repetitions stores how many (exactly value) times key value repeated.
Then, you need to find the record of mode (i.e. record with the highest time of repetitions (i.e. highest value)) and take this one. LINQ will help us - let's sort the array by value and take the last one...or sort it descending and take the first one. Actually, that's the same in terms of result and productivity.
var modeRecord = repetitions.OrderByDescending(x => x.Value).First();
// or
var modeRecord = repetitions.OrderBy(x => x.Value).Last();
Here it is! Here we have a mode:
List<int> myValues = new List<int>(new int[] { 1, 3, 3, 3, 7, 7 } );
Dictionary<int, int> repetitions = new Dictionary<int, int>();
foreach (var val in myValues)
{
if (repetitions.ContainsKey(val))
repetitions[val]++; // Met it one more time
else
repetitions.Add(val, 1); // Met it once, because it is not in dict.
}
var modeRecord = repetitions.OrderByDescending(x => x.Value).First();
Console.WriteLine("Mode is {0}. It meets {1} times in an list", modeRecord.Key, modeRecord.Value);
Your mode calculation logic is good. All you need is following your own instructions in a code :)
Here's an alternative LINQ approach:
var values = new int[] { 1, 3, 3, 3, 7, 7 };
var mode =
values
.Aggregate(
new { best = 0, best_length = 0, current = 0, current_length = 0 },
(a, n) =>
{
var current_length = 1 + (a.current == n ? a.current_length : 0);
var is_longer = current_length > a.best_length;
return new
{
best = is_longer ? n : a.best,
best_length = is_longer ? current_length : a.best_length,
current = n,
current_length,
};
}).best;
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.
This might sound lame, but I have not been able to find a really good explanation of Aggregate.
Good means short, descriptive, comprehensive with a small and clear example.
The easiest-to-understand definition of Aggregate is that it performs an operation on each element of the list taking into account the operations that have gone before. That is to say it performs the action on the first and second element and carries the result forward. Then it operates on the previous result and the third element and carries forward. etc.
Example 1. Summing numbers
var nums = new[]{1,2,3,4};
var sum = nums.Aggregate( (a,b) => a + b);
Console.WriteLine(sum); // output: 10 (1+2+3+4)
This adds 1 and 2 to make 3. Then adds 3 (result of previous) and 3 (next element in sequence) to make 6. Then adds 6 and 4 to make 10.
Example 2. create a csv from an array of strings
var chars = new []{"a","b","c","d"};
var csv = chars.Aggregate( (a,b) => a + ',' + b);
Console.WriteLine(csv); // Output a,b,c,d
This works in much the same way. Concatenate a a comma and b to make a,b. Then concatenates a,b with a comma and c to make a,b,c. and so on.
Example 3. Multiplying numbers using a seed
For completeness, there is an overload of Aggregate which takes a seed value.
var multipliers = new []{10,20,30,40};
var multiplied = multipliers.Aggregate(5, (a,b) => a * b);
Console.WriteLine(multiplied); //Output 1200000 ((((5*10)*20)*30)*40)
Much like the above examples, this starts with a value of 5 and multiplies it by the first element of the sequence 10 giving a result of 50. This result is carried forward and multiplied by the next number in the sequence 20 to give a result of 1000. This continues through the remaining 2 element of the sequence.
Live examples: http://rextester.com/ZXZ64749
Docs: http://msdn.microsoft.com/en-us/library/bb548651.aspx
Addendum
Example 2, above, uses string concatenation to create a list of values separated by a comma. This is a simplistic way to explain the use of Aggregate which was the intention of this answer. However, if using this technique to actually create a large amount of comma separated data, it would be more appropriate to use a StringBuilder, and this is entirely compatible with Aggregate using the seeded overload to initiate the StringBuilder.
var chars = new []{"a","b","c", "d"};
var csv = chars.Aggregate(new StringBuilder(), (a,b) => {
if(a.Length>0)
a.Append(",");
a.Append(b);
return a;
});
Console.WriteLine(csv);
Updated example: http://rextester.com/YZCVXV6464
It partly depends on which overload you're talking about, but the basic idea is:
Start with a seed as the "current value"
Iterate over the sequence. For each value in the sequence:
Apply a user-specified function to transform (currentValue, sequenceValue) into (nextValue)
Set currentValue = nextValue
Return the final currentValue
You may find the Aggregate post in my Edulinq series useful - it includes a more detailed description (including the various overloads) and implementations.
One simple example is using Aggregate as an alternative to Count:
// 0 is the seed, and for each item, we effectively increment the current value.
// In this case we can ignore "item" itself.
int count = sequence.Aggregate(0, (current, item) => current + 1);
Or perhaps summing all the lengths of strings in a sequence of strings:
int total = sequence.Aggregate(0, (current, item) => current + item.Length);
Personally I rarely find Aggregate useful - the "tailored" aggregation methods are usually good enough for me.
Super short
Aggregate works like fold in Haskell/ML/F#.
Slightly longer
.Max(), .Min(), .Sum(), .Average() all iterates over the elements in a sequence and aggregates them using the respective aggregate function. .Aggregate () is generalized aggregator in that it allows the developer to specify the start state (aka seed) and the aggregate function.
I know you asked for a short explaination but I figured as others gave a couple of short answers I figured you would perhaps be interested in a slightly longer one
Long version with code
One way to illustrate what does it could be show how you implement Sample Standard Deviation once using foreach and once using .Aggregate. Note: I haven't prioritized performance here so I iterate several times over the colleciton unnecessarily
First a helper function used to create a sum of quadratic distances:
static double SumOfQuadraticDistance (double average, int value, double state)
{
var diff = (value - average);
return state + diff * diff;
}
Then Sample Standard Deviation using ForEach:
static double SampleStandardDeviation_ForEach (
this IEnumerable<int> ints)
{
var length = ints.Count ();
if (length < 2)
{
return 0.0;
}
const double seed = 0.0;
var average = ints.Average ();
var state = seed;
foreach (var value in ints)
{
state = SumOfQuadraticDistance (average, value, state);
}
var sumOfQuadraticDistance = state;
return Math.Sqrt (sumOfQuadraticDistance / (length - 1));
}
Then once using .Aggregate:
static double SampleStandardDeviation_Aggregate (
this IEnumerable<int> ints)
{
var length = ints.Count ();
if (length < 2)
{
return 0.0;
}
const double seed = 0.0;
var average = ints.Average ();
var sumOfQuadraticDistance = ints
.Aggregate (
seed,
(state, value) => SumOfQuadraticDistance (average, value, state)
);
return Math.Sqrt (sumOfQuadraticDistance / (length - 1));
}
Note that these functions are identical except for how sumOfQuadraticDistance is calculated:
var state = seed;
foreach (var value in ints)
{
state = SumOfQuadraticDistance (average, value, state);
}
var sumOfQuadraticDistance = state;
Versus:
var sumOfQuadraticDistance = ints
.Aggregate (
seed,
(state, value) => SumOfQuadraticDistance (average, value, state)
);
So what .Aggregate does is that it encapsulates this aggregator pattern and I expect that the implementation of .Aggregate would look something like this:
public static TAggregate Aggregate<TAggregate, TValue> (
this IEnumerable<TValue> values,
TAggregate seed,
Func<TAggregate, TValue, TAggregate> aggregator
)
{
var state = seed;
foreach (var value in values)
{
state = aggregator (state, value);
}
return state;
}
Using the Standard deviation functions would look something like this:
var ints = new[] {3, 1, 4, 1, 5, 9, 2, 6, 5, 4};
var average = ints.Average ();
var sampleStandardDeviation = ints.SampleStandardDeviation_Aggregate ();
var sampleStandardDeviation2 = ints.SampleStandardDeviation_ForEach ();
Console.WriteLine (average);
Console.WriteLine (sampleStandardDeviation);
Console.WriteLine (sampleStandardDeviation2);
IMHO
So does .Aggregate help readability? In general I love LINQ because I think .Where, .Select, .OrderBy and so on greatly helps readability (if you avoid inlined hierarhical .Selects). Aggregate has to be in Linq for completeness reasons but personally I am not so convinced that .Aggregate adds readability compared to a well written foreach.
A picture is worth a thousand words
Reminder:
Func<X, Y, R> is a function with two inputs of type X and Y, that returns a result of type R.
Enumerable.Aggregate has three overloads:
Overload 1:
A Aggregate<A>(IEnumerable<A> a, Func<A, A, A> f)
Example:
new[]{1,2,3,4}.Aggregate((x, y) => x + y); // 10
This overload is simple, but it has the following limitations:
the sequence must contain at least one element,
otherwise the function will throw an InvalidOperationException.
elements and result must be of the same type.
Overload 2:
B Aggregate<A, B>(IEnumerable<A> a, B bIn, Func<B, A, B> f)
Example:
var hayStack = new[] {"straw", "needle", "straw", "straw", "needle"};
var nNeedles = hayStack.Aggregate(0, (n, e) => e == "needle" ? n+1 : n); // 2
This overload is more general:
a seed value must be provided (bIn).
the collection can be empty,
in this case, the function will yield the seed value as result.
elements and result can have different types.
Overload 3:
C Aggregate<A,B,C>(IEnumerable<A> a, B bIn, Func<B,A,B> f, Func<B,C> f2)
The third overload is not very useful IMO.
The same can be written more succinctly by using overload 2 followed by a function that transforms its result.
The illustrations are adapted from this excellent blogpost.
Aggregate is basically used to Group or Sum up data.
According to MSDN
"Aggregate Function Applies an accumulator function over a sequence."
Example 1: Add all the numbers in a array.
int[] numbers = new int[] { 1,2,3,4,5 };
int aggregatedValue = numbers.Aggregate((total, nextValue) => total + nextValue);
*important: The initial aggregate value by default is the 1 element in the sequence of collection.
i.e: the total variable initial value will be 1 by default.
variable explanation
total: it will hold the sum up value(aggregated value) returned by the func.
nextValue: it is the next value in the array sequence. This value is than added to the aggregated value i.e total.
Example 2: Add all items in an array. Also set the initial accumulator value to start adding with from 10.
int[] numbers = new int[] { 1,2,3,4,5 };
int aggregatedValue = numbers.Aggregate(10, (total, nextValue) => total + nextValue);
arguments explanation:
the first argument is the initial(starting value i.e seed value) which will be used to start addition with the next value in the array.
the second argument is a func which is a func that takes 2 int.
1.total: this will hold same as before the sum up value(aggregated value) returned by the func after the calculation.
2.nextValue: : it is the next value in the array sequence. This value is than added to the aggregated value i.e total.
Also debugging this code will give you a better understanding of how aggregate work.
In addition to all the great answers here already, I've also used it to walk an item through a series of transformation steps.
If a transformation is implemented as a Func<T,T>, you can add several transformations to a List<Func<T,T>> and use Aggregate to walk an instance of T through each step.
A more concrete example
You want to take a string value, and walk it through a series of text transformations that could be built programatically.
var transformationPipeLine = new List<Func<string, string>>();
transformationPipeLine.Add((input) => input.Trim());
transformationPipeLine.Add((input) => input.Substring(1));
transformationPipeLine.Add((input) => input.Substring(0, input.Length - 1));
transformationPipeLine.Add((input) => input.ToUpper());
var text = " cat ";
var output = transformationPipeLine.Aggregate(text, (input, transform)=> transform(input));
Console.WriteLine(output);
This will create a chain of transformations: Remove leading and trailing spaces -> remove first character -> remove last character -> convert to upper-case. Steps in this chain can be added, removed, or reordered as needed, to create whatever kind of transformation pipeline is required.
The end result of this specific pipeline, is that " cat " becomes "A".
This can become very powerful once you realize that T can be anything. This could be used for image transformations, like filters, using BitMap as an example;
Learned a lot from Jamiec's answer.
If the only need is to generate CSV string, you may try this.
var csv3 = string.Join(",",chars);
Here is a test with 1 million strings
0.28 seconds = Aggregate w/ String Builder
0.30 seconds = String.Join
Source code is here
Definition
Aggregate method is an extension method for generic collections. Aggregate method applies a function to each item of a collection. Not just only applies a function, but takes its result as initial value for the next iteration. So, as a result, we will get a computed value (min, max, avg, or other statistical value) from a collection.
Therefore, Aggregate method is a form of safe implementation of a recursive function.
Safe, because the recursion will iterate over each item of a collection and we can’t get any infinite loop suspension by wrong exit condition. Recursive, because the current function’s result is used as a parameter for the next function call.
Syntax:
collection.Aggregate(seed, func, resultSelector);
seed - initial value by default;
func - our recursive function. It can be a lambda-expression, a Func delegate or a function type T F(T result, T nextValue);
resultSelector - it can be a function like func or an expression to compute, transform, change, convert the final result.
How it works:
var nums = new[]{1, 2};
var result = nums.Aggregate(1, (result, n) => result + n); //result = (1 + 1) + 2 = 4
var result2 = nums.Aggregate(0, (result, n) => result + n, response => (decimal)response/2.0); //result2 = ((0 + 1) + 2)*1.0/2.0 = 3*1.0/2.0 = 3.0/2.0 = 1.5
Practical usage:
Find Factorial from a number n:
int n = 7;
var numbers = Enumerable.Range(1, n);
var factorial = numbers.Aggregate((result, x) => result * x);
which is doing the same thing as this function:
public static int Factorial(int n)
{
if (n < 1) return 1;
return n * Factorial(n - 1);
}
Aggregate() is one of the most powerful LINQ extension method, like Select() and Where(). We can use it to replace the Sum(), Min(). Max(), Avg() functionality, or to change it by implementing addition context:
var numbers = new[]{3, 2, 6, 4, 9, 5, 7};
var avg = numbers.Aggregate(0.0, (result, x) => result + x, response => (double)response/(double)numbers.Count());
var min = numbers.Aggregate((result, x) => (result < x)? result: x);
More complex usage of extension methods:
var path = #“c:\path-to-folder”;
string[] txtFiles = Directory.GetFiles(path).Where(f => f.EndsWith(“.txt”)).ToArray<string>();
var output = txtFiles.Select(f => File.ReadAllText(f, Encoding.Default)).Aggregate<string>((result, content) => result + content);
File.WriteAllText(path + “summary.txt”, output, Encoding.Default);
Console.WriteLine(“Text files merged into: {0}”, output); //or other log info
This is an explanation about using Aggregate on a Fluent API such as Linq Sorting.
var list = new List<Student>();
var sorted = list
.OrderBy(s => s.LastName)
.ThenBy(s => s.FirstName)
.ThenBy(s => s.Age)
.ThenBy(s => s.Grading)
.ThenBy(s => s.TotalCourses);
and lets see we want to implement a sort function that take a set of fields, this is very easy using Aggregate instead of a for-loop, like this:
public static IOrderedEnumerable<Student> MySort(
this List<Student> list,
params Func<Student, object>[] fields)
{
var firstField = fields.First();
var otherFields = fields.Skip(1);
var init = list.OrderBy(firstField);
return otherFields.Skip(1).Aggregate(init, (resultList, current) => resultList.ThenBy(current));
}
And we can use it like this:
var sorted = list.MySort(
s => s.LastName,
s => s.FirstName,
s => s.Age,
s => s.Grading,
s => s.TotalCourses);
Aggregate used to sum columns in a multi dimensional integer array
int[][] nonMagicSquare =
{
new int[] { 3, 1, 7, 8 },
new int[] { 2, 4, 16, 5 },
new int[] { 11, 6, 12, 15 },
new int[] { 9, 13, 10, 14 }
};
IEnumerable<int> rowSums = nonMagicSquare
.Select(row => row.Sum());
IEnumerable<int> colSums = nonMagicSquare
.Aggregate(
(priorSums, currentRow) =>
priorSums.Select((priorSum, index) => priorSum + currentRow[index]).ToArray()
);
Select with index is used within the Aggregate func to sum the matching columns and return a new Array; { 3 + 2 = 5, 1 + 4 = 5, 7 + 16 = 23, 8 + 5 = 13 }.
Console.WriteLine("rowSums: " + string.Join(", ", rowSums)); // rowSums: 19, 27, 44, 46
Console.WriteLine("colSums: " + string.Join(", ", colSums)); // colSums: 25, 24, 45, 42
But counting the number of trues in a Boolean array is more difficult since the accumulated type (int) differs from the source type (bool); here a seed is necessary in order to use the second overload.
bool[][] booleanTable =
{
new bool[] { true, true, true, false },
new bool[] { false, false, false, true },
new bool[] { true, false, false, true },
new bool[] { true, true, false, false }
};
IEnumerable<int> rowCounts = booleanTable
.Select(row => row.Select(value => value ? 1 : 0).Sum());
IEnumerable<int> seed = new int[booleanTable.First().Length];
IEnumerable<int> colCounts = booleanTable
.Aggregate(seed,
(priorSums, currentRow) =>
priorSums.Select((priorSum, index) => priorSum + (currentRow[index] ? 1 : 0)).ToArray()
);
Console.WriteLine("rowCounts: " + string.Join(", ", rowCounts)); // rowCounts: 3, 1, 2, 2
Console.WriteLine("colCounts: " + string.Join(", ", colCounts)); // colCounts: 3, 2, 1, 2
Everyone has given his explanation. My explanation is like that.
Aggregate method applies a function to each item of a collection. For example, let's have collection { 6, 2, 8, 3 } and the function Add (operator +) it does (((6+2)+8)+3) and returns 19
var numbers = new List<int> { 6, 2, 8, 3 };
int sum = numbers.Aggregate(func: (result, item) => result + item);
// sum: (((6+2)+8)+3) = 19
In this example there is passed named method Add instead of lambda expression.
var numbers = new List<int> { 6, 2, 8, 3 };
int sum = numbers.Aggregate(func: Add);
// sum: (((6+2)+8)+3) = 19
private static int Add(int x, int y) { return x + y; }
A short and essential definition might be this: Linq Aggregate extension method allows to declare a sort of recursive function applied on the elements of a list, the operands of whom are two: the elements in the order in which they are present into the list, one element at a time, and the result of the previous recursive iteration or nothing if not yet recursion.
In this way you can compute the factorial of numbers, or concatenate strings.
I have a List< int[] > myList, where I know that all the int[] arrays are the same length - for the sake of argument, let us say I have 500 arrays, each is 2048 elements long. I'd like to sum all 500 of these arrays, to give me a single array, 2048 elements long, where each element is the sum of all the same positions in all the other arrays.
Obviously this is trivial in imperative code:
int[] sums = new int[myList[0].Length];
foreach(int[] array in myList)
{
for(int i = 0; i < sums.Length; i++)
{
sums[i] += array[i];
}
}
But I was wondering if there was a nice Linq or Enumerable.xxx technique?
Edit: Ouch...This became a bit harder while I wasn't looking. Changing requirements can be a real PITA.
Okay, so take each position in the array, and sum it:
var sums = Enumerable.Range(0, myList[0].Length)
.Select(i => myList.Select(
nums => nums[i]
).Sum()
);
That's kind of ugly...but I think the statement version would be even worse.
EDIT: I've left this here for the sake of interest, but the accepted answer is much nicer.
EDIT: Okay, my previous attempt (see edit history) was basically completely wrong...
You can do this with a single line of LINQ, but it's horrible:
var results = myList.SelectMany(array => array.Select(
(value, index) => new { value, index })
.Aggregate(new int[myList[0].Length],
(result, item) => { result[item.index] += value; return result; });
I haven't tested it, but I think it should work. I wouldn't recommend it though. The SelectMany flattens all the data into a sequence of pairs - each pair is the value, and its index within its original array.
The Aggregate step is entirely non-pure - it modifies its accumulator as it goes, by adding the right value at the right point.
Unless anyone can think of a way of basically pivoting your original data (at which point my earlier answer is what you want) I suspect you're best off doing this the non-LINQ way.
This works with any 2 sequences, not just arrays:
var myList = new List<int[]>
{
new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 },
new int[] { 10, 20, 30, 40, 50, 60, 70, 80, 90 }
};
var sums =
from array in myList
from valueIndex in array.Select((value, index) => new { Value = value, Index = index })
group valueIndex by valueIndex.Index into indexGroups
select indexGroups.Select(indexGroup => indexGroup.Value).Sum()
foreach(var sum in sums)
{
Console.WriteLine(sum);
}
// Prints:
//
// 11
// 22
// 33
// 44
// 55
// 66
// 77
// 88
// 99
OK, assuming we can assume that the sum of the ints at each position over the list of arrays will itself fit into an int (which is a dodgy assumption, but I'll make it anyway to make the job easier):
int[] sums =
Enumerable.Range(0, listOfArrays[0].Length-1).
Select(sumTotal =>
Enumerable.Range(0, listOfArrays.Count-1).
Aggregate((total, listIndex) =>
total += listOfArrays[listIndex][sumTotal])).ToArray();
EDIT - D'oh. For some reason .Select evaded me originally. That's a bit better. It's a slight hack because sumTotal is acting as both the input (the position in the array which is used in the Aggregate call) and the output sum in the resulting IEnumerable, which is counter-intuitive.
Frankly this is far more horrible than doing it the old-fasioned way :-)
Here is one that trades the Linq statement simplicity with performance.
var colSums =
from col in array.Pivot()
select col.Sum();
public static class LinqExtensions {
public static IEnumerable<IEnumerable<T>> Pivot<T>( this IList<T[]> array ) {
for( int c = 0; c < array[ 0 ].Length; c++ )
yield return PivotColumn( array, c );
}
private static IEnumerable<T> PivotColumn<T>( IList<T[]> array, int c ) {
for( int r = 0; r < array.Count; r++ )
yield return array[ r ][ c ];
}
}
I would do it as follows … but this solution might actually be very slow so you might want to run a benchmark before deploying it in performance-critical sections.
var result = xs.Aggregate(
(a, b) => Enumerable.Range(0, a.Length).Select(i => a[i] + b[i]).ToArray()
);
It can be done with Zip and Aggregate. The question is so old that probably Zip was not around at the time. Anyway, here is my version, hoping it will help someone.
List<int[]> myListOfIntArrays = PopulateListOfArraysOf100Ints();
int[] totals = new int[100];
int[] allArraysSum = myListOfIntArrays.Aggregate(
totals,
(arrCumul, arrItem) => arrCumul.Zip(arrItem, (a, b) => a + b))
.ToArray();