Creating sub variables from a dynamic list of variables in bash? - bash

Question

Given a dynamic list of things, I want to add a counter variable for each thing in the list to be accessed from outside the loop like so - the count for $X would ideally be given by:
for THING in `cat list.txt`
#do stuff with $THING
done
echo $Xcount
Obviously "$THING"count=value doesn't work, nor did a couple iterations of $ and \$ give me what I want.
The command:
echo "$THING"count=value
is what I would like executed, but back ticking it alone yields this error:
echo "$THING"count=value
bash: Xcount=value: command not found
Quoting the value doesn't change results.
Is there a clean way to address this in this fashion?Note that each $THING will hold a string value, not a number and that the contents of the list of things will vary over time. If "W" appears I'll need a counter for it and if X vanishes from the list I'll still need a counter for it as well - so I don't think a pair of arrays (one for items, one for counts) is the way to go, even if it can be made to work by expanding and resorting it every time - I want O(1) access time, memory space is "free" in this example, IO and CPU are my optimization constraints.
Rici's suggestion is serviceable, (loop based example follows for those who are good enough with that solution):
unset COUNT
declare -A COUNT
for ITEM in X Y Z; do
THING=$ITEM;
((COUNT[$THING]+=1));
done
for i in "${!COUNT[#]}"
do
echo -n "key : $i" " ";
echo "value: ${COUNT[$i]}";
done
but my preference would be to actually be able to declare variables Xcount Ycount and Zcount if the list provided were X Y and Z. Is there a way to do that, i.e. to execute this from inside the loop:
echo "$THING"count=value
withOUT getting "bash: Xcount=value: command not found"
?

Answer 1

Here, the let command is useful to allow variable names to be dynamically created. To access the values, you might want indirect variables:
for thing in foo bar baz foo bar foo; do
let "${thing}count++"
done
for thing in foo bar baz; do
varname=${thing}count
echo "$varname = ${!varname}"
done
foocount = 3
barcount = 2
bazcount = 1
I would not actually recommend doing this: associative arrays are conceptually much easier to deal with.
Get out of the habit of using ALL_CAPS_VARNAMES: one day you'll use PATH and wonder you can't find any commands.

Answer 2

Make $COUNT an associative array and set
COUNT[$THING]=value
Silly example for the skeptical:
$ declare -A COUNT
$ THING=X
$ COUNT[$THING]=3
$ THING=Y
$ COUNT[$THING]=42
$ THING=X
$ ((COUNT[$THING]+=7))
$ echo ${COUNT[X]} ${COUNT[Y]}
10 42