shell script array won't populate from for loop

It happens because the value of string_1 is empty at the first iteration.

Example :

#!/bin/bash

firstline='[Event "Marchand Open"][Site "Rochester NY"][Date "2005.03.19"][Round "1"][White "Smith, Igor"][Black "Jones, Matt"][Result "1-0"][ECO "C01"][WhiteElo "2409"][BlackElo "1911"]'

unset cccr
declare -A cccr

(IFS='['; for word in $firstline; do

string_1=$( echo $word | cut -f1 -d'"' )
string_2=$( echo $word | cut -f2 -d'"' )

echo "$string_1 - $string_2"
#cccr[$string_1]=$string_2

done)

Output :

 - # Problem !
Event  - Marchand Open
Site  - Rochester NY
...

You have to modify your script to prevent the value of being empty.

A very simple workaround is to check the value of string_1 before using it.

Example :

# ...
string_1=$( echo $word | cut -f1 -d'"' )
string_2=$( echo $word | cut -f2 -d'"' )

if [ ! -z $string_1 ]; then # If $string_1 is not empty
    echo "$string_1 - $string_2"
    cccr[$string_1]=$string_2
fi
# ...

From the man page of [

-z STRING
              the length of STRING is zero

Output :

Event  - Marchand Open
Site  - Rochester NY
# ... No problem

EDIT

BTW, if look at the value of string_1, you will see that the value is Event' ' and not Event (there's a whitespace at the end of Event) So cccr[Event] does not exist, but cccr[Event ] exists. To fix that, you can delete the whitespaces in string_1 :

string_1=$(echo $word | cut -f1 -d'"' | tr -d ' ') # tr -d ' ' deletes all the whitespaces

EDIT 2

I forgot to tell you that it's normal if it does not work. Indeed, the loop is executed in a subshell environment. So the array is filled in the subshell, but not in the current shell. From the man page of bash :

(list) list  is  executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below).  Variable
              assignments and builtin commands that affect the shell's environment  do  not  remain  in  effect
              after the command completes.  The return status is the exit status of list.

So there are 2 solutions :

1. Don't run the loop in a subshell (remove the parentheses).

# ...
OLDIFS=$IFS
IFS='['
for word in $firstline; do
    string_1=$(echo $word | cut -f1 -d'"' | tr -d ' ')
    string_2=$(echo $word | cut -f2 -d'"')
    if [ ! -z $string_1 ]; then
        cccr[$string_1]=$string_2
    fi
done
IFS=$OLDIFS

echo "Event = ${cccr[Event]}"
echo "Site = ${cccr[Site]}"

Output :

Event = Marchand Open
Site = Rochester NY

2. Use your array in the subshell.

# ...
(IFS='['
for word in $firstline; do
    string_1=$(echo $word | cut -f1 -d'"' | tr -d ' ')
    string_2=$(echo $word | cut -f2 -d'"')
    if [ ! -z $string_1 ]; then # If $string_1 is not empty
    cccr[$string_1]=$string_2
    fi
done
echo "Event = ${cccr[Event]}"
echo "Site = ${cccr[Site]}"
)

Output :

Event = Marchand Open
Site = Rochester NY