How to number rows within a group in dataframe

dplyr

dplyr n()

dplyr row_number()

Published

January 27, 2022

In this tutorial, how to add numbers to rows with in each group in a dataframe in R. The picture below illustrates an example of adding row numbers per group. Add Numbers to rows within groups in dataframe ### Packages And Data We will first create sample dataframe using tibble function and then use dplyr’s functions to add sequence of numbers to each group in a dataframe based on a grouping variable in two ways. First, we will use dplyr’s row_number() function in combination with group_by() to add numbers within each group. And then we will use dplyr’s n() function to do the same.

Let us first load tidyverse.

library(tidyverse)

We create a a simple dataframe using tibble with two groups.

df   
## 1 g2         4
## 2 g1         5
## 3 g1        14
## 4 g2        19
## 5 g2         2
## 6 g2         8

How to add numbers to rows within each group in a data frame

TO add sequence of number to rows within each group we will first use group_by() function in dplyr on the column that contains the groups. In this example, “grp” column contains grouping variable. After applying group_by() we can mutate to add a column and use row_number() function to add numbers to each row within a group.

df %>%
  group_by(grp) %>%
  mutate(id=row_number())

## # A tibble: 6 × 3
## # Groups:   grp [2]
##   grp   counts    id
##      
## 1 g2         4     1
## 2 g1         5     1
## 3 g1        14     2
## 4 g2        19     2
## 5 g2         2     3
## 6 g2         8     4

Note that the grouping variable does not have be ordered for this to work.

df %>%
  group_by(grp) %>%
  mutate(id=row_number()) %>%
  arrange(grp)

## # A tibble: 6 × 3
## # Groups:   grp [2]
##   grp   counts    id
##      
## 1 g1         5     1
## 2 g1        14     2
## 3 g2         4     1
## 4 g2        19     2
## 5 g2         2     3
## 6 g2         8     4

Adding numbers to rows within each group in a data frame: Second way

Here is another way to add row numbers with in each group using dplyr functions in R. In this example, we use n() instead of row_number(). The dplyr function n() gets the number of rows and here it gives us the number of rows in the current grouped dataframe.

df %>%
  group_by(grp) %>%
  mutate(id=1:n())

## # A tibble: 6 × 3
## # Groups:   grp [2]
##   grp   counts    id
##      
## 1 g2         4     1
## 2 g1         5     1
## 3 g1        14     2
## 4 g2        19     2
## 5 g2         2     3
## 6 g2         8     4

You may be interested in learning dplyr row_number(): Add unique row number to a dataframe

--- title: "How to number rows within a group in dataframe" date: 2022-01-27 categories: ['dplyr', 'dplyr n()', 'dplyr row_number()'] format: html: code-fold: false code-tools: true --- In this tutorial, how to add numbers to rows with in each group in a dataframe in R. The picture below illustrates an example of adding row numbers per group. ![How to add row number with each group of a dataframe in R](https://rstats101.com/wp-content/uploads/2022/01/numbering_rows_within_groups_in_dataframe.png) Add Numbers to rows within groups in dataframe ### Packages And Data We will first create sample dataframe using tibble function and then use dplyr's functions to add sequence of numbers to each group in a dataframe based on a grouping variable in two ways. First, we will use [dplyr's row_number()](https://rstats101.com/dplyr-row_number-add-unique-row-number-to-a-dataframe/) function in combination with group_by() to add numbers within each group. And then we will use dplyr's n() function to do the same. Let us first load tidyverse. ```r library(tidyverse) ``` We create a a simple dataframe using tibble with two groups. ```r df ## 1 g2 4 ## 2 g1 5 ## 3 g1 14 ## 4 g2 19 ## 5 g2 2 ## 6 g2 8 ``` ### How to add numbers to rows within each group in a data frame TO add sequence of number to rows within each group we will first use group_by() function in dplyr on the column that contains the groups. In this example, "grp" column contains grouping variable. After applying group_by() we can mutate to add a column and use row_number() function to add numbers to each row within a group. ```r df %>% group_by(grp) %>% mutate(id=row_number()) ``` ```r ## # A tibble: 6 × 3 ## # Groups: grp [2] ## grp counts id ## ## 1 g2 4 1 ## 2 g1 5 1 ## 3 g1 14 2 ## 4 g2 19 2 ## 5 g2 2 3 ## 6 g2 8 4 ``` Note that the grouping variable does not have be ordered for this to work. ```r df %>% group_by(grp) %>% mutate(id=row_number()) %>% arrange(grp) ``` ```r ## # A tibble: 6 × 3 ## # Groups: grp [2] ## grp counts id ## ## 1 g1 5 1 ## 2 g1 14 2 ## 3 g2 4 1 ## 4 g2 19 2 ## 5 g2 2 3 ## 6 g2 8 4 ``` ### Adding numbers to rows within each group in a data frame: Second way Here is another way to add row numbers with in each group using dplyr functions in R. In this example, we use n() instead of row_number(). The dplyr function n() gets the number of rows and here it gives us the number of rows in the current grouped dataframe. ```r df %>% group_by(grp) %>% mutate(id=1:n()) ``` ```r ## # A tibble: 6 × 3 ## # Groups: grp [2] ## grp counts id ## ## 1 g2 4 1 ## 2 g1 5 1 ## 3 g1 14 2 ## 4 g2 19 2 ## 5 g2 2 3 ## 6 g2 8 4 ``` You may be interested in learning [dplyr row_number(): Add unique row number to a dataframe](https://rstats101.com/dplyr-row_number-add-unique-row-number-to-a-dataframe/)