How to add an if statement before calculation in AWK - bash

Question

I have a series of files that I am looping through and calculating the mean on a column within each file after performing a serious of filters. Each filter is piped in to the next, BEFORE calculating the mean on the final output. All of this is done within a sub shell to assign it to a variable for later use.
for example:
variable=$(filter1 | filter 2 | filter 3 | calculate mean)
to calculate the mean I use the following code
... | awk 'BEGIN{s=0;}{s=s+$5;}END{print s/NR;}'
So, my problem is that depending on the file, the number of rows after the final filter is reduced to 0, i.e. the pipe passes nothing to AWK and I end up with awk: fatal: division by zero attempted printed to screen, and the variable then remains empty. I later print the variable to file and in this case I end up with BLANK in a text file. Instead what I am attempting to do is state that if NR==0 then assign 0 to the variable so that my final output in the text file is 0.
To do this I have tried to add an if statement at the start of my awk command
... | awk '{if (NR==0) print 0}BEGIN{s=0;}{s=s+$5;}END{print s/NR;}'
but this doesn't change the output/ error and I am left with BLANKs
I did move the begin statement but this caused other errors (syntax and output errors)
Expected results:
given that column from a file has 5 lines and looks thus, I would filter on apple and pipe into the calculation
apple 10
apple 10
apple 10
apple 10
apple 10
code:
vairable=$(awk -F"\t" '{OFS="\t"; if($1 ~ /apple/) print $0}' file.in | awk 'BEGIN{s=0;}{s=s+$5;}END{print s/NR;}')
then I would expect the variable to be set to 10 (10*5/5 = 10)
In the following scenario where I filter on banana
vairable=$(awk -F"\t" '{OFS="\t"; if($1 ~ /banana/) print $0}' file.in | awk 'BEGIN{s=0;}{s=s+$5;}END{print s/NR;}')
given that the pipe passes nothing to AWK I would want the variable to be 0
is it just easier to accept the blank space and change it later when printed to file - i.e. replace BLANK with 0?

Answer 1

The default value of a variable which you treat as a number in AWK is 0, so you don't need BEGIN {s=0}.
You should put the condition in the END block. NR is not the number of all rows, but the index of the current row. So it will only give the number of rows there were at the end.
awk '{s += $5} END { if (NR == 0) { print 0 } else { print s/NR } }'
Or, using a ternary:
awk '{s += $5} END { print (NR == 0) ? 0 : s/NR }'
Also, a side note about your BEGIN{OFS='\t'} ($1 ~ /banana/) { print $0 } examples: most of that code is unnecessary. You can just pass the condition:
awk -F'\t' '$1 ~ /banana/'`
When an awk program is only a condition, it uses that as a condition for whether or not to print a line. So you can use conditions as a quick way to filter through the text.

Answer 2

The correct way to write:
awk -F"\t" '{OFS="\t"; if($1 ~ /banana/) print $0}' file.in | awk 'BEGIN{s=0;}{s=s+$5;}END{print s/NR;}'
is (assuming a regexp comparison for $1 really is appropriate, which it probably isn't):
awk 'BEGIN{FS=OFS="\t"} $1 ~ /banana/{ s+=$5; c++ } END{print (c ? s/c : 0)}' file.in
Is that what you're looking for?
Or are you trying to get the mean per column 1 like this:
awk 'BEGIN{FS=OFS="\t"} { s[$1]+=$5; c[$1]++ } END{ for (k in s) print k, s[k]/c[k] }' file.in
or something else?