# IDF [Obj,Obj]

### Description
Computes IDF of `T` in `DT`  object pairs

### Input
- `DT [OBJ,OBJ]`: a list of object-object pairs.
### Output
- `DTidf [OBJ,OBJ]`: IDF-weighted pairs
- `Tidf [OBJ]`: IDF-weighted objects

### Parameters
- `IDF`: the IDF weighting scheme variant. `P(t|d) = t_nd / d_N`, `n_D = number of documents`, `t_nd = number of documents with term t`.
  - `BASIC`: non-logaritmic IDF (`1 - P(t|d) = 1 - (t_nd / d_N)`)
  - `IDFB`: non-smoothed IDF (`-log( P(t|d) = log ( d_N / t_nd )`)
  - `IDFP`: smoothed IDF (`log( (d_N - t_nd + 0.5) / (t_nd + 0.5) )`)
  - `IDFPA`: alpha-smoothed IDF (`alpha * IDFP`) 
Output scores can be [aggregated](docs://score_aggregation) and/or [normalised](docs://score_normalisation).