7
#!/bin/bash 
q=$(bc <<< "scale=2;$p*100")
head -n$q numbers.txt > secondcoordinate.txt

That's just part of the script, but I think it's enough to clarify my intentions. p is a variable with just two decimals, so q should be an integer... Nevertheless, bc shows, for example, 10.00 instead of 10.

How can I solve this?

  • 1
    delete scale=2; – Ipor Sircer Nov 3 '16 at 0:15
  • @Ipor Sircer still doesn't work – Diego Nov 3 '16 at 0:20
  • Did you try scale=0? – Jeff Schaller Nov 3 '16 at 0:22
  • @JeffSchaller Yes, I did, but still shows two decimals – Diego Nov 3 '16 at 0:26
7

You can't do this with the obvious scale=0 because of the way that the scale is determined.

The documentation indirectly explains that dividing by one is sufficient to reset the output to match the value of scale, which defaults to zero:

expr1 / expr2 The result of the expression is the quotient of the two expressions. The scale of the result is the value of the variable scale.

p=12.34; echo "($p*100)" | bc
1234.00

p=12.34; echo "($p*100)/1" | bc
1234

If your version of bc does not handle this, pipe it through sed instead:

p=12.34; echo "($p*100)" | bc | sed 's!\.0*$!!'
1234

This RE will only strip trailing zeros from an integer number. So 3.00 will reduce to 3, but 3.10 will not reduce to 3.1. If you really need the full ability to strip the trailing zeros from a decimal number, a PCRE is required:

p=12.34; echo "($p*100)" | bc | perl -pe '/\./ && s/0+$/$1/ && s/\.$//'

But if you're going to use perl then you might as well dispense with bc in the first place:

p=12.34; perl -e '$p = shift; print $p * 100, "\n"' "$p"
| improve this answer | |
  • 2
    Perhaps using printf would be cleaner than piping through sed? e.g. printf '%.0f\n' $(bc <<< "$p*100") – steeldriver Nov 3 '16 at 0:36
  • @steeldriver that requires printf to interpret the string as a number and then reformat it. I'd have preferred to use the PCRE (\.[0-9]?*)0*$ (which would strip trailing zeros from values such as 12.3400) but sed can't handle that. – roaima Nov 3 '16 at 0:38
4

you can use awk to calculate the values

bash-3.2$ p=0.01
bash-3.2$ q=$(awk -vp_val="$p" 'BEGIN{print p_val*100}')
bash-3.2$ echo $q
1


bash-3.2$ p=0.02
bash-3.2$ q=$(awk -vp_val="$p" 'BEGIN{print p_val*100}')
bash-3.2$ echo $q
2


bash-3.2$ p=0.022
bash-3.2$ q=$(awk -vp_val="$p" 'BEGIN{print p_val*100}')
bash-3.2$ echo $q
2.2
| improve this answer | |
4

TL;DR

You have lots of options. bc has known behavior where scale=0 doesn't always do what you expect, but there are a lot of workarounds. Here are just a few.

printf

Use printf to limit your output to integers.

$ printf "%g\n" $(echo '12.34 * 100' | bc)
1234

bc with division

If you want to stick with bc scaling, you need to specify both a scale of zero and divide by 1 to reset the scale. This is known behavior, but I really can't explain the why of it.

$ echo '12.34 * 100 / 1' | scale=0 bc
1234

sed

Just strip off the unwanted trailing characters.

$ echo '12.34 * 100' | bc | sed 's/\.00$//'
1234

bash

Use a brace expansion to return the value before the decimal.

$ p='12.34'; q=$(bc <<< "scale=2; $p*100"); echo ${q%%.00}
1234
| improve this answer | |
  • 1
    Just one niggle - ${q%%.00} would usually be referred to as parameter expansion rather than "brace expansion", I think – steeldriver Nov 3 '16 at 11:30
  • 1
    Does a construct like scale=0 bc really set the scale? I can't get any change in effect from variations on that theme here – roaima Nov 3 '16 at 13:40
  • Using scale=0 bc sets the envvar scale which bc doesn't seem to read. It has to be included within the text piped to stdin. – jamadagni Dec 20 '19 at 4:17
-1

Here is a bash function to remove trailing zeroes.

remove_trailing_zeroes()
{
    declare -n n="$1"
    # Prepend a 0 if number starts with a dot.
    if [[ $n =~ ^[.] ]]; then
        n="0$n"
    fi
    # Remove trailing zeroes
    while [[ $n =~ [.].*0$ ]]; do
        n="${n%0}"
    done
    # Remove trailing dot if any
    if [[ $n =~ [.]$ ]]; then
        n="${n%.}"
    fi
}

You can then use it like this for your case:

q=$(bc <<< "scale=2;$p*100")
remove_trailing_zeroes q
head -n$q numbers.txt > secondcoordinate.txt
| improve this answer | |
  • le sigh. This may work but it is a terrible approach to ops problem...and likely overkill for any problem. – jesse_b Aug 2 '19 at 14:56
  • @Jesse_b: I just copied a function I had created and used myself to solve a similar but more general problem. I don't see why you say it's a terrible or overkill approach. – Frederic Aug 2 '19 at 18:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.