1: $n$ – int64int32nag_int scalar: Default: the dimension of the array x.
$n$ , the number of data values. The largest prime factor of n must not exceed $19$ , and the total number of prime factors of n, counting repetitions, must not exceed $20$ .

Constraint: $n > 1$ .

Output Parameters

1: $x (n)$ – double array: The discrete Fourier transform stored in Hermitian form. If the components of the transform ${\hat{z}}_{k}$ are written as $a_{k} + i b_{k}$ , and if x is declared with bounds $(0 : n - 1)$ in the function from which nag_sum_fft_real_1d_nowork (c06ea) is called, then for $0 \leq k \leq n / 2$ , $a_{k}$ is contained in $x (k)$ , and for $1 \leq k \leq (n - 1) / 2$ , $b_{k}$ is contained in $x (n - k)$ . (See also Real transforms in the C06 Chapter Introduction and Example.)
2: $ifail$ – int64int32nag_int scalar: $ifail = 0$ unless the function detects an error (see Error Indicators and Warnings).

Error Indicators and Warnings

Errors or warnings detected by the function:

$ifail = 1$: At least one of the prime factors of n is greater than $19$ .

$ifail = 2$: n has more than $20$ prime factors.

$ifail = 3$

On entry,

n \leq 1

$ifail = 4$: An unexpected error has occurred in an internal call. Check all function calls and array dimensions. Seek expert help.

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.

$ifail = - 999$: Dynamic memory allocation failed.

Accuracy

Some indication of accuracy can be obtained by performing a subsequent inverse transform and comparing the results with the original sequence (in exact arithmetic they would be identical).

Further Comments

The time taken is approximately proportional to

n \times \log (n)

, but also depends on the factorization of

n

. nag_sum_fft_real_1d_nowork (c06ea) is faster if the only prime factors of

n

are

2

3

5

; and fastest of all if

n

is a power of

2

On the other hand, nag_sum_fft_real_1d_nowork (c06ea) is particularly slow if

n

has several unpaired prime factors, i.e., if the ‘square-free’ part of

n

has several factors. For such values of

n

, nag_sum_fft_real_1d_rfmt (c06fa) (which requires additional double workspace) is considerably faster.

Example

This example reads in a sequence of real data values and prints their discrete Fourier transform (as computed by nag_sum_fft_real_1d_nowork (c06ea)), after expanding it from Hermitian form into a full complex sequence. It then performs an inverse transform using nag_sum_conjugate_hermitian_rfmt (c06gb) followed by nag_sum_fft_hermitian_1d_nowork (c06eb), and prints the sequence so obtained alongside the original data values.

Open in the MATLAB editor: c06ea_example

function c06ea_example


fprintf('c06ea example results\n\n');

% real data
n = 7;
x = [0.34907  0.54890  0.74776  0.94459  1.13850  1.32850  1.51370];

% transform
[xt, ifail] = c06ea(x);

% get result in form useful for printing.
zt = nag_herm2complex(xt);
disp('Discrete Fourier Transform of x:');
disp(transpose(zt));

% restore by conjugating and backtransforming
xt(floor(n/2)+2:n) = -xt(floor(n/2)+2:n);
[xr, ifail] = c06eb(xt);

fprintf('Original sequence as restored by inverse transform\n\n');
fprintf('       Original   Restored\n');
for j = 1:n
  fprintf('%3d   %7.4f    %7.4f\n',j, x(j),xr(j));
end




function [z] = nag_herm2complex(x);
  n = size(x,2);
  z(1) = complex(x(1));
  for j = 2:floor((n-1)/2) + 1
    z(j) = x(j) + i*x(n-j+2);
    z(n-j+2) = x(j) - i*x(n-j+2);
  end
  if (mod(n,2)==0)
    z(n/2+1) = complex(x(n/2+1));
  end

c06ea example results

Discrete Fourier Transform of x:
   2.4836 + 0.0000i
  -0.2660 + 0.5309i
  -0.2577 + 0.2030i
  -0.2564 + 0.0581i
  -0.2564 - 0.0581i
  -0.2577 - 0.2030i
  -0.2660 - 0.5309i

Original sequence as restored by inverse transform

       Original   Restored
  1    0.3491     0.3491
  2    0.5489     0.5489
  3    0.7478     0.7478
  4    0.9446     0.9446
  5    1.1385     1.1385
  6    1.3285     1.3285
  7    1.5137     1.5137

PDF version (NAG web site, 64-bit version, 64-bit version)

Chapter Contents

Chapter Introduction

NAG Toolbox