How to use awk '{print $1*Number}' from the second line or telling him to ignore NaN values? - linux

Question

I have a file called 'waterproofposters.jsonl' with this type of output:
Regular price
100
200
300
400
500
And I need to take out 2% of each value. I have used the following code:
awk '{print $1*0.98}' waterproofposters.jsonl
And then I have the following output:
0
98
196
294
392
490
And then I'm stuck because I need to have 'Regular price' in the first line instead '0'
I thought to replace '0' with 'Regular price using
find . -name "waterproof.jsonl" | xargs sed -i -e 's/0/Regular price/g'
But it will replace all the '0' by 'Regular price'

Answer 1

To print the first line as-is:
awk '{print (NR>1 ? $0*0.98 : $0)}'
To print lines that are not a number as-is:
awk '{print ($0+0 == $0 ? $0*0.98 : $0)}'
I'm using $0 instead of $1 in the multiplication because:
They're the same thing in your numerical input, and
I aesthetically prefer using the same value across the whole script rather than different values for the numeric vs non-numeric lines, and
When you use a specific field it causes awk to do field-splitting so it's a bit more efficient to not reference a field when the whole record will do.
Here's both of the above working with the posted sample input:
$ awk '{print (NR>1 ? $0*0.98 : $0)}' file
Regular price
98
196
294
392
490
$ awk '{print ($0+0 == $0 ? $0*0.98 : $0)}' file
Regular price
98
196
294
392
490
and here's the difference between the two given input that has a non-numeric value mid input file:
$ cat file
Regular price
100
200
foobar
400
500
$ awk '{print (NR>1 ? $0*0.98 : $0)}' file
Regular price
98
196
0
392
490
$ awk '{print ($0+0 == $0 ? $0*0.98 : $0)}' file
Regular price
98
196
foobar
392
490

Answer 2

You can certainly achieve what you need with a single awk call, but an answer to why your sed -i -e 's/0/Regular price/g' command did not work as expected is that you used 0 as the regex pattern. 0 matches any zero char inside the string.
You want to replace 0s that are the only char on a line.
Hence, you need to use ^ and $ anchors to match the start and end of the line respectively:
sed -i 's/^0$/Regular price/'
If you need to replace on the first line only add the 1 address before the substitution command:
sed -i '1 s/^0$/Regular price/'
Note you do not need g, since you only expect one replacement per line and g is only needed when performing multiple replacements on a line. By default, all lines will get processed.

Answer 3

How to use awk '{print $1Number}' from the second line or telling him to ignore NaN values?*
I would do it following way using GNU AWK, let file.txt content be
Regular price
100
200
300
400
500
then
awk 'NR==1{print}NR>=2{print $1*0.98}' file.txt
output
Regular price
98
196
294
392
490
Explanation: if it 1st line just print it, if it 2nd or later line print 0.98 of 1st column value
(tested in GNU Awk 5.0.1)