How to use associative array to calculate summary in Zsh

prod-summary.zsh

#!/bin/zsh
# Rewrite of http://www.atmarkit.co.jp/ait/articles/1208/17/news110_2.html

# unset summary;
typeset -A summary; 
while read prod amt; do
  ((summary[$prod]+=$amt))
done

for prod in ${(ko)summary}; do
  echo $prod $(date +%Y-%M-%d) $summary[$prod]
done