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	0.424737	0.782748	0.681777	0.572985	0.735879
2	0.428584	0.600395	0.539097	0.995162	0.757722
3	0.960558	0.167239	0.926269	0.00101525	0.797032

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	0.0926801	0.51047	0.0182201	0.282308	0.633224	0.218633	0.923353	0.577041	0.673746	0.59492
2	0.674586	0.442703	0.360177	0.977093	0.232776	0.743179	0.959031	0.474704	0.362828	0.299904
3	0.853672	0.219866	0.767345	0.803446	0.753585	0.278893	0.139847	0.19652	0.583675	0.447995
4	0.30419	0.528587	0.419393	0.872883	0.256235	0.0927021	0.733771	0.773304	0.35754	0.112215
5	0.00703228	0.563562	0.94804	0.543083	0.452466	0.60112	0.534152	0.169756	0.972281	0.511011
6	0.0205538	0.433061	0.184369	0.367608	0.533516	0.542916	0.00705235	0.470152	0.19433	0.00416603
7	0.688379	0.425821	0.790728	0.829473	0.374633	0.785347	0.0298765	0.939468	0.0746554	0.679708
8	0.34046	0.30185	0.537306	0.350657	0.882454	0.460346	0.780001	0.551189	0.728183	0.550581
9	0.0243611	0.286375	0.35574	0.622435	0.754108	0.91973	0.316334	0.0269321	0.26608	0.231496
10	0.373269	0.508587	0.345888	0.190497	0.754962	0.303547	0.660626	0.192616	0.385012	0.216885
11	0.720611	0.359379	0.788825	0.395271	0.973575	0.132564	0.723773	0.829181	0.590105	0.359539
12	0.311856	0.474905	0.310713	0.769761	0.533877	0.758357	0.661514	0.17868	0.963539	0.22141
13	0.117566	0.787011	0.271336	0.99351	0.470142	0.191422	0.936034	0.825118	0.864033	0.42115
⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮	⋮
999989	0.0962606	0.598586	0.732444	0.489372	0.241066	0.512523	0.527585	0.711133	0.923477	0.425483
999990	0.159721	0.802838	0.804383	0.295411	0.348738	0.0687883	0.0468217	0.883975	0.376613	0.0351025
999991	0.901358	0.734939	0.714723	0.60859	0.111339	0.603886	0.705214	0.0593379	0.212619	0.24508
999992	0.281053	0.771366	0.543346	0.869548	0.168224	0.141557	0.409522	0.760517	0.170061	0.693612
999993	0.0964515	0.550841	0.801289	0.351199	0.229217	0.501073	0.482301	0.502982	0.878444	0.671248
999994	0.453908	0.252602	0.513762	0.449409	0.604462	0.5066	0.765549	0.91116	0.643185	0.986821
999995	0.0667829	0.0153803	0.256034	0.330013	0.0346192	0.595964	0.844599	0.832714	0.979674	0.359995
999996	0.655738	0.571018	0.871706	0.514961	0.99357	0.580691	0.762547	0.307967	0.337729	0.940187
999997	0.118139	0.471918	0.857863	0.334845	0.754409	0.955712	0.305699	0.131203	0.0845749	0.235298
999998	0.821392	0.709429	0.950366	0.792938	0.204405	0.459165	0.924076	0.53253	0.127452	0.165834
999999	0.86303	0.135856	0.483179	0.765809	0.0529708	0.496276	0.206796	0.673471	0.583007	0.803619
1000000	0.630663	0.773273	0.144543	0.136788	0.978063	0.0167045	0.869717	0.0857667	0.478027	0.32786

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

  2.178086 seconds (60.08 M allocations: 1.055 GiB, 15.60% gc time, 3.73% compilation time)

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

  1.862223 seconds (59.99 M allocations: 1.050 GiB, 6.29% gc time)

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

  0.233865 seconds (428.99 k allocations: 30.089 MiB, 4.03% gc time, 91.72% compilation time: 27% of which was recompilation)

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

  0.009578 seconds (25 allocations: 7.631 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

  0.164873 seconds (684.31 k allocations: 41.595 MiB, 96.42% compilation time: 97% of which was recompilation)

1000000-element Vector{Float64}:
 4.524594656468376
 5.526979192486358
 5.044843910022011
 4.450819836455767
 5.302503012392829
 2.7577227261336805
 5.618088460709214
 5.483027953951166
 3.8035897887299397
 3.93188928765376
 ⋮
 4.808806298265378
 5.065044624374165
 6.087458020724531
 4.3157769856357
 6.536114733186674
 4.249661766586929
 5.687587649537533
 5.064013304031212
 4.44140391850883

Do it again

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

  0.005210 seconds (127 allocations: 7.634 MiB)

1000000-element Vector{Float64}:
 4.524594656468376
 5.526979192486358
 5.044843910022011
 4.450819836455767
 5.302503012392829
 2.7577227261336805
 5.618088460709214
 5.483027953951166
 3.8035897887299397
 3.93188928765376
 ⋮
 4.808806298265378
 5.065044624374165
 6.087458020724531
 4.3157769856357
 6.536114733186674
 4.249661766586929
 5.687587649537533
 5.064013304031212
 4.44140391850883

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This notebook was generated using Literate.jl.