bash: test if associative array entry exists, create if doesn't, increment value if does

Question

What would be the bash code after:

declare -A map

to do the following:

1) if a string does not yet exist in the map, create the entry for it and set its count (value) to 1

2) if the string already exists in the map, increment the count (value) by 1?

Am using post version 4.0 of Bash.

Based on @that other guy's answer, I tried:

#!/bin/bash
input=("a" "b" "a")
declare -a map

for i in ${input[@]}
do
  let 'map[$i]++'
  echo "map[$i]=${map[$i]}"
done

And when run it produces the following -- does NOT seem to work:

$ maptest
map[a]=1
map[b]=2
map[a]=3

declare -a map declares map to be a list (i.e. indexed by integers). It's not the same as declare -A map, which does create an associative array. — rici
– rici, Commented Aug 18, 2015 at 2:54
Thanks @rici. I was wrong on my version of Bash (from cygwin, it's 3.1.17, even though I just got cygwin installed.) — Ray
– Ray, Commented Aug 18, 2015 at 17:48

that other guy · Accepted Answer · 2015-08-18 18:59:08Z

2

The way to do this would be:

let '++map[$key]'

This relies on the fact that an unset value is considered 0.

edited Aug 18, 2015 at 18:59

answered Aug 18, 2015 at 0:40

that other guy

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4 Comments

Ray Over a year ago

Thanks, am trying what you said with an echo statement that is obviously wrong, using: for i in ${input[@]} do let 'map[$i]++' echo "map[$i]=$map[$i]" done

that other guy Over a year ago

@Ray In your question you said declare -A map while in your example you use declare -a map.

Ray Over a year ago

I see. I actually do have Bash 3.1.17. I falsely assumed that loading latest cygwin would give me a Bash shell that was more recent than 7+ years old. I might rev that but other part of team will be using the script. Is it possible that you edit your answer, just slightly change the text?

Ray Over a year ago

Sorry, that was confusing. I need you to edit the text just so I can award you point and mark it as an answer. Any slight text edit will do. Thanks.

Paul Tobias · Accepted Answer · 2016-07-06 17:02:46Z

0

The code sample in the (edited) question works correctly when used with declare -A instead of declare -a. So the accepted answer is correct.

However if used with set -u aka set -o nounset, it will fail with map: unbound variable

The solution is to use declare -iA and the += operator, like this:

#!/bin/bash
set -o nounset

input=("a" "b" "a")
declare -iA map

for i in ${input[@]}
do
  map[$i]+=1
  echo "map[$i]=${map[$i]}"
done

Which produces the following output:

map[a]=1
map[b]=1
map[a]=2

answered Jul 6, 2016 at 17:02

Paul Tobias

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