awk with empty field in columns - awk

Question

Here my file.dat
1 A 1 4
2 2 4
3 4 4
3 7 B
1 U 2
Running awk '{print $2}' file.dat gives:
A
2
4
7
U
But I would like to keep the empty field:
A
4
U
How to do it?
I must add that between :
column 1 and 2 there is 3 whitespaces field separator
column 2 and 3 and between column 3 and 4 one whitespace field separator
So in column 2 there are 2 fields missing (lines 2 and 4) and in column 4
there are also 2 fields missing (lines 3 and 5)

Answer 1

If this isn't all you need:
$ awk -F'[ ]' '{print $4}' file
A
4
U
then edit your question to provide a more truly representative example and clearer requirements.

Answer 2

If the input is fixed-width columns, you can use substr to extract the slice you want. I have assumed that you want a single character at index 5:
awk '{ print(substr($0,5,1)) }' file

Answer 3

Your awk code is missing field separators.
Your example file doesn't clearly show what that field separator is.
From observation your file appears to have 5 columns.
You need to determine what your field separator is first.
This example code expects \t which means <TAB> as the field separator.
awk -F'\t' '{print $3}' OFS='\t' file.dat
This outputs the 3rd column from the file. This is the 'read in' field separator -F'\t' and OFS='\t' is the 'read out'.
A
4
U

Answer 4

For GNU awk. It processes the file twice. On the first time it examines all records for which string indexes have only space and considers continuous space sequences as separator strings building up FIELDWIDTHS variable. On the second time it uses that for fixed width processing of the data.
a[i]:s get valus 0/1 and h (header) with this input will be 100010101 and that leads to FIELDWIDTHS="4 2 2 1":
1 A 1 4
2 2 4
3 4 4
3 7 B
1 U 2
| | | |
100010101 - while(match(h,/10*/))
\ /|/|/|
4 2 2 1
Script:
$ awk '
NR==FNR {
for(i=1;i<=length;i++) # all record chars
a[i]=((a[i]!~/^(0|)$/) || substr($0,i,1)!=" ") # keep track of all space places
if(--i>m)
m=i # max record length...
next
}
BEGINFILE {
if(NR!=0) { # only do this once
for(i=1;i<=m;i++) # ... used here
h=h a[i] # h=100010101
while(match(h,/10*/)) { # build FIELDWIDTHS
FIELDWIDTHS=FIELDWIDTHS " " RLENGTH # qnd
h=substr(h,RSTART+RLENGTH)
}
}
}
{
print $2 # and output
}' file file
And output:
A
4
U
You need to trim off the space from the fields, though.