Indexing

General rules

The following rules explain target functionality of how getindex, setindex!, view, and broadcasting are intended to work with DataFrame, SubDataFrame and DataFrameRow objects.

The rules for a valid type of index into a column are the following:

• a value, later denoted as col:
  • a Symbol;
  • an Integer that is not Bool;
• a vector, later denoted as cols:
  • a vector of Symbol (does not have to be a subtype of AbstractVector{Symbol});
  • a vector of Integer other than Bool (does not have to be a subtype of AbstractVector{<:Integer});
  • a vector of Bool that has to be a subtype of AbstractVector{Bool};
  • a regular expression, which gets expanded to a vector of matching column names;
  • a Not expression (see InvertedIndices.jl);
  • an All or Between expression (see DataAPI.jl);
  • a colon literal :.

The rules for a valid type of index into a row are the following:

• a value, later denoted as row:
  • an Integer that is not Bool;
• a vector, later denoted as rows:
  • a vector of Integer other than Bool (does not have to be a subtype of AbstractVector{<:Integer});
  • a vector of Bool that has to be a subtype of AbstractVector{Bool};
  • a Not expression;
  • a colon literal :;
• an exclamation mark !.

Additionally it is allowed to index into an AbstractDataFrame using a two-dimensional CartesianIndex.

In the descriptions below df represents a DataFrame, sdf is a SubDataFrame and dfr is a DataFrameRow.

: always expands to axes(df, 1) or axes(sdf, 1).

df.col works like df[!, col] and sdf.col works like sdf[!, col] in all cases except that df.col .= v and sdf.col .= v perform in-place broadcasting if col is present in df/sdf and is a valid identifier.

getindex and view

The following list specifies the behavior of getindex and view operations depending on argument types.

In particular a description explicitly mentions that the data is copied or reused without copying.

For performance reasons, accessing, via getindex or view, a single row and multiple cols of a DataFrame, a SubDataFrame or a DataFrameRow always returns a DataFrameRow (which is a view type).

getindex on DataFrame:

• df[row, col] -> the value contained in row row of column col, the same as df[!, col][row];
• df[CartesianIndex(row, col)] -> the same as df[row,col];
• df[row, cols] -> a DataFrameRow with parent df;
• df[rows, col] -> a copy of the vector df[!, col] with only the entries corresponding to rows selected, the same as df[!, col][rows];
• df[rows, cols] -> a DataFrame containing copies of columns cols with only the entries corresponding to rows selected;
• df[!, col] -> the vector contained in column col returned without copying; the same as df.col if col is a valid identifier.
• df[!, cols] -> create a new DataFrame with columns cols without copying of columns; the same as select(df, cols, copycols=false).

view on DataFrame:

• @view df[row, col] -> a 0-dimensional view into df[!, col] in row row, the same as view(df[!, col], row);
• @view df[CartesianIndex(row, col)] -> the same as @view df[row, col];
• @view df[row, cols] -> the same as df[row, cols];
• @view df[rows, col] -> a view into df[!, col] with rows selected, the same as view(df[!, col], rows);
• @view df[rows, cols] -> a SubDataFrame with rows selected with parent df;
• @view df[!, col] -> a view into df[!, col] with all rows.
• @view df[!, cols] -> the same as @view df[:, cols].

getindex on SubDataFrame:

• sdf[row, col] -> a value contained in row row of column col;
• sdf[CartesianIndex(row, col)] -> the same as sdf[row,col];
• sdf[row, cols] -> a DataFrameRow with parent parent(sdf);
• sdf[rows, col] -> a copy of sdf[!, col] with only rows rows selected, the same as sdf[!, col][rows];
• sdf[rows, cols] -> a DataFrame containing columns cols and sdf[rows, col] as a vector for each col in cols;
• sdf[!, col] -> a view of entries corresponding to sdf in the vector parent(sdf)[!, col]; the same as sdf.col if col is a valid identifier.
• sdf[!, cols] -> create a new SubDataFrame with columns cols, the same parent as sdf, and the same rows selected; the same as select(sdf, cols, copycols=false).

view on SubDataFrame:

• @view sdf[row, col] -> a 0-dimensional view into df[!, col] at row row, the same as view(sdf[!, col], row);
• @view sdf[CartesianIndex(row, col)] -> the same as @view sdf[row, col];
• @view sdf[row, cols] -> a DataFrameRow with parent parent(sdf);
• @view sdf[rows, col] -> a view into sdf[!, col] vector with rows selected, the same as view(sdf[!, col], rows);
• @view sdf[rows, cols] -> a SubDataFrame with parent parent(sdf);
• @view sdf[!, col] -> a view into sdf[!, col] vector with all rows.
• @view sdf[!, cols] -> the same as @view sdf[:, cols].

getindex on DataFrameRow:

• dfr[col] -> the value contained in column col of dfr; the same as dfr.col if col is a valid identifier;
• dfr[cols] -> a DataFrameRow with parent parent(dfr);

view on DataFrameRow:

• @view dfr[col] -> a 0-dimensional view into parent(dfr)[DataFrames.row(dfr), col];
• @view dfr[cols] -> a DataFrameRow with parent parent(dfr);

Note that views created with columns selector set to : change their columns' count if columns are added/removed/renamed in the parent; if column selector is other than : then view points to selected columns by their number at the moment of creation of the view.

setindex!

The following list specifies the behavior of setindex! operations depending on argument types.

In particular a description explicitly mentions if the assignment is in-place.

Note that if a setindex! operation throws an error the target data frame may be partially changed so it is unsafe to use it afterwards (the column length correctness will be preserved).

setindex! on DataFrame:

• df[row, col] = v -> set value of col in row row to v in-place;
• df[CartesianIndex(row, col)] = v -> the same as df[row, col] = v;
• df[row, cols] = v -> set row row of columns cols in-place; the same as dfr = df[row, cols]; dfr[:] = v;
• df[rows, col] = v -> set rows rows of column col in-place; v must be an AbstractVector; if rows is : and col is a Symbol that is not present in df then a new column in df is created and holds a copy of v; equivalent to df.col = copy(v) if col is a valid identifier;
• df[rows, cols] = v -> set rows rows of columns cols in-place; v must be an AbstractMatrix or an AbstractDataFrame (in this case column names must match);
• df[!, col] = v -> replaces col with v without copying (with the exception that if v is an AbstractRange it gets converted to a Vector); also if col is a Symbol that is not present in df then a new column in df is created and holds v; equivalent to df.col = v if col is a valid identifier; this is allowed if ncol(df) == 0 || length(v) == nrow(df);
• df[!, cols] = v -> replaces existing columns cols in data frame df with copying; v must be an AbstractMatrix or an AbstractDataFrame (in the latter case column names must match);

setindex! on SubDataFrame:

• sdf[row, col] = v -> set value of col in row row to v in-place;
• sdf[CartesianIndex(row, col)] = v -> the same as sdf[row, col] = v;
• sdf[row, cols] = v -> the same as dfr = df[row, cols]; dfr[:] = v in-place;
• sdf[rows, col] = v -> set rows rows of column col, in-place; v must be an abstract vector;
• sdf[rows, cols] = v -> set rows rows of columns cols in-place; v can be an AbstractMatrix or v can be AbstractDataFrame when column names must match;

Note that sdf[!, col] = v, sdf[!, cols] = v and sdf.col = v are not allowed as sdf can be only modified in-place.

setindex! on DataFrameRow:

• dfr[col] = v -> set value of col in row row to v in-place; equivalent to dfr.col = v if col is a valid identifier;
• dfr[cols] = v -> set values of entries in columns cols in dfr by elements of v in place; v can be: 1) a Tuple or an AbstractArray, in which cases it must have a number of elements equal to length(dfr), 2) an AbstractDict, in which case column names must match, 3) a NamedTuple or DataFrameRow, in which case column names and order must match;

Broadcasting

The following broadcasting rules apply to AbstractDataFrame objects:

• AbstractDataFrame behaves in broadcasting like a two-dimensional collection compatible with matrices.
• If an AbstractDataFrame takes part in broadcasting then a DataFrame is always produced as a result. In this case the requested broadcasting operation produces an object with exactly two dimensions. An exception is when an AbstractDataFrame is used only as a source of broadcast assignment into an object of dimensionality higher than two.
• If multiple AbstractDataFrame objects take part in broadcasting then they have to have identical column names.

Note that if broadcasting assignment operation throws an error the target data frame may be partially changed so it is unsafe to use it afterwards (the column length correctness will be preserved).

Broadcasting DataFrameRow is currently not allowed (which is consistent with NamedTuple).

It is possible to assign a value to AbstractDataFrame and DataFrameRow objects using the .= operator. In such an operation AbstractDataFrame is considered as two-dimensional and DataFrameRow as single-dimensional.

Additional rules:

• in the df[CartesianIndex(row, col)] .= v, df[row, col] .= v syntaxes v is broadcasted into the contents of df[row, col] (this is consistent with Julia Base);
• in the df[row, cols] .= v syntaxes the assignment to df is performed in-place;
• in the df[rows, col] .= v and df[rows, cols] .= v syntaxes the assignment to df is performed in-place; if rows is : and col is Symbol and it is missing from df then a new column is allocated and added; the length of the column is always the value of nrow(df) before the assignment takes place;
• in the df[!, col] .= v syntax column col is replaced by a freshly allocated vector; if col is Symbol and it is missing from df then a new column is allocated added; the length of the column is always the value of nrow(df) before the assignment takes place;
• the df[!, cols] .= v syntax replaces existing columns cols in data frame df with freshly allocated vectors;
• df.col .= v syntax is allowed and performs in-place assignment to an existing vector df.col.
• in the sdf[CartesianIndex(row, col)] .= v, sdf[row, col] .= v and sdf[row, cols] .= v syntaxes the assignment to sdf is performed in-place;
• in the sdf[rows, col] .= v and sdf[rows, cols] .= v syntaxes the assignment to sdf is performed in-place;
• sdf.col .= v syntax is allowed and performs in-place assignment to an existing vector sdf.col.
• dfr.col .= v syntax is allowed and performs in-place assignment to a value extracted by dfr.col.

Note that sdf[!, col] .= v and sdf[!, cols] .= v syntaxes are not allowed as sdf can be only modified in-place.

If column indexing using Symbol names in cols is performed, the order of columns in the operation is specified by the order of names.

Indexing GroupedDataFrames

A GroupedDataFrame can behave as either an AbstractVector or AbstractDict depending on the type of index used. Integers (or arrays of them) trigger vector-like indexing while Tupless and NamedTuples trigger dictionary-like indexing. An intermediate between the two is the GroupKey type returned by keys(::GroupedDataFrame), which behaves similarly to a NamedTuple but has performance on par with integer indexing.

The elements of a GroupedDataFrame are SubDataFrames of its parent.

• gd[i::Integer] -> Get the ith group.
• gd[a::AbstractArray{<:Integer}] -> Get a reduced GroupedDataFrame containing only the groups with indices in a.
• gd[key::NamedTuple] -> Get the group corresponding to the given values of the grouping columns. The fields of the NamedTuple must match the grouping columns columns passed to groupby (including order).
• gd[key::Tuple] -> Same as previous, but omitting the names on key.
• get(gd, key::Union{Tuple, NamedTuple}, default) -> Get group for key key, returning default if it does not exist.
• gd[key::GroupKey] -> Get the group corresponding to the GroupKeykey (one of the elements of the vector returned by keys(::GroupedDataFrame)). This should be nearly as fast as integer indexing.