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Popularfan
Asked: 2020-06-21 03:05:14 +0800 CST 2020-06-21 03:05:14 +0800 CST

Given a value I need to know up to which number it corresponds from a list of numbers and its index

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I have a list of numbers ordered ascending. Given a value I need to know up to what number of the array and its index corresponds.

List<string> myLista = new List<string>(new[] { "2", "9", "17", "25", "35", "42", "70" });

Being the initial index = 0. For a value = 5, the index would be 1 and the number 9 For a value = 17, the index would be 2 and the number 17 For a value = 0, the index would be 0 and the number 2

This is a possible approximation but I am only interested in knowing the index and value of the last element of the list:

myLista.Where(x => Int32.Parse(x) <= Int32.Parse("31")).ToList()
c#

2 Answers

  1. If I understand you correctly, you need the index of the last number in the list that is less than or equal to the value you are checking. This method will give you:

    int IndiceDeUltimolNumeroMenorQue(List<string> valores, int valorAComprobar)
{
    for(int i=0; i<valores.Count; i++)
    {
        if(valorAComprobar <= int.Parse(valores[i]))
            return i;
    }

    return valores.Count - 1;
}

    Once you have the index, you can easily retrieve the value with valores[indice].

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  2. Best Answer

    I see that in Konamiman's answer you ask for a solution in Linq. The only thing I can think of is the following:

    We can use the overload of Select<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,TResult>)which incorporates the index of the elements, to create a result using a new anonymous class that returns both the value and the index within the list.

    This is an example that I think does what you ask for:

    var listaConIndices=myLista.Select((v, i) => new { Valor = v, Indice = i }).Where(x => Int32.Parse(x.Valor) >= Int32.Parse("0")).FirstOrDefault();

    As you can see, we first use Selectit to create a class that has the properties Valore Indice. Subsequently, we search for the elements that are greater than or equal to the searched element, and we keep the first one.

    Keep in mind that if the value passed to it is greater than the largest value in the list, the result in listaConIndiceswould be null, so we should check it later.

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