Awk – High Precision Arithmetic in Awk

arithmeticawkfloating point

I am looking for a way to tell awk to do high-precision arithmetic in a substitution operation. This involves, reading a field from a file and substituting it with a 1% increment on that value. However, I am losing precision there. Here is a simplified reproduction of the problem:

 $ echo 0.4970436865354813 | awk '{gsub($1, $1*1.1)}; {print}'
   0.546748

Here, I have a 16 digit after decimal precision but awk gives only six. Using printf, I am getting the same result:

$ echo 0.4970436865354813 | awk '{gsub($1, $1*1.1)}; {printf("%.16G\n", $1)}'
0.546748

Any suggestions on to how to get the desired precision?

Best Answer

$ echo 0.4970436865354813 | awk -v CONVFMT=%.17g '{gsub($1, $1*1.1)}; {print}'
0.54674805518902947

Or rather here:

$ echo 0.4970436865354813 | awk '{printf "%.17g\n", $1*1.1}'
0.54674805518902947

is probably the best you can achieve. Use bc instead for arbitrary precision.

$ echo '0.4970436865354813 * 1.1' | bc -l
.54674805518902943

Related Solutions

Bash – Arithmetic operations with expr and variables

bash cannot do floating point operations, only integers. I suggest to use bc -l instead:

$ mPercent='2.72'
$ fPercent='3.14'
$ bc -l <<< $(echo "$mPercent / 100 "*" .482 + $fPercent / 100 "*" .518")
.02937560000000000000

Arithmetic operations between files

A paste and bc combination is a good choice for simple arithmetic:

paste -d/ file1 file2 | bc -l

Output:

.2000000000
.0300000000
.0040000000

A more advanced example

With some trickery you can get a bit more complicated. Say file3 contains:

6
7
8

You can do (file1 + file3) / file2 like this:

paste -d'(+)/' /dev/null file1 file3 /dev/null file2

Output:

(2+6)/10
(3+7)/100
(4+8)/1000

This works because paste cycles through its delimiter list for each line.

React to divide-by-zero

Illigal operations sent to bc result in a warning being sent to standard error. You could redirect these to a different file and decide program flow based on its content, e.g.:

paste -d/ file1 file2 | bc -l > resultfile 2> errorfile
if grep -q 'Divide by zero' errorfile; then
  echo "Error in calculation"
else
  echo "All is well"
fi

Or if there was any error:

paste -d/ file1 file2 | bc -l > resultfile 2> errorfile
if ! file errorfile | grep -q empty; then
  echo "Error in calculation"
else
  echo "All is well"
fi