How do I get bc(1) to print the leading zero?

Question

I do something like the following in a Makefile:

echo "0.1 + 0.1" | bc

(in the real file the numbers are dynamic, of course)

It prints .2 but I want it to print 0.2.

I would like to do this without resorting to sed but I can't seem to find how to get bc to print the zero. Or is bc just not able to do this?

Answer 1

After a quick look at the source (see bc_out_num(), line 1461), I don't see an obvious way to make the leading 0 get printed if the integer portion is 0. Unless I missed something, this behaviour is not dependent on a parameter which can be changed using command-line flag.

Short answer: no, I don't think there's a way to make bc print numbers the way you want.

I don't see anything wrong with using sed if you still want to use bc. The following doesn't look that ghastly, IMHO:

[me@home]$ echo "0.1 + 0.1" | bc | sed 's/^\./0./'
0.2

If you really want to avoid sed, both eljunior's and choroba's suggestions are pretty neat, but they require value-dependent tweaking to avoid trailing zeros. That may or may not be an issue for you.

Answer 2

Building on potongs answer,

For fractional results:

echo "x=0.1 + 0.1; if(x<1 && x > 0) print 0; x" | bc -l

Note that negative results will not be displayed correctly. Aquarius Power has a solution for that.

Answer 3

echo "$a / $b" | bc -l | sed -e 's/^-\./-0./' -e 's/^\./0./'

This should work for all cases where the results are:

• "-.123"
• ".123"
• "-1.23"
• "1.23"

Explanation:

1. For everything that only starts with -., replace -. with -0.

2. For everything that only starts with ., replace . with 0.

Answer 4

Probably, bc isn't really the best "bench calculator" for the modern age. Other languages will give you more control. Here are working examples that print values in the range (-1.0..+1.0) with a leading zero. These examples use bc, AWK, and Python 3.

#!/bin/bash

echo "using bc"
time for (( i=-2; i<=+2; i++ ))
   {
   echo $(bc<<<"scale=1; x=$i/2; if (x==0||x<=-1||x>=1) { print x } else { if (x<0) { print \"-0\";-x } else { print \"0\";x } } ")
   }
echo

echo "using awk"
time for (( i=-2; i<=+2; i++ ))
   {
   echo $(echo|awk "{printf \"%.1f\",$i/2}")
   }  
echo

echo "using Python"
time for (( i=-2; i<=+2; i++ ))
   {
   echo $(python3<<<"print($i/2)")
   }

Note that the Python version is about 10x slower, if that matters.

Answer 5

$ bc -l <<< 'x=-1/2; if (length (x) == scale (x) && x != 0) { if (x < 0) print "-",0,-x else print 0,x } else print x'

This one is pure bc. It detects the leading zero by comparing the result of the length with the scale of the expression. It works on both positive and negative number.