Get previous element in foreach

Question

It is possible to take an element from the previous to current position using the repeat loop foreach?

For example, whenever I need to take an element previous to the current position of the loop, I use the loop for, as follows:

List<int> ListaDados = new List<int>();

ListaDados.Add(1); ListaDados.Add(2); ListaDados.Add(3); ListaDados.Add(5);

for(int i=1; i < ListaDados.Count; i++) {      
    var elementoAtual = ListaDados[i];
    var elementoAnterior = ListaDados[i-1];
}

Answer 1

Yes, it is possible:

using static System.Console;
using System.Collections.Generic;

public class Program {
    public static void Main() {
        var lista = new List<int>() { 1, 2, 3, 4 };
        var anterior = 0;
        foreach (var item in lista) {
            WriteLine($"Soma o atual e anterior { anterior + item }");
            anterior = item;
        }
    }
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

The exact way to implement this depends on your need. In some cases it may be interesting to put a comparison to avoid the first execution, since C# does not treat this execution differently and does not intend to have this in the language.

In structures you create or can extend it is possible to create a specific enumerator that gives you access to the previous element, but that is another matter.

I don’t like the other answer, it is absurdly slower, with possibly quadratic complexity (in List certainly, and it’s double). And I think it’s even wrong because if you have duplicate items will give error.

Answer 2

As Gabriel commented on, "by definition the foreach will invoke the Ienumerable.Getenumerator to make its increment, it does not use the index as the for".

But there is an alternative (which I believe is not very viable) if you really want to use the foreach:

List<int> ListaDados = new List<int>();

ListaDados.Add(1); ListaDados.Add(2); ListaDados.Add(3); ListaDados.Add(5);

freach (int elementoAtual in ListaDados){

    if (ListaDados.IndexOf(elementoAtual) == 0)
        // não existe elemento anterior
    else
    {
        var elementoAnterior = ListaDados.ElementAt(ListaDados.IndexOf(elementoAtual));

        //todo: seu código
    }
}