重击-在列中用以下值填充空单元格

Question

I have a long tab-delimited CSV file and I am trying to paste in a cell a value that comes later on the column. For instance, input.txt:

0 
1 
1.345 B
2
2.86 A
3
4

I would like an output such as:

0 B
1 B
1.345 B
2 A
2.86 A
3 B
4 B

I've been tinkering with code from other threads like this awk solution , but the problem is that the value I want is not before the empty cell, but after, kind of a .FillUp in Excel.

Additional information:

• input file may have different number of lines
• "A" and "B" in input file may be at different rows and not evenly separated
• second column may have only two values
• last cell in second column may not have value
• [EDIT] for the last two rows in input.txt, B is known to be in the second column, as all rows after 2.86 are not A.

Thanks in advance.

Answer 1

$ tac input.txt | awk -v V=B '{if ($2) V=$2; else $2=V; print}' | tac
0 B
1 B
1.345 B
2 A
2.86 A
3 B
4 B

tac ( cat backwards) prints a file in reverse. Reverse the file, fill in the missing values, and then reverse it again.

Answer 2

This allows you to process the file in a single pass as long as you know the first value to fill. It should be quite a bit faster than reversing the file twice.

awk 'BEGIN {fillvalue="B"} $2 {fillvalue=$2=="A"?"B":"A"} !$2 {$2=fillvalue} 1' input.txt

Note that this assumes knowledge about the nature of that second column being only 'A' or 'B' or blank.