R: Binary Search

binsearch {gtools} | R Documentation

## Binary Search

### Description

Search within a specified range to locate an integer parameter which results in the the specified monotonic function obtaining a given value.

### Usage

    binsearch(fun, range, ..., target = 0, lower = ceiling(min(range)),
              upper = floor(max(range)), maxiter = 100, showiter = FALSE)

### Arguments

`fun` | Monotonic function over which the search will be performed.
`range` | 2-element vector giving the range for the search.
`...` | Additional parameters to the function `fun`.
`target` | Target value for `fun`. Defaults to 0.
`lower` | Lower limit of search range. Defaults to `min(range)`.
`upper` | Upper limit of search range. Defaults to `max(range)`.
`maxiter` | Maximum number of search iterations. Defaults to 100.
`showiter` | Boolean flag indicating whether the algorithm state should be printed at each iteration. Defaults to FALSE.

### Details

This function implements an extension to the standard binary search algorithm for searching a sorted list. The algorithm has been extended to cope with cases where an exact match is not possible, to detect whether that the function may be monotonic increasing or decreasing and act appropriately, and to detect when the target value is outside the specified range.

The algorithm initializes two variable `lo` and `high` to the extremes values of `range`. It then generates a new value `center` halfway between `lo` and `hi`. If the value of `fun` at `center` exceeds `target`, it becomes the new value for `lo`, otherwise it becomes the new value for `hi`. This process is iterated until `lo` and `hi` are adjacent. If the function at one or the other equals the target, this value is returned, otherwise `lo`, `hi`, and the function value at both are returned.

Note that when the specified target value falls between integers, the two closest values are returned. If the specified target falls outside of the specified `range`, the closest endpoint of the range will be returned, and an warning message will be generated. If the maximum number if iterations was reached, the endpoints of the current subset of the range under consideration will be returned.

### Value

A list containing:

`call` | How the function was called.
`numiter` | The number of iterations performed
`flag` | One of the strings, "Found", "Between Elements", "Maximum number of iterations reached", "Reached lower boundary", or "Reached upper boundary."
`where` | One or two values indicating where the search terminated.
`value` | Value of the function `fun` at the values of `where`.

### Note

This function often returns two values for `where` and `value`. Be sure to check the `flag` parameter to see what these values mean.

### Author(s)

Gregory R. Warnes greg@warnes.net

### See Also

`optim`, `optimize`, `uniroot`

### Examples

    ### Toy examples

    # search for x=10
    binsearch( function(x) x-10, range=c(0,20) )

    # search for x=10.1
    binsearch( function(x) x-10.1, range=c(0,20) )

    ### Classical toy example

    # binary search for the index of 'M' among the sorted letters
    fun <- function(X) ifelse(LETTERS[X] > 'M', 1,
                              ifelse(LETTERS[X] < 'M', -1, 0 ) )

    binsearch( fun, range=1:26 )
    # returns $where=13
    LETTERS[13]

    ### Substantive example, from genetics
    ## Not run:
    library(genetics)
    # Determine the necessary sample size to detect all alleles with
    # frequency 0.07 or greater with probability 0.95.
    power.fun <- function(N) 1 - gregorius(N=N, freq=0.07)$missprob

    binsearch( power.fun, range=c(0,100), target=0.95 )

    # equivalent to
    gregorius( freq=0.07, missprob=0.05)

    ## End(Not run)

[Package gtools version 3.8.1 Index]