Bash: iteration through array names - arrays

Question

I am trying to write code to break up a large array into many different small arrays. Eventually the array I would be passed is one of unknown size, this is just my test subject. I have gotten this far:
#!/bin/bash
num=(10 3 12 3 4 4)
inArray=${#num[#]}
numArrays=$(($inArray/2))
remain=$(($inArray%2))
echo $numArrays
echo $remain
nun=0
if test $remain -gt $nun; then
numArrays=$(($numArrays+1))
fi
array=(1 2)
j=0
for ((i=0;i<$numArrays;i++, j=j+2)); do
array=("${num[#]:$j:2}")
echo "The array says: ${array[#]}"
echo "The size? ${#array[#]}"
done
What I am really having a problem with is : I would like to make the variable 'array' be able to change names slightly every time, so each array is kept and has a unique name after the loop. I have tried making the name array_$i but that returns:
[Stephanie#~]$ ./tmp.sh
3
0
./tmp.sh: line 16: syntax error near unexpected token `"${num[#]:$j:2}"'
./tmp.sh: line 16: ` array_$i=("${num[#]:$j:2}")'
[Stephanie#RDT00069 ~]$ ./tmp.sh
3
0
./tmp.sh: line 16: syntax error near unexpected token `$i'
./tmp.sh: line 16: ` array($i)=("${num[#]:$j:2}")'
Does anyone have any advice?
Thanks

Answer 1

I don't think you can really avoid eval here, but you might be able to do it safely if you're careful. Here's my approach:
for name in "${!array_*}"; do # Get all names starting with array_
i="${name#array_*}" # Get the part after array_
if [[ $i != *[^0-9]* ]]; then # Check that it's a number.
printf '%s is not a valid subarray name\n' "$name"
else
# Create a variable named "statement" that contains code you want to eval.
printf -v statement 'cur_array=( "${%s[#]}" )' "$name"
eval "$statement"
# Do interesting things with $cur_array
fi
done
Before this, when you're just creating the array, you know what $name should be, so just use the printf -v part.
To make it even safer, you could save all the allowed array names in another array and check that $name is a member.

Answer 2

With simple variables, you can use the declare keyword to make indirect assignments:
v=foo
declare $v=5
echo $foo # Prints 5
This doesn't extend to arrays in the obvious (to me, anyway) sense:
i=2
# This produces a syntax error
declare -a array_$i=("${num[#]:$j:2}")
Instead, you can declare an empty array
declare -a array_$i
or assign items one at a time:
declare -a array_$i[0]=item1 array_$i[1]=item2
Here's an example of using a for-loop to copy, say, the 3rd
and 4th letters of a big array into a smaller one. We use
i as the dynamic part of the name of the smaller array, and
j as the index into that array.
letters=(a b c d e f)
i=1
j=0
for letter in "${letters[#]:2:2}"; do
# E.g., i=0 and j=1 would result in
# declare -a array_0[1]=c
declare -a array_$i[$j]=$letter
let j+=1
done
done
echo ${array_1[#]}; # c d
${foo[#]:x:y} gives us elements x, x+1, ..., x+y-1 from foo, and
You can wrap the whole thing inside another for-loop to accomplish the goal of splitting letters into 3 smaller arrays:
# We'll create array_0, array_1, and array_2
for i in 0 1 2; do
# Just like our subset above, but start at position i*2 instead of
# a constant.
for letter in "${letters[#]:$((i*2)):2}"; do
declare -a array_$i[$j]=$letter
done
done
Once you manage to populate your three arrays, how do you access them without eval? Bash has syntax for indirect access:
v=foo
foo=5
echo ${!v} # echoes 5!
The exclamation point says to use the word that follows as a variable whose value should be used as the name of the parameter to expand. Knowing that, you might think you could do the following, but you'd be wrong.
i=1
v=array_$i # array_1
echo ${!v[0]} # array_1[0] is c, so prints c, right? Wrong.
In the above, bash tries to find a variable called v[0] and expand it to get the name of a parameter to expand. We actually have to treat our array plus its index as a single name:
i=1
v=array_$i[0]
echo ${!v} # This does print c

Answer 3

This should work, but this is not a good solution, another language may be better bash does not support multi dimensional arrays
eval array_$i='('"${num[#]:$j:2}"')'
And then, for example
eval 'echo "${array_'$i'[0]}"'