# fftw_flags

set method for fft planner algorithm selection

### Syntax

[a,[S]]=fftw_flags(flag)

### Arguments

- flag
a string or an integer. Specifies the planner algorithm. See below.

- a
an integer. The planner code. See below.

- S
a character string. The planner name.

### Description

This function enables the selection of the algorithm used to determine the fftw planner algorithm. The planner is used to determine an efficient way to compute the fft.

Warning: the default value "FFTW_ESTIMATE" gives generally
quite efficient plans. Try to change it only if the fft efficiency is
really important and you have a lot of similar calls to do. |

Accepted entries are :

{FFTW_ESTIMATE} or 64. Specifies that, instead of actual measurements of different algorithms, a simple heuristic is used to pick a (probably sub-optimal) plan quickly. With this flag, the input/output arrays are not overwritten during planning. It is the default value

FFTW_MEASURE or 0. tells FFTW to find an optimized plan by actually computing several FFTs and measuring their execution time. Depending on your machine, this can take some time (often a few seconds).

FFTW_PATIENT or 32. It is like "FFTW_MEASURE", but considers a wider range of algorithms and often produces a “more optimal” plan (especially for large transforms), but at the expense of several times longer planning time (especially for large transforms).

FFTW_EXHAUSTIVE or 8. It is like "FFTW_PATIENT", but considers an even wider range of algorithms, including many that we think are unlikely to be fast, to produce the most optimal plan but with a substantially increased planning time.

Remark : when using FFTW_MEASURE/FFTW_PATIENT/FFTW_EXHAUSTIVE you must call fftw twice. (first call for initialization, second and others calls for computation)

### Examples

A=rand(1, 2^9 + 2^15); fftw_forget_wisdom(); fftw_flags("FFTW_ESTIMATE"); timer(); y=fft(A); timer()//first call determines the plan timer(); y=fft(A); timer() //subsequent similar calls fftw_flags("FFTW_MEASURE"); fftw_forget_wisdom(); timer(); y=fft(A); timer()//first call determines the plan, this takes about 5min timer(); y=fft(A); timer() //subsequent similar calls

### See also

- fft — Direct or inverse Fast Fourier Transform of a vector, matrix, or hypermatrix
- set_fftw_wisdom — set fftw wisdom
- get_fftw_wisdom — return fftw wisdom
- fftw_forget_wisdom — Reset fftw wisdom

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