dtrain[, .N, by = item_condition_id] %>%
    ggplot(aes(x = as.factor(item_condition_id), y = N/1000)) +
    geom_bar(stat = 'identity', fill = 'cyan2') +
    labs(x = 'Item condition', y = 'Number of items (000s)', title = 'Number of items by condition category')
请问代码第一行中[]内的代码和>是什么意思呢