Conas a oibríonn slicing i Python?

Question

Accepted Answer

**Slicing** iolrú de sheicheamh (liosta, tuple, teanga) ag baint úsáide as an chomhréir `sequence[start:stop:step]`. Is gné chumhachtach, ghearr-fhocal í a úsáidtear go leanúnach i Python.

## Slicing bunúsach

```python
nums = [0, 1, 2, 3, 4, 5]

nums[1:4]     # [1, 2, 3]   — start at index 1, up to (NOT including) 4
nums[:3]      # [0, 1, 2]   — from the start (start defaults to 0)
nums[3:]      # [3, 4, 5]   — to the end (stop defaults to len)
nums[:]       # [0, 1, 2, 3, 4, 5] — a full (shallow) COPY
```

An riail bhunúsach: is **inclusive** a bhíonn `start`, is **exclusive** a bhíonn `stop` (`nums[1:4]` tugann na hinnéacsanna 1, 2, 3 — ní 4).

## Innéacsanna diúltacha a bhíonn ag comhaireamh ó dheireadh

```python
nums[-1]      # 5  — last element
nums[-2]      # 4  — second to last
nums[-3:]     # [3, 4, 5] — last three elements
nums[:-1]     # [0, 1, 2, 3, 4] — everything except the last
```

## Paraiméadar an chéime

```python
nums[::2]     # [0, 2, 4]      — every 2nd element
nums[1::2]    # [1, 3, 5]      — every 2nd, starting at index 1
nums[::-1]    # [5, 4, 3, 2, 1, 0] — REVERSE the sequence (a famous idiom)
```

`nums[::-1]` ag aisiompú seicheamh is ceann de na guth is feasach Python.

## Slicing teangacha

```python
s = "Hello"
s[1:4]        # "ell"
s[::-1]       # "olleH" — reverse a string
s[0]          # "H"
```

## Shannadh slicing (do liostai inmhalartaithe)

```python
nums = [0, 1, 2, 3, 4]
nums[1:3] = [10, 20, 30]   # replace a slice (can change length)
# nums → [0, 10, 20, 30, 3, 4]
```

## Slicing slán (gan IndexError)

```python
nums[10:20]   # [] — out-of-range slices return empty, don't crash (unlike nums[10])
```

Murab ionann agus rochtain singil-innéacs (a thógann `IndexError`), filleann slicing go grástúil ar an méid atá ar fáil.

## Cén fáth a bhíonn sé tábhachtach

Is gné omnipresent, ar bhealach Python, slicing do bhreiseán, chóipeáil, agus seichimh a aisiompú go soiléir.

An tuiscint ar an chomhréir `start:stop:step` (go háirithe inclusive-start/exclusive-stop), innéacsanna diúltacha (comhaireamh ó dheireadh), an idiom inbhéartaithe (`[::-1]`), agus go bhfuil slices slán (gan tubaiste ar raon-as-raon) a ligeann duit liostai agus teangacha a bhogadh go gearr agus go ceart.

Cuireann sé in ionad lúpanna fhoclach do ghnáth-oibríochtaí agus feictear ar fud an chóid Python idiomatic.