associate multiple values for one key in array in bash

Question

I have a text file that looks like:

1 aaaa
2 bbbb
3 cccc
4 ffffdd
2 eeee
2 ffff
4 gggg

I would like to map these into some sort of associative ar

Answer 1

Here's a solution with Shell Parameter Expansion and Associative Arrays:

# store
declare -A array # this is the only update
while read key value; do
    array[$key]="${array[$key]}${array[$key]:+,}$value"
done < file
# print
for key in "${!array[@]}"; do echo "$key->${array[$key]}"; done

---

Explanation

array[$key]="${array[$key]}${array[$key]:+,}$value"

saves each $value in array[$key] separated by ,:

• ${array[$key]} save previous value(s) (if any).
• ${array[$key]:+,} adds a , if there's a previous value.
• $value adds the new read value.

---

for key in "${!array[@]}"; do echo "$key->${array[$key]}"; done

prints the values associated to each $key.

---

From man bash:

${parameter:+word}
If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys) assigned in name. If name is not an array, expands to 0 if name is set and null otherwise. When ‘@’ is used and the expansion appears within double quotes, each key expands to a separate word.

---

Example

$ cat file
1 aaaa
2 bbbb
3 cccc
4 ffffdd
2 eeee
2 ffff
4 gggg

$ ./script.sh 
1->aaaa
2->bbbb,eeee,ffff
3->cccc
4->ffffdd,gggg