bash operations on arrays using bc

Question

from reading the man page of bc, it seems that bc can accept simple variables, but also arrays as input.

However, if I try to add two arrays, I only get a single element as an output

a=(1 2 3)
b=(10 11 12)
c=`echo "$a + $b" | bc`

Then c only contains 11. If there a way to get bc to operate on all elements in the arrays to produce (11 13 15) as an output? Or do I need to do a loop?

Your assignment will only ever make c a single string. Note that "$a" when a is an array refers implictly to "${a[0]}". — Charles Duffy
– Charles Duffy, Commented Feb 25, 2019 at 13:30
bc isn't part of bash, so it can't read internal bash datastructures -- all communication between bash and bc is via generation and parsing of strings. Thus, echo "$a + $b" is generating a single string; bc can't see the original variables and has no way of knowing what their values were. — Charles Duffy
– Charles Duffy, Commented Feb 25, 2019 at 13:31
...thus, bc has support for arrays in the same sense that Python or awk support arrays -- they do, but those arrays have nothing whatsoever to do with bash arrays. — Charles Duffy
– Charles Duffy, Commented Feb 25, 2019 at 13:32

Charles Duffy · Accepted Answer · 2019-02-25 18:42:50Z

5

bc can't natively access bash arrays, but you can generate from your two arrays a stream of addition operations, and read their results back into a third array (thus only needing to invoke bc once, rather than running a separate copy of bc per loop entry):

a=(1 2 3)
b=(10 11 12)

readarray -t c < <(for idx in "${!a[@]}"; do
  echo "${a[$idx]} + ${b[$idx]}"
done | bc)
declare -p c              # print output as an array definition
printf '%s\n' "${c[@]}"   # print output one entry per line

See this running at https://ideone.com/YuPhQP, properly emitting as output:

declare -a c=([0]="11" [1]="13" [2]="15")
11
13
15

edited Feb 25, 2019 at 18:42

answered Feb 25, 2019 at 13:35

Charles Duffy

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