If the goal is to calculate the correct column width, you have to do one pass through the data before writing the first row.
If the file can be read multiple times (not a UNIX stream), you can just read the file twice.
If the file is a stream, instead of retaining the entire dataset in memory, you can write to a temporary file and re-parse it after calculating the widths.
The correct column width is calculated from the first n rows not the full file.
A stream does not work for tv because a stream does not know how many rows are in the file a priori. Displaying the dimensions of the file is a priority for `tv`. I am very happy with that trade-off. I would rather know the dimensions of a file than have a file stream of unknown dimensions.
If you did it the way he's talking about you would stream through the file to find how many rows and write the file as a temp file that you could re-parse for the actual data.
I'm not saying you should or shouldn't, but your use case doesn't bar you from using streams.
If the file can be read multiple times (not a UNIX stream), you can just read the file twice.
If the file is a stream, instead of retaining the entire dataset in memory, you can write to a temporary file and re-parse it after calculating the widths.