So I am trying to use bc to calculate some logarithms but I also need to use it to calculate the modulus for something. Whilst making my script, I launched bc to test it.
Without any flags, bc <<< "3%5" of course returns 3.
But with bc -l (loads math library so I can compute logarithms) any calculation of a%b returns 0 where a and b can be any number but 0.
What's happening?
That's because, from the manual:
expr % expr
The result of the expression is the "remainder" and it is com‐
puted in the following way. To compute a%b, first a/b is com‐
puted to scale digits. That result is used to compute a-(a/b)*b
to the scale of the maximum of scale+scale(b) and scale(a). If
scale is set to zero and both expressions are integers this
expression is the integer remainder function.
When you run bc with the -l flag, scale is set to 20. To fix this:
bc -l <<< "oldscale=scale; scale=0; 3%5; scale=oldscale; l(2)"
We first save scale in variable oldscale, then set scale to 0 to perform some arithmetic operations, and to compute a ln we set scale back to its old value. This will output:
3
.69314718055994530941
as wanted.