General
PromptBeginner5 minmarkdown
- Duplicate column names allowed only with `check.names = FALSE`
but many operations will de-dup them silently.
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Coding & Debugging
PromptBeginner5 minmarkdown
- `as.numeric(f)` returns **integer codes**
not original values. Use `as.numeric(levels(f))[f]` or `as.numeric(as.character(f))`.
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General
PromptBeginner5 minmarkdown
- `duplicated` marks the **second and later** occurrences as TRUE
not the first. Use `fromLast = TRUE` to reverse.
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Data
PromptBeginner5 minmarkdown
- For data frames
operates on whole rows. For lists
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General
PromptBeginner5 minmarkdown
- Atomic vectors recycle to match longest column
but only if exact multiple. Protect with `I()` to prevent conversion.
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General
PromptBeginner5 minmarkdown
- `cbind(NULL)` returns `NULL` (not a matrix). For consistency
`rbind(NULL)` also returns `NULL`.
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Data
PromptBeginner5 minmarkdown
- `rbind` on data frames matches columns **by name**
not position. Missing columns get `NA`.
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Data
PromptBeginner5 minmarkdown
- Result is always an **array** (even 1D)
class table. Convert to data frame with `as.data.frame(tbl)`.
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Data
PromptBeginner5 minmarkdown
- `cbind` on data frames calls `data.frame(...)`
not `cbind.matrix`. Mixing matrices and data frames can give unexpected results.
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General
PromptBeginner5 minmarkdown
- Supports formula syntax: `split(df
~ Month)`.
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General
PromptBeginner5 minmarkdown
- If `f` is a list of factors
interaction is used; levels containing `.` can cause unexpected splits unless `sep` is changed.
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General
PromptBeginner5 minmarkdown
- `...` args to FUN are **not** divided into cells — they apply globally
so FUN should not expect additional args with same length as X.
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General
PromptBeginner5 minmarkdown
- `by = 0` or `by = row.names` merges on row names
adding a Row.names column.
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Data
PromptBeginner5 minmarkdown
- Default `by` is `intersect(names(x)
names(y))` — can silently merge on unintended columns if data frames share column names.
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Data
PromptBeginner5 minmarkdown
- Returns an **array** (not a data frame). Class info on return values is **discarded** (e.g.
Date objects become numeric).
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General
PromptBeginner5 minmarkdown
- `...` args cannot share names with `X`
`MARGIN`
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General
PromptBeginner5 minmarkdown
- `sapply` can return a vector
matrix
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General
PromptBeginner5 minmarkdown
- Return value orientation is transposed: if FUN returns length-n vector
result has dim `c(n
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Data
PromptBeginner5 minmarkdown
- On a **data frame**
`apply` coerces to matrix via `as.matrix` first — mixed types become character.
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General
PromptBeginner5 minmarkdown
- Factors
raw vectors
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Data
PromptBeginner5 minmarkdown
- Matrix indexing a data frame (`df[cbind(i
j)]`) coerces to matrix first — avoid.
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Data
PromptBeginner5 minmarkdown
- Data frame `[` with single column: `df[
1]` returns a **vector** (drop=TRUE default for columns)
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Data
PromptBeginner5 minmarkdown
- `subset` argument uses **non-standard evaluation** — column names are resolved in the data frame
which can silently pick up wrong variables in programmatic use. Use `[` with explicit logic in functions.
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Coding & Debugging
PromptBeginner5 minmarkdown
- Factor indexing: `x[f]` uses integer codes of factor `f`
not its character labels. Use `x[as.character(f)]` for label-based indexing.
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