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up vote 2 down vote favorite

Bash doesn't natively support two-dimensional arrays, but I would like to simulate one. As a minimal working example, suppose that I have two arrays, a0 and a1:

a0=(1 2 3 4)
a1=(5 6 7 8)

I want to write a for loop that will print the third element of a0 and a1. Of course, I could do this manually with two explicit calls to echo:

echo ${a0[2]}
echo ${a1[2]}

But, I want to generalize this with a for loop. How can I do this?

I tried the following:

for i in ${a0[@]} ${a1[@]}
do
echo {$i}[2]
echo ${i[2]}
echo ${i}[2]
echo ${$i[2]}
echo ${${i}[2]}
done

But none of those attempts are successful; I get this output:

{1}[2]

1[2]
chgreen.sh: line 30: ${$i[2]}: bad substitution

Do you have any ideas?

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1  
What’s wrong with the accepted answer at the question you linked to? –  Scott Nov 6 '14 at 1:22

2 Answers 2

up vote 2 down vote accepted

As you’ve presumably learned by now from your research, bash doesn’t support multi-dimensional arrays per se, but it does support “associative” arrays.  These are basically indexed by a string, rather than a number, so you can have, for example,

grade[John]=100
grade[Paul]=100
grade[George]=90
grade[Ringo]=80

As demonstrated (but not explained very well) in the accepted answer of the question you linked to, indices of associative arrays can contain commas, and so a common trick is to concatenate your individual indices (0-1 × 0-3) into a string, separated by commas.  While this is more cumbersome than ordinary arrays, it can be effective:

$ declare -A a              <-- Create the associative array.
$ a[0,0]=1
$ a[0,1]=2
$ a[0,2]=3
$ a[0,3]=4
$ a[1,0]=5
$ a[1,1]=6
$ a[1,2]=7
$ a[1,3]=8
$ for i in 0 1
> do
>     echo ${a[$i,2]}
> done
3                           <-- And here’s your output.
7
share|improve this answer
up vote 2 down vote

You're iterating over the wrong thing. Your for saves each element of the array as $i, not the array's indices. What you want is something like

#!/usr/bin/env bash
a0=(1 2 3 4)
a1=(5 6 7 8)

for ((i=0;i<${#a0[@]};i++))
do
    echo ${a0[$i]} ${a1[$i]};
done
share|improve this answer
1  
You seem to be missing the point of the question. “I want to write a for loop that will print the third element of a0 and a1.” The OP wants a multi-dimensional array, and he wants to be able to iterate over any index (dimension). In particular, he wants to print ${a0[2]} and ${a1[2]} in a way that would scale readily to a situation where he has a0, a1, a2, a3, a4, … –  Scott Nov 6 '14 at 1:23
    
@Scott so you're thinking the OP wants to iterate over arrays? You are very likely right, that's not what I had understood at all. If so, your solution is just about perfect. –  terdon♦ Nov 6 '14 at 12:01

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