search for a string , and add if it matches

Question

I have a file that has 2 columns as given below....

101   6
102   23
103   45
109   36
101   42
108   21
102   24
109   67

and so on......

I want to write a script that adds the values from 2nd column if their corresponding first column matches

for example add all 2nd column values if it's 1st column is 101
add all 2nd column values if it's 1st colummn is 102
add all 2nd column values if it's 1st colummn is 103 and so on ...

i wrote my script like this , but i'm not getting the correct result

awk '{print $1}' data.txt > col1.txt
while read line
do
    awk ' if [$1 == $line] sum+=$2; END {print "Sum for time stamp", $line"=", sum}; sum=0' data.txt
    done < col1.txt

Answer 1

awk '{array[$1]+=$2} END { for (i in array) {print "Sum for time stamp",i,"=", array[i]}}' data.txt

Answer 2

Pure Bash :

declare -a sum
while read -a line ; do
  (( sum[${line[0]}] += line[1] ))
done < "$infile"

for index in ${!sum[@]}; do
  echo -e "$index ${sum[$index]}"
done

The output:

101 48
102 47
103 45
108 21
109 103