Print line numbers starting at zero using awk

Question

Can anyone tell me how to print line numbers including zero using awk?

Here is my input file stackfile2.txt

when I run the below awk command I get actual_output.txt

awk '{print NR,$0}' stackfile2.txt | tr " ", "," > actual_output.txt 

whereas my expected output is file.txt

How do I print the line numbers starting with zero (0)?

Answer 1

NR starts at 1, so use

awk '{print NR-1 "," $0}' 

Answer 2

Using awk.

i starts at 0, i++ will increment the value of i, but return the original value that i held before being incremented.

awk '{print i++ "," $0}' file