Using a pipe inside a for loop. How do I preserve the value of the variable inside the for loop

I've been stuck for quite some time with the following code. It works but the variable loses the value that was set during the iterations.

I have the following code

checkIfLetterInsideWord(){

for (( i=1; i <= $word_length; i++))
    do
       if [[ "$1:$i-1:1}" = ${letter} ]]; then

          exists=1

          answer_guess=$(echo $answer_guess | sed "s/-/${letter}/{i}" }

       fi

 done

}

I want to preserve the value of the variable answer_guess. I understand that it loses the value because of the usage of a pipeline inside the loop but I don't know to approach this problem.