Referencing Row Number in R

Question

How do I reference the row number of an observation  For example  if you have a data frame called  data  and want to create a variable data rownumber equal to each observation s row number  how would you do it without using a loop

User · Answer

Simply    data rownumber   1 nrow Data

User · Answer

This is probably the simplest way   data rownumber   1 dim data  1    It s probably worth noting that if you want to select a row by its row index  you can do this with simple bracket notation  data 3    vs   data data rownumber  3     So I m not really sure what this new column accomplishes

User · Answer

These are present by default as rownames when you create a data frame   R gt  df   data frame  a    rnorm 10    b    runif 10    c    letters 1 10   R gt  df             a          b c 1   0 3336944 0 39746731 a 2  -0 2334404 0 12242856 b 3   1 4886706 0 07984085 c 4  -1 4853724 0 83163342 d 5   0 7291344 0 10981827 e 6   0 1786753 0 47401690 f 7  -0 9173701 0 73992239 g 8   0 7805941 0 91925413 h 9   0 2469860 0 87979229 i 10  1 2810961 0 53289335 j   and you can access them via the rownames command    R gt  rownames df    1   1    2    3    4    5    6    7    8    9    10    if you need them as numbers  simply coerce to numeric by adding as numeric  as in as numeric rownames df      You don t need to add them  as if you know what you are looking for  say item df c     i   you can use the which command   R gt  which df c    i    1  9   or if you don t know the column  R gt  which df     i   arr ind T       row col  1     9   3   you may access the element using df 9   c    or df c 9     If you wanted to add them you could use df rownumber  lt - as numeric rownames df    though this may be less robust than df rownumber  lt - 1 nrow df  as there are cases when you might have assigned to rownames so they will no longer be the default index numbers  the which command will continue to return index numbers even if you do assign to rownames

User · Answer

Perhaps with dataframes one of the most easy and practical solution is   data   dplyr  mutate data  rownum row number

[r] Referencing Row Number in R

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