Constructs a shifted index reference for ordered sets (e.g., YEAR, SEASON).
Used for GAMS lag/lead operators like y-1 (previous year) or y+1 (next year).
Negative offset represents backward shift (lag), positive represents forward shift (lead).
Usage
ast_shift(symbol, offset, ...)
Arguments
- symbol
A character string representing the index symbol (e.g., "y", "ls", "ld").
- offset
An integer offset. Negative for backward shift (e.g., -1 for y-1), positive for forward shift (e.g., +1 for y+1).
Value
A shift S3 object (subclass of ast).
Examples
ast_shift("y", -1) # y-1 (previous year)
#> <ast>
#> [1] "y" "-1"
ast_shift("y", 1) # y+1 (next year)
#> <ast>
#> [1] "y" "1"
ast_shift("ls", -1) # ls-1 (previous season)
#> <ast>
#> [1] "ls" "-1"
