I have a collection of integers that I want to order by descending value, with the exception of keeping a 0 value as first in the list.
For example:
0,1,2,3,4,5,6,7,8
Should result in:
0,8,7,6,5,4,3,2,1
Thanks!
var input = new [] {0,1,2,3,4,5,6,7,8};
Two sortings which work with both negative and positive numbers:
var result = input.OrderBy(i => i == 0? 0 : 1).ThenByDescending(i => i);
or this if all your numbers are non-negative:
var result = input.OrderByDescending(i => i == 0? int.MaxValue : i);
or some really weird solution if you have both negative and positive numbers but you don't want to sort twice (as I'm doing in first solution):
var result = input
.GroupBy(i => i == 0 ? 0 : 1)
.OrderBy(g => g.Key)
.Select(g => g.Key == 0 ? g : g.OrderByDescending(i => i)
.SelectMany(g => g);
Related
Given two numbers A and B, what is the minimum number of steps to transform number A to become number B.
A step can either be A *= 2, A++ or A /= 2 if and only if A is an even number.
What is the most efficient algorithm to achieve this?
Suppose A and B can be really large numbers.
Here's my take, done in C#.
var a = 2;
var b = 15;
var found = new HashSet<int>() { a };
var operations = new (string operation, Func<int, bool> condition, Func<int, int> projection)[]
{
("/2", x => x % 2 == 0, x => x / 2),
("*2", x => x <= int.MaxValue / 2, x => x *2),
("+1", x => true, x => x + 1),
};
IEnumerable<(int count, string operations, int value)> Project((int count, string operations, int value) current)
{
foreach (var operation in operations)
{
if (operation.condition(current.value))
{
var value = operation.projection(current.value);
if (!found.Contains(value))
{
found.Add(value);
yield return (current.count + 1, $"{current.operations}, {operation.operation}", value);
}
}
}
}
var candidates = new[] { (count: 0, operations: $"{a}", value: a) };
while (!found.Contains(b))
{
candidates =
candidates
.SelectMany(c => Project(c))
.ToArray();
}
var result = candidates.Where(x => x.value == b).First();
Console.WriteLine($"{result.count} operations: {result.operations} = {result.value}");
That outputs:
5 operations: 2, +1, *2, +1, *2, +1 = 15
Basically, this is starting with a at the zeroth step. It then takes this generation and produces all possible values from the operations to create the next generation. If it produces a value that it has already seen it discards the value as there is an equal or faster operation to produce the value. It keeps repeating until b is found.
I have a data set of values that I want to summarise in groups. For each group, I want to create an array big enough to contain the values of the largest group. When a group contains less than this maximum number, I want to insert a default value of zero for the empty key values.
Dataset
Col1 Col2 Value
--------------------
A X 10
A Z 15
B X 9
B Y 12
B Z 6
Desired result
X, [10, 9]
Y, [0, 12]
Z, [15, 6]
Note that value "A" in Col1 in the dataset has no value for "Y" in Col2. Value "A" is first group in the outer series, therefore it is the first element that is missing.
The following query creates the result dataset, but does not insert the default zero values for the Y group.
result = data.GroupBy(item => item.Col2)
.Select(group => new
{
name = group.Key,
data = group.Select(item => item.Value)
.ToArray()
})
Actual result
X, [10, 9]
Y, [12]
Z, [15, 6]
What do I need to do to insert a zero as the missing group value?
Here is how I understand it.
Let say we have this
class Data
{
public string Col1, Col2;
public decimal Value;
}
Data[] source =
{
new Data { Col1="A", Col2 = "X", Value = 10 },
new Data { Col1="A", Col2 = "Z", Value = 15 },
new Data { Col1="B", Col2 = "X", Value = 9 },
new Data { Col1="B", Col2 = "Y", Value = 12 },
new Data { Col1="B", Col2 = "Z", Value = 6 },
};
First we need to determine the "fixed" part
var columns = source.Select(e => e.Col1).Distinct().OrderBy(c => c).ToList();
Then we can process with the normal grouping, but inside the group we will left join the columns with group elements which will allow us to achieve the desired behavior
var result = source.GroupBy(e => e.Col2, (key, elements) => new
{
Key = key,
Elements = (from c in columns
join e in elements on c equals e.Col1 into g
from e in g.DefaultIfEmpty()
select e != null ? e.Value : 0).ToList()
})
.OrderBy(e => e.Key)
.ToList();
It won't be pretty, but you can do something like this:
var groups = data.GroupBy(d => d.Col2, d => d.Value)
.Select(g => new { g, count = g.Count() })
.ToList();
int maxG = groups.Max(p => p.count);
var paddedGroups = groups.Select(p => new {
name = p.g.Key,
data = p.g.Concat(Enumerable.Repeat(0, maxG - p.count)).ToArray() });
You can do it like this:-
int maxCount = 0;
var result = data.GroupBy(x => x.Col2)
.OrderByDescending(x => x.Count())
.Select(x =>
{
if (maxCount == 0)
maxCount = x.Count();
var Value = x.Select(z => z.Value);
return new
{
name = x.Key,
data = maxCount == x.Count() ? Value.ToArray() :
Value.Concat(new int[maxCount - Value.Count()]).ToArray()
};
});
Code Explanation:-
Since you need to append default zeros in case when you have less items in any group, I am storing the maxCount (which any group can produce in a variable maxCount) for this I am ordering the items in descending order. Next I am storing the maximum count which the item can producr in maxCount variable. While projecting I am simply checking if number of items in the group is not equal to maxCount then create an integer array of size (maxCount - x.Count) i.e. maximum count minus number of items in current group and appending it to the array.
Working Fiddle.
I have a collection
List<int> periods = new List<int>();
periods.Add(0);
periods.Add(30);
periods.Add(60);
periods.Add(90);
periods.Add(120);
periods.Add(180);
var overDueDays = 31;
I have a variable over due days. When the vale is between 0 to 29 then I want to return the index of 0. When between 30 - 59 I want to return index 1. The periods list is from db so its not hard coded and values can be different from what are here. What is the best way to to it using LINQ in one statement.
It's not really what Linq is designed for, but (assuming that the range is not fixed) you could do the following to get the index
List<int> periods = new List<int>();
periods.Add(0);
periods.Add(30);
periods.Add(60);
periods.Add(90);
periods.Add(120);
periods.Add(180);
var overDueDays = 31;
var result = periods.IndexOf(periods.First(n => overDueDays < n)) - 1;
You can use .TakeWhile():
int periodIndex = periods.TakeWhile(p => p <= overDueDays).Count() - 1;
how about this ?
var qPeriods = periods.Where(v => v <= overDueDays)
.Select((result, i) => new { index = i })
.Last();
Assuming that periods is sorted, you can use the following approach:
var result = periods.Skip(1)
.Select((o, i) => new { Index = i, Value = o })
.FirstOrDefault(o => overDueDays < o.Value);
if (result != null)
{
Console.WriteLine(result.Index);
}
else
{
Console.WriteLine("Matching range not found!");
}
The first value is skipped since we're interested in comparing with the upper value of the range. By skipping it, the indices fall into place without the need to subtract 1. FirstOrDefault is used in case overDueDays doesn't fall between any of the available ranges.
I have the following LINQ statement:
var result = CommisionDataContext.COMMISSIONS
.Where(c.PRODUCT == 'Computer')
.GroupBy(g => new
{
CostCenter = g.COST_CENTER,
Product = g.PRODUCT,
}
.Select(group => new
{
Revenue = group.Sum(p => p.REVENUE),
Volume = group.Sum(p => p.VOLUME),
Avg = group.Sum(p => p.REVENUE) / group.Sum(p => p.VOLUME),
});
How can I guard against the Divide by zero exception which could happen, and if it does I just want Avg to be equal to 0.
To avoid summing more than necessary, you should be able to do something like this (note that your code does not compile, however):
...
.Select(group => new
{
Revenue = group.Sum(p => p.REVENUE),
Volume = group.Sum(p => p.VOLUME)
})
.Select(item => new
{
item.Revenue,
item.Volume,
Avg = item.Volume == 0 ? 0 : item.Revenue / item.Volume
});
Be careful because C# Double can handle Infinity values, in fact if you try
var x = 5.0 / 0.0
you will find that x = Infinity (without Exception) and this is really different from 0! So, if you want your AVG = 0 be sure it's correct from a mathematical and representational point of view (for example I work with marketing formulas and charts and it's important to know if my AVG is a true 0 or an approximation to avoid wrong charts representation and misconception).
Anyway, this code avoids a Divide by zero Exception (you need to set AVG as Double) if you find useful the exact value of AVG operation taking care of the Infinity return value if the volumes sum is 0.
Avg = Convert.ToDouble(group.Sum(p => p.REVENUE)) / group.Sum(p => p.VOLUME))
Otherwise you can use the "check before" approach with a single evaluation.
var x = Convert.ToDouble(group.Sum(p => p.REVENUE)) / group.Sum(p => p.VOLUME));
Avg = Double.IsInfinity(x)? 0 : x;
Just replace
Avg = group.Sum(p => p.REVENUE) / group.Sum(p => p.VOLUME),
with
Avg = group.Sum(p => p.VOLUME) == 0
? 0
: group.Sum(p => p.REVENUE) / group.Sum(p => p.VOLUME),
I have a list of duplicate numbers:
Enumerable.Range(1,3).Select(o => Enumerable.Repeat(o, 3)).SelectMany(o => o)
// {1,1,1,2,2,2,3,3,3}
I group them and get quantity of occurance:
Enumerable.Range(1,3).Select(o => Enumerable.Repeat(o, 3)).SelectMany(o => o)
.GroupBy(o => o).Select(o => new { Qty = o.Count(), Num = o.Key })
Qty Num
3 1
3 2
3 3
What I really need is to limit the quantity per group to some number. If the limit is 2 the result for the above grouping would be:
Qty Num
2 1
1 1
2 2
1 2
2 3
1 3
So, if Qty = 10 and limit is 4, the result is 3 rows (4, 4, 2). The Qty of each number is not equal like in example. The specified Qty limit is the same for whole list (doesn't differ based on number).
Thanks
Some of the other answers are making the LINQ query far more complex than it needs to be. Using a foreach loop is certainly faster and more efficient, but the LINQ alternative is still fairly straightforward.
var input = Enumerable.Range(1, 3).SelectMany(x => Enumerable.Repeat(x, 10));
int limit = 4;
var query =
input.GroupBy(x => x)
.SelectMany(g => g.Select((x, i) => new { Val = x, Grp = i / limit }))
.GroupBy(x => x, x => x.Val)
.Select(g => new { Qty = g.Count(), Num = g.Key.Val });
There was a similar question that came up recently asking how to do this in SQL - there's no really elegant solution and unless this is Linq to SQL or Entity Framework (i.e. being translated into a SQL query), I'd really suggest that you not try to solve this problem with Linq and instead write an iterative solution; it's going to be a great deal more efficient and easier to maintain.
That said, if you absolutely must use a set-based ("Linq") method, this is one way you could do it:
var grouped =
from n in nums
group n by n into g
select new { Num = g.Key, Qty = g.Count() };
int maxPerGroup = 2;
var portioned =
from x in grouped
from i in Enumerable.Range(1, grouped.Max(g => g.Qty))
where (x.Qty % maxPerGroup) == (i % maxPerGroup)
let tempQty = (x.Qty / maxPerGroup) == (i / maxPerGroup) ?
(x.Qty % maxPerGroup) : maxPerGroup
select new
{
Num = x.Num,
Qty = (tempQty > 0) ? tempQty : maxPerGroup
};
Compare with the simpler and faster iterative version:
foreach (var g in grouped)
{
int remaining = g.Qty;
while (remaining > 0)
{
int allotted = Math.Min(remaining, maxPerGroup);
yield return new MyGroup(g.Num, allotted);
remaining -= allotted;
}
}
Aaronaught's excellent answer doesn't cover the possibility of getting the best of both worlds... using an extension method to provide an iterative solution.
Untested:
public static IEnumerable<IEnumerable<U>> SplitByMax<T, U>(
this IEnumerable<T> source,
int max,
Func<T, int> maxSelector,
Func<T, int, U> resultSelector
)
{
foreach(T x in source)
{
int number = maxSelector(x);
List<U> result = new List<U>();
do
{
int allotted = Math.Min(number, max);
result.Add(resultSelector(x, allotted));
number -= allotted
} while (number > 0 && max > 0);
yield return result;
}
}
Called by:
var query = grouped.SplitByMax(
10,
o => o.Qty,
(o, i) => new {Num = o.Num, Qty = i}
)
.SelectMany(split => split);