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Parse Formula Components for Longitudinal Plotting

Source: R/formula-parsing.R
parse_formula.Rd

Parses an R formula object into its constituent components for use in longitudinal plotting functions. This function extracts variables for plotting observed values and changes over time, with optional grouping and faceting specifications.

Usage

parse_formula(formula)

Arguments

formula

An R formula object. The formula can have the following forms:

  • y ~ x (simple x-y relationship)

  • y ~ x | group (with grouping)

  • y ~ x | group ~ facet (with grouping and single facet)

  • y ~ x | group ~ facet1 + facet2 (with grouping and multiple facets)

Value

A list with four components:

  • y: character string, the dependent variable name (e.g., measure value)

  • x: character string, the independent variable name (typically time or visit)

  • group: character string or NULL, the grouping variable name if specified

  • facets: character vector or NULL, the faceting variable name(s) if specified

Details

The function processes formula components in the following order:

  1. Extracts the y-variable (dependent variable) from the left-hand side of the first '~'

  2. Extracts the x-variable (typically time/visit) from before the '|' on the right-hand side

  3. If present, extracts the grouping variable after the '|'

  4. If present, extracts faceting variables after the second '~'

Multiple faceting variables should be separated by '+'. The function is designed to work with both continuous and categorical x-variables, supporting the flexible plotting capabilities of the zzlongplot package.

Examples

# Simple longitudinal measurement
parse_formula(score ~ visit)
#> $y
#> [1] "score"
#> 
#> $x
#> [1] "visit"
#> 
#> $group
#> NULL
#> 
#> $facets
#> NULL
#> 

# With treatment group
parse_formula(score ~ visit | treatment)
#> $y
#> [1] "score"
#> 
#> $x
#> [1] "visit"
#> 
#> $group
#> [1] "treatment"
#> 
#> $facets
#> NULL
#> 

# With treatment group and site facet
parse_formula(score ~ visit | treatment ~ site)
#> $y
#> [1] "score"
#> 
#> $x
#> [1] "visit"
#> 
#> $group
#> [1] "treatment"
#> 
#> $facets
#> [1] "site"
#> 

# With treatment group and multiple facets
parse_formula(score ~ visit | treatment ~ site + gender)
#> $y
#> [1] "score"
#> 
#> $x
#> [1] "visit"
#> 
#> $group
#> [1] "treatment"
#> 
#> $facets
#> [1] "site"   "gender"
#>