Description
Subtract one from a valid index and reduce modulo positive length for predecessor.
An index map tracks where one item lands after a structural list operation. Zero-based element positions differ from boundary cursors, and missing destinations need a sentinel.
Subtract one from a valid index and reduce modulo positive length for predecessor.
When to use this tool
Use List Cycle One Step Predecessor when you need one item's position under an explicit list transformation.
How to use the tool and how it works
- Enter length, index. Use the units and conventions below.
- Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
- Read predecessor. Use the output convention below.
Subtract one from a valid index and reduce modulo positive length for predecessor.1
| Input field | Meaning and units | Accepted input |
|---|---|---|
| length | Length, count, or code length under the stated domain and unit convention. | Required. Safe integer; minimum 1. |
| index | Zero-based position or diagnostic index under the calculation above; inspect any sentinel policy. | Required. Safe integer; minimum 0. |
| Output field | Interpretation | Returned shape |
|---|---|---|
| predecessor | Predecessor | Required. Safe integer. |
{
"input": {
"length": 5,
"index": 0
},
"output": {
"predecessor": 4
}
}Limitations and assumptions
- A singleton is its own predecessor; no list value is retrieved.
- Inputs must satisfy both the field types and the cross-field conditions above. Invalid input causes an error; a successful false flag or documented placeholder is a different result.
- Numeric calculations use binary floating point unless exact decimal-integer arithmetic is explicitly stated. No hidden measurement tolerance or decimal rounding is added; nonfinite or out-of-contract results are rejected. Numeric signed-zero results are exported as zero.
Alternative or Complementary methods
Apply the operation to a small representative list when checking a consuming language's indexing and slicing semantics.
References
Similar or alternative tools
- Cyclic List Neighbor Pair
Require a nonempty values list, normalize signed index modulo its length, and return that item and its cyclic successor.
- Cyclic List Phase Remainder
Subtract origin from index using exact integers and reduce modulo positive length into phase in [0, length).