Why can't I append to array? - arrays

Question

I'm not sure whats going on here
#!/bin/bash
STRING_PREFIX="foo"
STRING_IDX="1,2,3,4,5"
declare -a STRING_ARRAY
main() {
assemble_strings
for i in "${STRING_ARRAY[#]}"; do
echo "TEST: $i"
done
}
assemble_strings() {
IFS=,
while IFS= read idx; do
STRING_ARRAY+=("${STRING_PREFIX}${idx}")
done < <(echo $STRING_IDX)
}
main
I expect an array of 5 strings each prepended with 'foo'. Instead I get an array of 1 string
TEST: foo1 2 3 4 5
For bonus points, how can I avoid the loop entirely? I can't figure out how to create an array from an expression in bash.

Answer 1

First: Because you put IFS= at the front of your read, the prior IFS=, does nothing (insofar as that read is concerned).
Second: Because you aren't setting -d , in your read, it's using the default -- newline -- value as record terminator. (IFS determines the field separator, not the record terminator; with an empty IFS value, your records have only one field in them anyhow). Thus, when you call read, it reads the whole record -- up to the newline -- so your loop only runs once.
One approach, using read -a to read directly to an array (in this case, treating the entire input stream as a single record, with fields separated by commas):
string_idx=1,2,3,4,5
string_prefix=foo
# use read to directly populate the array
IFS=, read -r -d '' -a string_array <<<"$string_idx"
# go back through and tack on prefixes
for idx in "${!string_array[#]}"; do
string_array[$idx]="${string_prefix}${string_array[$idx]}"
done
# print values
printf ' entry: %s\n' "${string_array[#]}"
Another, making the smallest change to your existing code -- treating the input stream as a series of single-field comma-separated records:
string_idx=1,2,3,4,5
string_prefix=foo
string_array=( )
while IFS= read -r -d , idx; do
string_array+=( "${string_prefix}${idx}" )
done <<<"$string_idx,"