Subtracting a constant value from a column without round-up - perl

Question

I have a file like this.
Column1 Column2
-3500 value1
-3480 value2
-3460 value 3
9920 value 50
9940
9960
10000
10020
40000 Last value
Look at this example:
awk 'NR>1{$1=$1-4.91}1' file
Column1 Column2
-3504.91 value1
-3484.91 value2
-3464.91 value 3
9915.09 value 50
9935.09
9955.09
9995.09
10015.1
39995.1 Last value
I would like to have the correct value, not the rounded like this:
Column1 Column2
-3504.91 value1
-3484.91 value2
-3464.91 value 3
9915.09 value 50
9935.09
9955.09
9995.09
10015.09
39995.09 Last value
I am trying to subtract a constant value 4.91 from the first column. Everything works fine until 9980, but starting at 10000, awk is subtracting only 4.9 from the data and gives values 0.01 lesser than the original ones.I think it is rounding upto some decimal places, but I don't want the rounded values.Can anybody suggest me a workaround..
Anyother suggestions from shell script or Perl are welcome.

Answer 1

Can be done in perl:
perl -pe 's/^([-0-9]+)/$1 - 4.91/e' your_file
Details:
-p reads file line by line and prints it
-e runs perl code with line content in $_
s/.../.../e - replace regexp by expression
^([-0-9]+) - matched any digit and/or - sign. Also captures matched fragment.
$1 - 4.91 - does the work using value captured by regexp.

Answer 2

Here is how to do it with awk:
You simply set how many decimal you like using the printf function.
awk 'NR>1{printf "%.2f\n",$1-4.91}' file
-3504.91
-3484.91
-3464.91
9915.09
9935.09
9955.09
9995.09
10015.09
39995.09

Answer 3

$ awk 'BEGIN{CONVFMT="%.2f"} NR>1{$1=$1-4.91}1' file
Column1 Column2
-3504.91 value1
-3484.91 value2
-3464.91 value 3
9915.09 value 50
9935.09
9955.09
9995.09
10015.09
39995.09 Last value
or if you prefer:
$ awk 'NR>1{$1=sprintf("%.2f",$1-4.91)}1' file
Column1 Column2
-3504.91 value1
-3484.91 value2
-3464.91 value 3
9915.09 value 50
9935.09
9955.09
9995.09
10015.09
39995.09 Last value