Possible pitfalls

using DataFrames
using BenchmarkTools

Know what is copied when creating a DataFrame

x = DataFrame(rand(3, 5), :auto)

3×5 DataFrame

Row	x1	x2	x3	x4	x5
	Float64	Float64	Float64	Float64	Float64
1	0.560891	0.566409	0.604743	0.606931	0.761472
2	0.255647	0.636852	0.45487	0.199889	0.898372
3	0.300935	0.310988	0.673843	0.512502	0.585906

x and y are not the same object

y = copy(x)
x === y

false

x and y are not the same object

y = DataFrame(x)
x === y

false

the columns are also not the same

any(x[!, i] === y[!, i] for i in ncol(x))

false

x and y are not the same object

y = DataFrame(x, copycols=false)
x === y

false

But the columns are the same

all(x[!, i] === y[!, i] for i in ncol(x))

true

the same when creating data frames using kwarg syntax

x = 1:3;
y = [1, 2, 3];
df = DataFrame(x=x, y=y)

3×2 DataFrame

Row	x	y
	Int64	Int64
1	1	1
2	2	2
3	3	3

different object

y === df.y

false

range is converted to a vector

typeof(x), typeof(df.x)

(UnitRange{Int64}, Vector{Int64})

slicing rows always creates a copy

y === df[:, :y]

false

you can avoid copying by using copycols=false keyword argument in functions.

df = DataFrame(x=x, y=y, copycols=false)

3×2 DataFrame

Row	x	y
	Int64	Int64
1	1	1
2	2	2
3	3	3

now it is the same

y === df.y

true

not the same object

select(df, :y)[!, 1] === y

false

the same object

select(df, :y, copycols=false)[!, 1] === y

true

Do not modify the parent of GroupedDataFrame or view

x = DataFrame(id=repeat([1, 2], outer=3), x=1:6)
g = groupby(x, :id)

x[1:3, 1] = [2, 2, 2]
g ## well - it is wrong now, g is only a view

GroupedDataFrame with 2 groups based on key: id

First Group (3 rows): id = 2

Row	id	x
	Int64	Int64
1	2	1
2	2	3
3	1	5

⋮

Last Group (3 rows): id = 2

Row	id	x
	Int64	Int64
1	2	2
2	2	4
3	2	6

s = view(x, 5:6, :)

2×2 SubDataFrame

Row	id	x
	Int64	Int64
1	1	5
2	2	6

delete!(x, 3:6)

2×2 DataFrame

Row	id	x
	Int64	Int64
1	2	1
2	2	2

This is an error

s ## Will return BoundsError

Single column selection for a DataFrame

Single column selection for a DataFrame creates aliases with ! and getproperty syntax and copies with :

x = DataFrame(a=1:3)
x.b = x[!, 1] ## alias
x.c = x[:, 1] ## copy
x.d = x[!, 1][:] ## copy
x.e = copy(x[!, 1]) ## explicit copy
display(x)

3×5 DataFrame

Row	a	b	c	d	e
	Int64	Int64	Int64	Int64	Int64
1	1	1	1	1	1
2	2	2	2	2	2
3	3	3	3	3	3

x[1, 1] = 100
display(x)

3×5 DataFrame

Row	a	b	c	d	e
	Int64	Int64	Int64	Int64	Int64
1	100	100	1	1	1
2	2	2	2	2	2
3	3	3	3	3	3

When iterating rows of a data frame

• use eachrow to avoid compilation cost in wide tables,

• but Tables.namedtupleiterator for fast execution in tall tables

The table below is tall:

df2 = DataFrame(rand(10^6, 10), :auto)

1000000×10 DataFrame

999975 rows omitted

Row	x1	x2	x3	x4	x5	x6	x7	x8	x9	x10
	Float64	Float64	Float64	Float64	Float64	Float64	Float64	Float64	Float64	Float64
1	0.173259	0.868429	0.199462	0.144989	0.140205	0.246803	0.246632	0.942353	0.926668	0.911074
2	0.123954	0.996035	0.484935	0.392219	0.203026	0.459265	0.177535	0.343015	0.484788	0.790101
3	0.154781	0.186644	0.799731	0.997506	0.955073	0.445179	0.619125	0.0335754	0.607219	0.865513
4	0.816031	0.868959	0.770971	0.581648	0.913118	0.241069	0.185114	0.959924	0.329531	0.144141
5	0.814599	0.801111	0.956055	0.248121	0.241139	0.150823	0.404526	0.322152	0.165737	0.185206
6	0.582423	0.119731	0.607285	0.0600893	0.827207	0.467929	0.855228	0.136935	0.866942	0.495018
7	0.592345	0.28729	0.530249	0.671263	0.387001	0.478387	0.270147	0.00583683	0.169445	0.92732
8	0.566305	0.466944	0.433918	0.509695	0.125568	0.540185	0.0345033	0.215133	0.247376	0.742053
9	0.711011	0.156912	0.966428	0.373472	0.779452	0.171457	0.9646	0.205437	0.0636527	0.92508
10	0.4733	0.639631	0.758602	0.342212	0.446266	0.940815	0.354516	0.846175	0.255437	0.04738
11	0.36807	0.944925	0.904134	0.648487	0.894707	0.423242	0.925946	0.727968	0.658193	0.688444
12	0.604438	0.472574	0.969352	0.831801	0.454789	0.811133	0.718755	0.733665	0.3699	0.438884
13	0.883801	0.398876	0.944276	0.466476	0.102224	0.77286	0.733994	0.652745	0.819884	0.883364
⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮
999989	0.890034	0.0641943	0.499185	0.196725	0.420027	0.0912085	0.00682904	0.53108	0.637768	0.533981
999990	0.810706	0.403768	0.0845722	0.664577	0.521997	0.580567	0.415557	0.702156	0.204175	0.965622
999991	0.456027	0.77377	0.598199	0.122355	0.448871	0.792457	0.827398	0.148164	0.28031	0.158536
999992	0.846266	0.679415	0.277502	0.94522	0.908634	0.0268256	0.573818	0.794065	0.396007	0.0417267
999993	0.77744	0.399099	0.127973	0.304421	0.710879	0.635667	0.324141	0.728154	0.0727041	0.606665
999994	0.756423	0.272729	0.13272	0.0142115	0.614726	0.115551	0.619445	0.438747	0.460739	0.663496
999995	0.287939	0.881226	0.350966	0.540559	0.70547	0.544379	0.0550066	0.71417	0.134671	0.81261
999996	0.849627	0.789437	0.127928	0.507435	0.319254	0.978655	0.00634836	0.803388	0.783744	0.67073
999997	0.118957	0.254107	0.854678	0.871965	0.747361	0.911129	0.152809	0.388875	0.538347	0.296838
999998	0.859681	0.547856	0.52048	0.0714623	0.849722	0.690608	0.347115	0.8185	0.311047	0.280407
999999	0.69592	0.131087	0.547444	0.261241	0.741731	0.796596	0.223215	0.750396	0.61934	0.495444
1000000	0.317247	0.632787	0.264964	0.94906	0.877184	0.632165	0.254215	0.0337684	0.0369622	0.905203

@time map(sum, eachrow(df2));

  3.542418 seconds (60.11 M allocations: 1.056 GiB, 9.67% gc time, 3.42% compilation time)

@time map(sum, eachrow(df2));

  3.169916 seconds (59.99 M allocations: 1.050 GiB, 4.13% gc time)

@time map(sum, Tables.namedtupleiterator(df2));

  0.249135 seconds (361.13 k allocations: 26.568 MiB, 4.86% gc time, 85.77% compilation time)

@time map(sum, Tables.namedtupleiterator(df2));

  0.012652 seconds (20 allocations: 7.630 MiB)

as you can see - this time it is much faster to iterate a type stable container still you might want to use the select syntax, which is optimized for such reductions:

this includes compilation time

@time select(df2, AsTable(:) => ByRow(sum) => "sum").sum

  0.011790 seconds (6.67 k allocations: 7.969 MiB, 50.48% compilation time)

1000000-element Vector{Float64}:
 4.799871994364215
 4.454871793224532
 5.664348582520293
 5.810507372086148
 4.289469346132967
 5.0187854176762725
 4.3192849190286875
 3.8816819363205237
 5.317501511565184
 5.104333118795934
 ⋮
 5.489477995118583
 4.687143261221065
 4.088786137198144
 5.026998053622702
 5.8365470064870415
 5.135065469480588
 5.29687884751163
 5.262414871594562
 4.90355610068893

Do it again

@time select(df2, AsTable(:) => ByRow(sum) => "sum").sum

  0.006313 seconds (123 allocations: 7.634 MiB)

1000000-element Vector{Float64}:
 4.799871994364215
 4.454871793224532
 5.664348582520293
 5.810507372086148
 4.289469346132967
 5.0187854176762725
 4.3192849190286875
 3.8816819363205237
 5.317501511565184
 5.104333118795934
 ⋮
 5.489477995118583
 4.687143261221065
 4.088786137198144
 5.026998053622702
 5.8365470064870415
 5.135065469480588
 5.29687884751163
 5.262414871594562
 4.90355610068893

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This notebook was generated using Literate.jl.