Data Analysis

Aggregate

Grouped Totals

aggregate computes summaries by group. Formula syntax names the value column and grouping column.

Program

Play the script to sum amounts by region and read the east total.

aggregate_summary.R
Replay: real traced execution (multi-file project)
sales <- data.frame(region = c("east", "west", "east"), amount = c(10, 7, 5))
summary <- aggregate(amount ~ region, sales, sum)
east_total <- summary$amount[summary$region == "east"]
cat(east_total, "\n", sep = "")

  1. sales ← 3 rows x 2 cols

    1sales <- data.frame(region = c("east", "west", "east"), amount = c(10, 7, 5))2summary <- aggregate(amount ~ region, sales, sum)
    values this step3 rows x 2 colssales

  2. summary ← 2 rows x 2 cols

    1sales <- data.frame(region = c("east", "west", "east"), amount = c(10, 7, 5))2summary <- aggregate(amount ~ region, sales, sum)3east_total <- summary$amount[summary$region == "east"]
    values this step2 rows x 2 colssummary3 rows x 2 colssales

  3. east_total ← 15

    2summary <- aggregate(amount ~ region, sales, sum)3east_total <- summary$amount[summary$region == "east"]4cat(east_total, "\n", sep = "")
    values this step15east_totaleast, westsummary$region

  4. cat(east_total, " ", sep = "")

    3east_total <- summary$amount[summary$region == "east"]4cat(east_total, "\n", sep = "")
    output15
    values this step15east_total

Follow the Groups

  1. sales starts with three rows: east 10, west 7, and east 5.
  2. aggregate(amount ~ region, sales, sum) groups rows by region.
  3. The two east rows add to 15.
  4. The west row stays 7.
  5. east_total selects the east summary, so the script prints 15. | region | source amounts | summary amount | | --- | --- | --- | | east | 10, 5 | 15 | | west | 7 | 7 |
aggregate `aggregate(amount ~ region, sales, sum)` groups by region and sums amounts.
formula `amount ~ region` reads as summarize amount by region.
grouped summary Grouped summaries reduce many rows to one row per group.

Exercise: aggregate_summary.R

Reproduce the output 15, then trace which two east rows add together to make the east summary.