On the Foreach Loop Get Previous Value - asp.net

Question

How can I implement the below statement using a foreach loop
for(int i = 0; i<=dt.Rows.Count -1; i++)
{
var quote = Convert.ToDouble(dt.Rows[i]["Data"]) - Convert.ToDouble(dt.Rows[i-1]["Data"])
}
when I use a foreach loop:
foreach (DataRow row in dt.Rows)
{
// how can i write this statement like the loop above.
}

Answer 1

You can store the previous value on each iteration of the loop to use in the next iteration.
var prev = 0;
foreach (DataRow row in dt.Rows)
{
var quote = Convert.ToDouble(row["Data"]) - Convert.ToDouble(prev);
prev = row["Data"];
}
How to set quote with an initial value of 0:
double quote = 0.0;
var prev = 0;
foreach (DataRow row in dt.Rows)
{
quote = Convert.ToDouble(row["Data"]) - Convert.ToDouble(prev);
prev = row["Data"];
}
Another way to set quote to 0 on the first iteration of the loop:
var prev = 0;
var i = 0;
foreach (DataRow row in dt.Rows)
{
double quote;
if(i == 0)
{
quote = 0.0;
}
else
{
quote = Convert.ToDouble(row["Data"]) - Convert.ToDouble(prev);
}
prev = row["Data"];
i++;
}
NOTE: this last example, based on OP's comments to my answer, is really just a stab in the dark because the OP has given no clear definition of what is needed. These add-on examples are really beyond the scope of the original question.

Answer 2

If you use a foreach-loop you don't have an index but only the current item. So if you want to access the previous DataRow you should use a for-loop as in your first approach. But you could write it more readable and efficient:
for(int i = 1; i < dt.Rows.Count -1; i+=2)
{
double thisData = dt.Rows[i].Field<double>("Data");
double prevData = dt.Rows[i-1].Field<double>("Data");
double quote = thisData - prevData;
}