# Jagged, Ragged, Awkward Arrays!#

*Originally presented as part of HSF Scikit-HEP training on March 28, 2022.*

NumPy can’t represent an array of variable-length lists without resorting to arrays of objects.

```
import numpy as np
# generates a ValueError
np.array([[0.0, 1.1, 2.2], [], [3.3, 4.4], [5.5], [6.6, 7.7, 8.8, 9.9]])
```

```
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
Cell In[1], line 4
1 import numpy as np
3 # generates a ValueError
----> 4 np.array([[0.0, 1.1, 2.2], [], [3.3, 4.4], [5.5], [6.6, 7.7, 8.8, 9.9]])
ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 1 dimensions. The detected shape was (5,) + inhomogeneous part.
```

Awkward Array is intended to fill this gap:

```
import awkward as ak
ak.Array([[0.0, 1.1, 2.2], [], [3.3, 4.4], [5.5], [6.6, 7.7, 8.8, 9.9]])
```

[[0, 1.1, 2.2], [], [3.3, 4.4], [5.5], [6.6, 7.7, 8.8, 9.9]] ----------------------- type: 5 * var * float64

Arrays like this are sometimes called “jagged arrays” and sometimes “ragged arrays.”

## Slicing in Awkward Array#

Basic slices are a generalization of NumPy’s—what NumPy would do if it had variable-length lists.

```
array = ak.Array([[0.0, 1.1, 2.2], [], [3.3, 4.4], [5.5], [6.6, 7.7, 8.8, 9.9]])
array
```

[[0, 1.1, 2.2], [], [3.3, 4.4], [5.5], [6.6, 7.7, 8.8, 9.9]] ----------------------- type: 5 * var * float64

```
array[2]
```

[3.3, 4.4] ----------------- type: 2 * float64

```
array[-1, 1]
```

```
7.7
```

```
array[2:, 0]
```

[3.3, 5.5, 6.6] ----------------- type: 3 * float64

```
array[2:, 1:]
```

[[4.4], [], [7.7, 8.8, 9.9]] ----------------------- type: 3 * var * float64

```
array[:, 0]
```

```
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
Cell In[8], line 1
----> 1 array[:, 0]
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/highlevel.py:1066, in Array.__getitem__(self, where)
636 """
637 Args:
638 where (many types supported; see below): Index of positions to
(...)
1062 have the same dimension as the array being indexed.
1063 """
1064 with ak._errors.SlicingErrorContext(self, where):
1065 return wrap_layout(
-> 1066 prepare_layout(self._layout[where]),
1067 self._behavior,
1068 allow_other=True,
1069 attrs=self._attrs,
1070 )
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:512, in Content.__getitem__(self, where)
511 def __getitem__(self, where):
--> 512 return self._getitem(where)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:557, in Content._getitem(self, where)
548 nextwhere = ak._slicing.prepare_advanced_indexing(items, backend)
550 next = ak.contents.RegularArray(
551 this,
552 this.length,
553 1,
554 parameters=None,
555 )
--> 557 out = next._getitem_next(nextwhere[0], nextwhere[1:], None)
559 if out.length is not unknown_length and out.length == 0:
560 return out._getitem_nothing()
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/regulararray.py:519, in RegularArray._getitem_next(self, head, tail, advanced)
513 nextcontent = self._content._carry(nextcarry, True)
515 if advanced is None or (
516 advanced.length is not unknown_length and advanced.length == 0
517 ):
518 return RegularArray(
--> 519 nextcontent._getitem_next(nexthead, nexttail, advanced),
520 nextsize,
521 self._length,
522 parameters=self._parameters,
523 )
524 else:
525 nextadvanced = ak.index.Index64.empty(nextcarry.length, index_nplike)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/listarray.py:720, in ListArray._getitem_next(self, head, tail, advanced)
714 nextcarry = ak.index.Index64.empty(lenstarts, self._backend.index_nplike)
715 assert (
716 nextcarry.nplike is self._backend.index_nplike
717 and self._starts.nplike is self._backend.index_nplike
718 and self._stops.nplike is self._backend.index_nplike
719 )
--> 720 self._maybe_index_error(
721 self._backend[
722 "awkward_ListArray_getitem_next_at",
723 nextcarry.dtype.type,
724 self._starts.dtype.type,
725 self._stops.dtype.type,
726 ](
727 nextcarry.data,
728 self._starts.data,
729 self._stops.data,
730 lenstarts,
731 head,
732 ),
733 slicer=head,
734 )
735 nextcontent = self._content._carry(nextcarry, True)
736 return nextcontent._getitem_next(nexthead, nexttail, advanced)
File ~/micromamba/envs/awkward-docs/lib/python3.11/site-packages/awkward/contents/content.py:277, in Content._maybe_index_error(self, error, slicer)
275 else:
276 message = self._backend.format_kernel_error(error)
--> 277 raise ak._errors.index_error(self, slicer, message)
IndexError: cannot slice ListArray (of length 5) with array(0): index out of range while attempting to get index 0 (in compiled code: https://github.com/scikit-hep/awkward/blob/awkward-cpp-35/awkward-cpp/src/cpu-kernels/awkward_ListArray_getitem_next_at.cpp#L21)
This error occurred while attempting to slice
<Array [[0, 1.1, 2.2], ..., [6.6, 7.7, ..., 9.9]] type='5 * var * float64'>
with
(:, 0)
```

**Quick quiz:** why does the last one raise an error?

Boolean and integer slices work, too:

```
array[[True, False, True, False, True]]
```

[[0, 1.1, 2.2], [3.3, 4.4], [6.6, 7.7, 8.8, 9.9]] ----------------------- type: 3 * var * float64

```
array[[2, 3, 3, 1]]
```

[[3.3, 4.4], [5.5], [5.5], []] ----------------------- type: 4 * var * float64

Like NumPy, boolean arrays for slices can be computed, and functions like ak.num are helpful for that.

```
ak.num(array)
```

[3, 0, 2, 1, 4] --------------- type: 5 * int64

```
ak.num(array) > 0
```

[True, False, True, True, True] -------------- type: 5 * bool

```
array[ak.num(array) > 0, 0]
```

[0, 3.3, 5.5, 6.6] ----------------- type: 4 * float64

```
array[ak.num(array) > 1, 1]
```

[1.1, 4.4, 7.7] ----------------- type: 3 * float64

Now consider this (similar to an example from the first lesson):

```
cut = array * 10 % 2 == 0
cut
```

[[True, False, True], [], [False, True], [False], [True, False, True, False]] ---------------------------- type: 5 * var * bool

```
array[cut]
```

[[0, 2.2], [], [4.4], [], [6.6, 8.8]] ----------------------- type: 5 * var * float64

This array, `cut`

, is not just an array of booleans. It’s a jagged array of booleans. All of its nested lists fit into `array`

’s nested lists, so it can deeply select numbers, rather than selecting lists.

## Application: selecting particles, rather than events#

Returning to the big TTree from the previous lesson,

```
import uproot
file = uproot.open(
"https://github.com/jpivarski-talks/2023-12-18-hsf-india-tutorial-bhubaneswar/raw/main/data/SMHiggsToZZTo4L.root"
)
tree = file["Events"]
muon_pt = tree["Muon_pt"].array(entry_stop=10)
```

This jagged array of booleans selects all *muons* with at least 20 GeV:

```
particle_cut = muon_pt > 20
```

```
muon_pt[particle_cut]
```

[[63, 38.1], [], [], [54.3, 23.5, 52.9], [], [38.5, 47], [], [], [], []] ------------------------ type: 10 * var * float32

and this non-jagged array of booleans (made with ak.any) selects all events *that have* a muon with at least 20 GeV:

```
event_cut = ak.any(muon_pt > 20, axis=1)
```

```
muon_pt[event_cut]
```

[[63, 38.1, 4.05], [54.3, 23.5, 52.9, 4.33, 5.35, 8.39, 3.49], [38.5, 47]] -------------------------------------------- type: 3 * var * float32

**Quick quiz:** construct exactly the same `event_cut`

using ak.max.

**Quick quiz:** apply both cuts; that is, select muons with over 20 GeV from events that have them.

Hint: you’ll want to make a

```
cleaned = muon_pt[particle_cut]
```

intermediary and you can’t use the variable `event_cut`

, as-is.

**Hint:** the final result should be a jagged array, just like muon_pt, but with fewer lists and fewer items in those lists.

## Combinatorics in Awkward Array#

Variable-length lists present more problems than just slicing and computing formulas array-at-a-time. Often, we want to combine particles in all possible pairs (within each event) to look for decay chains.

### Pairs from two arrays, pairs from a single array#

Awkward Array has functions that generate these combinations. For instance, ak.cartesian takes a Cartesian product per event (when `axis=1`

, the default).

```
numbers = ak.Array([[1, 2, 3], [], [5, 7], [11]])
letters = ak.Array([["a", "b"], ["c"], ["d"], ["e", "f"]])
```

```
pairs = ak.cartesian((numbers, letters))
```

These `pairs`

are 2-tuples, which are like records in how they’re sliced out of an array: using strings.

```
pairs["0"]
```

[[1, 1, 2, 2, 3, 3], [], [5, 7], [11, 11]] --------------------- type: 4 * var * int64

```
pairs["1"]
```

[['a', 'b', 'a', 'b', 'a', 'b'], [], ['d', 'd'], ['e', 'f']] -------------------------------- type: 4 * var * string

There’s also ak.unzip, which extracts every field into a separate array (opposite of ak.zip).

```
lefts, rights = ak.unzip(pairs)
```

```
lefts
```

[[1, 1, 2, 2, 3, 3], [], [5, 7], [11, 11]] --------------------- type: 4 * var * int64

```
rights
```

[['a', 'b', 'a', 'b', 'a', 'b'], [], ['d', 'd'], ['e', 'f']] -------------------------------- type: 4 * var * string

Note that these `lefts`

and `rights`

are not the original `numbers`

and `letters`

: they have been duplicated and have the same shape.

The Cartesian product is equivalent to this C++ `for`

loop over two collections:

```
for (int i = 0; i < numbers.size(); i++) {
for (int j = 0; j < letters.size(); j++) {
// compute formula with numbers[i] and letters[j]
}
}
```

Sometimes, though, we want to find all pairs within a single collection, without repetition. That would be equivalent to this C++ `for`

loop:

```
for (int i = 0; i < numbers.size(); i++) {
for (int j = i + 1; i < numbers.size(); j++) {
// compute formula with numbers[i] and numbers[j]
}
}
```

The Awkward function for this case is ak.combinations.

```
pairs = ak.combinations(numbers, 2)
pairs
```

[[(1, 2), (1, 3), (2, 3)], [], [(5, 7)], []] -------------------------- type: 4 * var * ( int64, int64 )

```
lefts, rights = ak.unzip(pairs)
```

```
lefts * rights # they line up, so we can compute formulas
```

[[2, 3, 6], [], [35], []] --------------------- type: 4 * var * int64

## Application to dimuons#

The dimuon search in the previous lesson was a little naive in that we required *exactly two* muons to exist in every event and only computed the mass of that combination. If a third muon were present because it’s a complex electroweak decay or because something was mismeasured, we would be blind to the other two muons. They might be real dimuons.

A better procedure would be to look for all pairs of muons in an event and apply some criteria for selecting them.

In this example, we’ll ak.zip the muon variables together into records.

```
import uproot
import awkward as ak
file = uproot.open(
"https://github.com/jpivarski-talks/2023-12-18-hsf-india-tutorial-bhubaneswar/raw/main/data/SMHiggsToZZTo4L.root"
)
tree = file["Events"]
arrays = tree.arrays(filter_name="/Muon_(pt|eta|phi|charge)/", entry_stop=10000)
muons = ak.zip(
{
"pt": arrays["Muon_pt"],
"eta": arrays["Muon_eta"],
"phi": arrays["Muon_phi"],
"charge": arrays["Muon_charge"],
}
)
```

```
arrays.type.show()
```

```
10000 * {
Muon_pt: var * float32,
Muon_eta: var * float32,
Muon_phi: var * float32,
Muon_charge: var * int32
}
```

```
muons.type.show()
```

```
10000 * var * {
pt: float32,
eta: float32,
phi: float32,
charge: int32
}
```

The difference between `arrays`

and `muons`

is that `arrays`

contains separate lists of `"Muon_pt"`

, `"Muon_eta"`

, `"Muon_phi"`

, `"Muon_charge"`

, while `muons`

contains lists of records with `"pt"`

, `"eta"`

, `"phi"`

, `"charge"`

fields.

Now we can compute pairs of muon *objects*

```
pairs = ak.combinations(muons, 2)
pairs.type.show()
```

```
10000 * var * (
{
pt: float32,
eta: float32,
phi: float32,
charge: int32
},
{
pt: float32,
eta: float32,
phi: float32,
charge: int32
}
)
```

and separate them into arrays of the first muon and the second muon in each pair.

```
mu1, mu2 = ak.unzip(pairs)
```

**Quick quiz:** how would you ensure that all lists of records in `mu1`

and `mu2`

have the same lengths? Hint: see ak.num and ak.all.

Since they do have the same lengths, we can use them in a formula.

```
import numpy as np
mass = np.sqrt(
2 * mu1.pt * mu2.pt * (np.cosh(mu1.eta - mu2.eta) - np.cos(mu1.phi - mu2.phi))
)
```

**Quick quiz:** how many masses do we have in each event? How does this compare with `muons`

, `mu1`

, and `mu2`

?

## Plotting the jagged array#

Since this `mass`

is a jagged array, it can’t be directly histogrammed. Histograms take a set of *numbers* as inputs, but this array contains *lists*.

Supposing you just want to plot the numbers from the lists, you can use ak.flatten to flatten one level of list or ak.ravel to flatten all levels.

```
import hist
hist.Hist(hist.axis.Regular(120, 0, 120, label="mass [GeV]")).fill(
ak.ravel(mass)
).plot()
None
```

Alternatively, suppose you want to plot the *maximum* mass-candidate in each event, biasing it toward Z bosons? ak.max is a different function that picks one element from each list, when used with `axis=1`

.

```
ak.max(mass, axis=1)
```

[89.5, None, None, 98.7, None, 87.1, None, None, None, None, ..., 90.6, 12, None, None, 91.7, None, None, None, 15.6] ---------------------- type: 10000 * ?float32

Some values are `None`

because there is no maximum of an empty list. ak.flatten/ak.ravel remove missing values (`None`

) as well as squashing lists,

```
ak.flatten(ak.max(mass, axis=1), axis=0)
```

[89.5, 98.7, 87.1, 90.5, 89.2, 18.8, 94.5, 8.54, 4.57, 31.6, ..., 88.3, 93.3, 86.9, 92.6, 101, 90.6, 12, 91.7, 15.6] -------------------- type: 4825 * float32

but so does removing the empty lists in the first place.

```
ak.max(mass[ak.num(mass) > 0], axis=1)
```

[89.5, 98.7, 87.1, 90.5, 89.2, 18.8, 94.5, 8.54, 4.57, 31.6, ..., 88.3, 93.3, 86.9, 92.6, 101, 90.6, 12, 91.7, 15.6] --------------------- type: 4825 * ?float32

## Exercise: select pairs of muons with opposite charges#

This is neither an event-level cut nor a particle-level cut, it is a cut on particle *pairs*.

### Solution#

The `mu1`

and `mu2`

variables are the left and right halves of muon pairs. Therefore,

```
cut = (mu1.charge != mu2.charge)
```

has the right multiplicity to be applied to the `mass`

array.

```
hist.Hist(hist.axis.Regular(120, 0, 120, label="mass [GeV]")).fill(
ak.ravel(mass[cut])
).plot()
None
```

plots the cleaned muon pairs.

## Exercise (harder): plot the one mass candidate per event that is strictly closest to the Z mass#

Instead of just taking the maximum mass in each event, find the one with the minimum difference between computed mass and `zmass = 91`

.

**Hint:** use ak.argmin with `keepdims=True`

.

Anticipating one of the future lessons, you could get a more accurate mass by asking the Particle library:

```
import particle, hepunits
zmass = particle.Particle.findall("Z0")[0].mass / hepunits.GeV
```

### Solution#

Instead of maximizing `mass`

, we want to minimize `abs(mass - zmass)`

and apply that choice to `mass`

. ak.argmin returns the *index position* of this minimum difference, which we can then apply to the original `mass`

. However, without `keepdims=True`

, ak.argmin removes the dimension we would need for this array to have the same nested shape as `mass`

. Therefore, we `keepdims=True`

and then use ak.ravel to get rid of missing values and flatten lists.

The last step would require two applications of ak.flatten: one for squashing lists at the first level and another for removing `None`

at the second level.

```
which = ak.argmin(abs(mass - zmass), axis=1, keepdims=True)
hist.Hist(hist.axis.Regular(120, 0, 120, label="mass [GeV]")).fill(
ak.flatten(mass[which], axis=None)
).plot()
None
```