how do i loop through an array of numbers and update the array in bash using the array values

In the loop iteration you are using array values instead of array indices, which is the core of the problem. So the TL;DR is: "${!array[@]}".

array=(2 0 1)
tag=(H H H)
echo "${array[@]@A}; ${tag[@]@A};"

for index in "${!array[@]}"; do
  ((++array[index]))
  tag[index]=C
  echo "${array[@]@A}; ${tag[@]@A};"
done

Output:

declare -a array=([0]="2" [1]="0" [2]="1"); declare -a tag=([0]="H" [1]="H" [2]="H");
declare -a array=([0]="3" [1]="0" [2]="1"); declare -a tag=([0]="C" [1]="H" [2]="H");
declare -a array=([0]="3" [1]="1" [2]="1"); declare -a tag=([0]="C" [1]="C" [2]="H");
declare -a array=([0]="3" [1]="1" [2]="2"); declare -a tag=([0]="C" [1]="C" [2]="C");

One problem is that the values you are retrieving with the for loop are integers that are being used as indexes to update the values of the array. A second problem is that your conditional statement is actually an assignment, so its exit code is always 0 (true), so $count, though incrementing, affects nothing.

First time through, $i==2, the third element of array is incremented (array[2]==2), the third element of the tag array is set to changed

The second time through, $i==0, the first element of array is incremented (array[0]==3), the first element of tag array is set to changed.

The third time through, $i==1 (see comment below), the second element of array is incremented (array[1]==1), and the second element of the tag array is set to changed.

Promised comment: In the third iteration, other languages would have $i==2 because array[2] had been incremented in the first loop. Bash is apparently iterating over the original values of the array, despite subsequent changes.

I think what you want to do is:

declare -a array=(2 0 1)
declare -a tag=("here" "here" "here")
declare -i count=0
declare -i i

echo "$count:  ${array[*]}"
echo "   ${tag[*]}"

for (( i=0; i<${#array[*]}; ++i )); do
    (( array[i]++ ))   # no need for '$' within (( ))
    tag[$i]="changed"
    (( count++ ))
    echo "$count:  ${array[*]}"
    echo "   ${tag[*]}"
done

I didn't include your conditional because I can't figure out what you're trying to do with it.

I added the echo statements to create output similar to the output you claimed in your example.

FYI: Bash indexing starts from 0 (so to change the 2-nd element you should consider index No. 1)

Note: Here the 0 in array is somehow equal to 3 - last element! (so, using 3 would be more user-friendly).

Assuming the $array values define the numbering element in $tag to be changed (ex. 2 for 2nd, 3 for 3rd, and so on) accordingly, the following script will do the job.

script.sh

#!/bin/bash
#array=(2 0 1) 

# Advantage of the logic of this script is that you may have more elements
# in $tag array but are able to change only a few of them that is mentioned in $array.
array=(2 3 1) # Change 2nd, 3rd, 1st elements accordingly.
tag=(H H H)

for i in `seq 1 ${#array[@]}` ;do
  (( i-- ))
  ii="${array[i]}"
  (( ii-- ))
  tag[$ii]="C"
  echo -e "$i:\t${tag[@]}"
done

Output of the script.sh:

0:  H C H
1:  H C C
2:  C C C

Update:

Just seen your comment, will try to update my script.

I want it to start from the array value 0, which is is the middle. So it should look like this, H C H > H C C > C C C

Script Updated, please check again!

I guess that you need two nesting loops in this case.

• The outer loop is to scan the array once using count.
• The inner loop is to identify the element that have value equal to count (0, then 1, then 2).
• The tag array is to avoid increasing the same element twice.

OK, putting all that in code:

#!/bin/bash
array=(2 0 1)
tag=(H H H)
n=${#array[*]}                             ## array size
echo "${array[@]@A}; ${tag[@]@A};"         ## for testing
for (( count=0; count<n; ++count )); do
  for (( i=0; i<n; ++i )); do
    if [[ ${array[$i]} -eq $count && ${tag[$i]} = "H" ]]; then
      (( array[i]++ ))
      tag[$i]="C"                          ## C=changed
      break                                ## break inner loop
    fi
  done
  echo "${array[@]@A}; ${tag[@]@A};"       ## for testing
done

Results:

declare -a array=([0]="2" [1]="0" [2]="1"); declare -a tag=([0]="H" [1]="H" [2]="H");
declare -a array=([0]="2" [1]="1" [2]="1"); declare -a tag=([0]="H" [1]="C" [2]="H");
declare -a array=([0]="2" [1]="1" [2]="2"); declare -a tag=([0]="H" [1]="C" [2]="C");
declare -a array=([0]="3" [1]="1" [2]="2"); declare -a tag=([0]="C" [1]="C" [2]="C");

(sorry for my broken English)