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I want to calculate the difference between two negative floating numbers and subsequently check if the result is below a certain threshold. The numbers are stored in the file.dat, since this calculation will be done many times the calculation has to rely on getting the values directly from a file without manual formatting. The file.dat looks for example like this.

-.63354975E+03 -.63354982E+03

Now since bash can't handle floats I've tried bc in the following way:

paste -sd- file.dat | bc >> file.dat

The paste command turns the input into

-.63354975E+03--.63354982E+03

bc should subract them, however I get the error message (standard_in) 1: syntax error. I could use sed to change one number to positive and then add them, but is there a way to do it directly with bc?


A different problem occurs when I use paste -sd+ file.dat | bc >> file.dat. This should add the numbers, the result is however 4.932900412, way off the expected 1267.09957. When I type in the calculation with the numbers formatted as -633.54975+-633.54982 I get the expected result. So it seems bc does not process numbers with exponents properly. Is there a way to resolve this?

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    The problem is the lack of whitespace (to delimit the numbers into separate tokens) I think - you could try replacing paste -sd- by pr -T -S' - ' -2 (pr allows string delimiters, whereas AFAIK paste only allows a single character) – steeldriver Jan 16 at 13:39
  • Alright, this does the trick, but the second problem kicks in. The result from the example is 6.000000070 instead of 0.00007. When I change the number format to -633.54... the result is correct. Do you know how to resolve this? Then the answer would be complete. – And Jan 16 at 13:45
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    for number format with expopnent see stackoverflow.com/questions/12882611/… There is an additional problem when you use the same file file.dat both as input argument and for output redirection. – Bodo Jan 16 at 13:46
  • @steeldriver Alright in this case please formulate an answer so I can accept it. – And Jan 16 at 13:51
  • If you're going to use sed to modify the exponent, then you may as well use it for inserting the operator as well e.g. sed -e '$!N;s/\n/ - /' -e '...' – steeldriver Jan 16 at 16:48
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awk solution:

awk 'NR==1{a=$0};NR==2{b=$0};END{print (a)-(b)}' file.dat

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