What is foreach loop exactly doing here...help me to understand it precisely

Question

Can anybody help me understand what these lines of codes really means...i understand it but not exactly...i mean what foreach is exactly doing here???

if (dt.Rows.Count > 0)
    {
        //GridView1.Visible = true;
        Gridview1.DataSource = dt;
        Gridview1.DataBind();
        StringBuilder sb = new StringBuilder();

        foreach (DataColumn col in dt.Columns)
        {
            sb.Append(col.ColumnName + ",");
        }

        sb.Remove(sb.Length - 1, 1);
        sb.Append(Environment.NewLine);

        foreach (DataRow row in dt.Rows)
        {
            for (int i = 0; i < dt.Columns.Count; i++)
            {
                sb.Append(row[i].ToString() + ",");
            }

            sb.Append(Environment.NewLine);
        }

        Response.ClearHeaders();
        Response.AppendHeader("Content-Disposition", "attachment; filename=CDR OF " + TextBox1.Text + ".csv");
        Response.AppendHeader("Content-Length", sb.Length.ToString());
        Response.ContentType = "text/csv";
        Response.Write(sb.ToString());
        Response.Flush();
        Response.End();
    }
    else
    {
        //GridView1.Visible = false;
        Show("No CDR Found!");
    }

And can we use StringWriter or string in place of StringBuilder ???

first `foreach`: iterating through the columns and adding the column names to `sb`. second `foreach`: iterating through the rows — bansi, Apr 06 '16 at 06:07
Well, [foreach](https://msdn.microsoft.com/library/ttw7t8t6.aspx) itself should be a very simple to understand concept. You have some "things" and want to do something *for each* one of those "things". Now, exactly *what* you are doing inside can range from very simple to very complicated stuff. In both of your cases, you basically gather data and building a string, containing all the data. This is what `StringBuilder` was created for. You *could* use a `string`, but each time you add something to an existing `string`, you create a new one, which would be bad. — Corak, Apr 06 '16 at 06:12

score 1 · Answer 1 · answered Apr 06 '16 at 06:07

1

The first foreach is appending the column header names. The second is appending the cell values. Both in comma separated way. Ao you are converting a GridView to a csv.

Why don't use a simple converter like the CSV helper. Has everything you need. CSV Helper

answered Apr 06 '16 at 06:07

Franki1986

1,320
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@MridulKoul - you *could* use a `string`, but that would have a major impact on performance. See http://stackoverflow.com/questions/73883/string-vs-stringbuilder for example for further information. – Corak Apr 06 '16 at 06:17

score 1 · Answer 2 · answered Apr 06 '16 at 06:12

Looks like the code is trying to generate csv file with headers for the data table. The first for each loops through the columns in DataTable and generates a comma separated column name string (referred by StringBuffer).

 sb.Remove(sb.Length - 1, 1);
 sb.Append(Environment.NewLine);

The above code snippet removes the last comma from the comma separated column name string and then adds a new line

 foreach (DataRow row in dt.Rows)
    {
        for (int i = 0; i < dt.Columns.Count; i++)
        {
            sb.Append(row[i].ToString() + ",");
        }

        sb.Append(Environment.NewLine);
    }

the loop above iterates through each row of DataTable, and creates a comma separated string for each column value of the. After each row is processed, a new line is added so that the next row contents are added as comma separated string in new line.

score 1 · Accepted Answer · answered Apr 06 '16 at 06:13

1

The first loop lists all your column names with a comma:

colname1,colname2,colname3,

The Remove then removes the last comma and adds a newline

colname1,colname2,colname3\n

The second loop lists all your values of each column of each row

colname1,colname2,colname3\n row1col1value,row1col2value,row1col3value,\n -- note that the comma is NOT removed here row2col1value,row2col2value,row2col3value,\n row3col1value,row3col2value,row3col3value,\n row4col1val etc...

And yes, I think you can use a string instead of the string builder.

answered Apr 06 '16 at 06:13

Tjab

368
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why comma is removed in coloumn section and not removed in rows section...? – Mridul Koul Apr 06 '16 at 06:19
2

Because a line like `sb.Remove(sb.Length - 1, 1);` (i.e. "remove the last character" or more precisely "start at the position in `sb` that is one less than its length and remove one character") is missing in the second `foreach`. Now **why** it is missing, we cannot tell. It *might* be a bug. – Corak Apr 06 '16 at 06:20
if it remove comma separating columns..does it make any diffrence...i mean i tried removing sb.Remove(sb.Length - 1, 1); ....but the downloaded csv file doesnt seems to be changed a bit... – Mridul Koul Apr 06 '16 at 06:30
1

In CSV, `"data,data,data,"` indicates that there are *four* columns (the last one is empty), while `"data,data,data"` (note the missing last comma) indicates that there are *three* columns. Now if this is a big deal or not completely depends on how the CSV is read back in again. – Corak Apr 06 '16 at 06:38
Not sure how, but I accentually down voted this answer and cannot redo the action... – Kilise Apr 07 '16 at 17:33

score 1 · Answer 4 · answered Apr 06 '16 at 06:47

The first loop is taking each column name from Datatable and appending with a separator comma to a stringBuilder. Then it is trimming the last comma appended and adding a new line to it. The second loop is taking all the data values of DataRow(row by row) , then comma and appending to the same stringbuilder.

What is foreach loop exactly doing here...help me to understand it precisely

4 Answers4