6

I am having trouble formatting nested for loops for long strings of input:

I am currently using the below nested for loops to name output files from a separate program that I have omitted:

#!/bin/bash
for hemi in lh rh
do
    for measure in thickness area volume meancurv gauscurv foldind curvind
    do
        echo ${hemi}"--"${measure}
    done
done

I would like to be able to format the loops like below to facilitate running multiple different loops with much longer array lists that may need to be modified. Only needing to modify the list in one place will greatly reduce the chance for errors/omissions etc: 

#!/bin/bash

hemi=( lh rh )
measure=( thickness area volume meancurv gauscurv foldind curvind )

for((i=${hemi[${i}]};i<2;i++));do
    for((j=${measure[${j}]};j<7;j++));do
        echo ${hemi[${i}]}"--"${measure[${j}]}
    done
done

This is the output I am getting and I am not able to figure out what I am doing wrong:

lh--thickness
lh--area
lh--volume
lh--meancurv
lh--gauscurv
lh--foldind
lh--curvind
./for_loop_testing.sh: line 7: ((: j=: syntax error: operand expected (error token is "=")
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3 Answers 3

Reset to default
15

Let me offer a simplified version:

for i in "${hemi[@]}"; do
    for j in "${measure[@]}"; do
        echo "$i--$j"
    done
done

The point is that "${array[@]}" expands to all array elements, so here we just iterate over all elements, put them into variables i and j, and later print them.

As a side note: don't forget to put double quotes around variables and arrays, especially if any element can contain whitespaces.

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3
  • 1
    In the numeric loop, use (( i=0; i < ${#hemi[@]}; i++ )) -- better yet, remove the first part of your answer and keep the superior simple version Commented Aug 20, 2015 at 1:56
  • This works too. This should also work for the purpose that I want to use it for. Commented Aug 20, 2015 at 2:22
  • This is much more elegant solution. Commented Aug 20, 2015 at 3:01
7

You need to initialize i and j to 0 instead of the value you are assigning. so, the loop will look like:

for((i=0;i<${#hemi[@]};i++));do
        for((j=0;j<${#measure[@]};j++));do
            echo "${hemi[$i]}--${measure[$j]}"
        done
    done

${#array[@]} gives you the number of elements in the array. (Note that bash arrays can have holes (non-contiguous elements), so only use this if you know they don't.)

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5
  • 1
    Within the [ ] index delimiters, you don't need to specify the ${ } part of the index variable. Just as for the (( )) pairis, you can more simply use the variable's name (only), eg. ${hemi[i]}"--"${measure[j]} Commented Aug 20, 2015 at 2:46
  • I was trying to do the least modification to OP's code to make sense for him. But I do like your suggestion. Commented Aug 20, 2015 at 3:00
  • @Peter.O: it's probably less confusing for beginners to write "${measure[$i]}". An array index is an arithmetic evaluation context where you can use variables without $ (and even stuff like i++), but not everyone's going to remember that special case. In my suggested-edit (to quote the expansion of hemi and measure), I went with [$i]. Commented Aug 20, 2015 at 19:12
  • 1
    @PeterCordes: My comment was just that - a comment (on simplicity), and not a statement of one way being better than another. Even with experience, all those {}s can be a pain in the bracket, and I believe they are far more likely to be "confusing". Once someone knows about the alternatives ([${i}] vs [$i] vs [i]), then it is just a matter of choice. I presented [i], you presented [$i] ... They all work.... BTW [i] is not a special case.. Commented Aug 20, 2015 at 23:23
  • @Peter.O: yup, I was just rambling, not really disagreeing with anything you said. Absolutely agree about dropping the extra {}, though. Maybe special case is the wrong word, but it's not obvious that array[] is an arithmetic context where you don't need $, it's just something you have to remember if you want to use it. I tend to remember it, but I had someone comment on another post a while ago that they found it hard to remember that rule, so didn't both with it. Commented Aug 20, 2015 at 23:26
3

If you don't mind reforming your lists with commas, you could just do this:

#!/bin/bash
printf "%s\n" {lh,rh}--{thickness,area,volume,meancurv,gauscurv,foldind,curvind}

There are some ways to get this expansions to use variables with eval, not that easy, but doable. Ask if needed.

Edit 1:

Just in case this was the reason for a downvote.
I'll clarify that this proposal does manage spaces, asterisks and some other difficult names. Try:

printf "%s\n" {lh,rh}--{"hello world",foldind," * ",curvind}

Is a format strikingly similar to "Comma separated values".

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