Append CSV Column

Manually editing CSV files in a text editor to insert a column is error-prone. A misplaced comma or an unescaped quote character corrupts the entire dataset downstream. This tool parses your CSV using an RFC 4180-compliant state machine, correctly handling quoted fields containing commas, newlines, and escaped double-quotes (""). It then appends your new column data to every row, preserving structural integrity. The parser auto-detects the delimiter by frequency analysis across the first 10 lines, supporting comma, semicolon, tab, and pipe separators.

Three fill modes are available: a single broadcast value applied to all rows, line-by-line values mapped 1:1 to existing rows, or an auto-increment sequence starting from any integer. Row count mismatches between your file and line-by-line input are handled by padding empty cells or truncating excess values. The tool processes files entirely in-browser. No data leaves your machine. Note: for files exceeding 50 MB, browser memory limits may apply depending on device RAM.

The delimiter auto-detection algorithm counts occurrences of each candidate delimiter across the first N lines (default N = 10). The delimiter producing the most consistent column count wins:

Where d is a candidate delimiter, counts_d is the array of per-line occurrence counts, and σ is the standard deviation. Lower variance means more consistent column structure. The delimiter with the highest score is selected.

For the auto-increment fill mode, the value for row i is computed as:

Where start is the user-defined initial integer and step is the increment (default 1). Row indexing begins at 1 for data rows (header row receives the column name, not a number).

CSV field serialization follows RFC 4180: if a field contains the delimiter, a double-quote, or a newline, the entire field is wrapped in double-quotes, and any internal double-quote is escaped by doubling it (" → "").