Bug of matlab coder (2013a) for IFFT?

Dear all,

I am trying to use the matlab coder to generate C++ code for IFFT. However, results from matlab and C++ code have greate difference. Could anyone help me to take a look at this problem?

The matlab code to do IFFT is as follows:

z=ifft(y,[],1);

The data of y are as follows:

487924.35768878186  
481026.24091831001  
478940.94803639117  
480926.64214804117  
478909.92810891091  
475591.66558611894  
472961.12419697485  
478082.98773687298  
475963.06721005152  
471624.91585664771  
473035.01798060897  
470885.04387918825  
465204.72500935785  
465559.98854017764  
468284.20173525374  
467779.54004903068  
467358.62894582795  
461658.53205369966  
461058.87787927384  
461185.53453021083  
455868.37107306172  
462911.95616388740  
459834.12223788921  
458194.44688116410  
456595.68046306813  
451450.72108931403  
448915.17218837264  
451224.11133871856  
448603.78107835044  
442943.27244258055  
445878.56931863062  
451008.71128154272  
451212.54731590918  
455779.62766682287  
454335.89646260266  
450941.74302688910  
451440.15563114837  
451448.07972477260  
449064.57034377591  
448331.01187180565  
449871.73269351595  
449496.69114039827  
445229.11575907044  
444080.89526586136  
443461.78518676467  
437449.62073759746  
443516.19085396524  
438574.10116169654  
434403.16106846917  
433751.71037962160  
426817.29095258669  
422633.17624841497  
418137.67043422104  
413457.43016094179  
411045.60664053162  
403724.29173170537  
392643.02660994284  
380096.57725000178  
357723.60756075662  
332111.19753204676  
317106.73298398359  
311326.13889686763  
311656.30946454400  
310753.51039134728  
313637.04296473821  
311065.22361899650  
310994.64764692046  
309178.16192473430  
310382.78521443135  
310153.53409929451  
308257.54974676436  
309423.90546237369  
310570.45094392239  
308946.62377444573  
310466.56800921442  
309782.13247536187  
320356.73878161400  
347570.20226357155  
368694.03200706805  
384869.13643517962  
398566.44765287120  
407737.20091712585  
412582.04329780280  
418070.05794431345  
422272.98431790201  
424134.63093139383  
430451.22874984005  
433175.44194491615  
437468.43240701430  
438150.54869395244  
439138.45124420297  
443049.79674182995  
441973.82723235723  
440130.96007651964  
447706.58685186348  
449457.42500166676  
454555.58068421762  
449613.45878018625  
452690.29414154071  
451600.11924471724  
452466.32575548667  
454145.23503918055  
449159.24071435159  
456451.88715437474  
450130.32872447267  
450516.57997100998  
452118.30987127440  
449933.06388969702  
452284.90910795948  
452236.75252272276  
453562.39540488424  
455035.18161905993  
456251.33671980369  
457868.23836029979  
460014.57253253541  
462557.04696245730  
461771.33764667553  
462317.90683617146  
466403.38912295533  
467626.46975267958  
466153.36142087687  
467724.10360542370  
470285.06758713548  
469912.31306916947  
469812.71429890068  
469016.11740732624  
470695.70313803683  
473460.18103648809  
475000.22541118169  
476990.88013428752  
477611.31089565484  
475078.08124162792  
479902.43694122758  
478081.34493697528  
480799.95328534150  
484503.82281941688

Thanks, Tang Laoya

Answer 1

[Sean de Wolski]

I'm not surprised to see a small difference. Building a mex file from this:

function y = doIFFT(x)

      y = ifft(x,2^nextpow2(numel(x)),1);
      %I had to add the nextpow2 part to get it to build
  end

Then looking at the differences:

norm(doIFFT(x)-doIFFT_mex(x))
ans =
   4.695591604029106e-11

The difference is very small. In fact it's close to:

eps(487924.35768878186)

So this is just a small numerical round-off difference in the approach taken by the libraries MATLAB sits on and the generated c-code.

Answer 2

[laoya]

Hi Sean de Wolski,

Thanks for your quick reply. However, from here the difference is not so small. The results from matlab are: 1.0e+005 *

4.313512092938006                     
0.300430732843556 - 0.005783095388433i
-0.147813810099868 + 0.000951173782191i
0.133108569308995 - 0.003373860065190i
-0.061656447655613 + 0.003328454544711i
0.027192760859384 - 0.000603671006620i
-0.005797878506124 - 0.000492192018536i
-0.001061055121786 + 0.001998424724582i
0.021577859002924 - 0.002456894682518i
-0.021811429158957 + 0.003207842695919i
0.017735324252322 - 0.003028416146602i
-0.014305192477317 - 0.000474115939438i
0.013515213478997 - 0.002166798336642i
-0.004684409887853 + 0.003175414468610i
-0.000443185935397 + 0.000308035524811i
0.003853153787452 - 0.002023631130418i
-0.005850698801703 + 0.000216651950420i
0.012856303304599 - 0.001080645604734i
-0.005791634751852 + 0.002526085886894i
0.005578657992319 - 0.000926823744157i
-0.000112906707825 + 0.001805996486923i
0.001274493397202 + 0.000127337940853i
-0.001556944764524 - 0.000427023062860i
-0.002102718086437 - 0.000349736278996i
0.002944493706193 - 0.003170105190536i
-0.001861635715701 - 0.000525536225213i
0.002975662968139 - 0.000719539330124i
-0.002191858687055 + 0.000974062580574i
0.000491769705110 - 0.001428164102981i
-0.000607238680120 - 0.000256915753116i
0.002503276464045 + 0.001201049865073i
0.000502580943466 - 0.000828435382671i
-0.001255667660083 + 0.000254113854892i
0.001911608261996 - 0.002132432317272i
-0.000122452068500 - 0.001590327883673i
0.000678876401114 - 0.002190333520453i
0.000850110098716 + 0.001294448469562i
0.002221809217307 - 0.000687101621501i
0.001690063348324 - 0.000139630162640i
0.001283562042216 + 0.000855341907948i
0.002625543849526 - 0.001037104423102i
0.000443377947227 + 0.002073096558817i
-0.000133656504791 - 0.000054353863241i
-0.001507985869583 - 0.001123830171823i
0.001803796928052 - 0.000319702526871i
0.000802884469623 + 0.001769241259875i
0.000017848403353 + 0.000495263516297i
-0.000878046846166 - 0.002090322061183i
-0.001684884158707 + 0.000640259092086i
0.000349058948051 - 0.002674201272202i
0.001317964739399 - 0.000682460501570i
0.000136079066329 - 0.000196295344922i
-0.000751139021633 - 0.000771625053355i
0.001165570869108 + 0.001735364213629i
-0.000174024655019 + 0.001713751491137i
-0.000142209151696 - 0.001924788489301i
-0.001616076044371 - 0.000170224894442i
0.000779708646816 + 0.001697818033402i
0.002037664934465 - 0.000305846247259i
0.000752313211467 - 0.000118969236097i
-0.001010826774948 + 0.001686222917862i
-0.000086432763696 + 0.000856642401964i
0.001333583110647 + 0.000499768614078i
-0.000941810898597 - 0.000492021314954i
0.000162814303691 + 0.000686033402377i
0.001251368703072 + 0.000230732674254i
0.000737206223204 + 0.000479604300745i
-0.000537515690459 + 0.000473585154103i
0.000927698765764                     
-0.000537515690459 - 0.000473585154103i
0.000737206223204 - 0.000479604300745i
0.001251368703072 - 0.000230732674254i
0.000162814303691 - 0.000686033402377i
-0.000941810898597 + 0.000492021314954i
0.001333583110647 - 0.000499768614078i
-0.000086432763696 - 0.000856642401964i
-0.001010826774948 - 0.001686222917862i
0.000752313211467 + 0.000118969236097i
0.002037664934465 + 0.000305846247259i
0.000779708646816 - 0.001697818033402i
-0.001616076044371 + 0.000170224894442i
-0.000142209151696 + 0.001924788489301i
-0.000174024655019 - 0.001713751491137i
0.001165570869108 - 0.001735364213629i
-0.000751139021633 + 0.000771625053355i
0.000136079066329 + 0.000196295344922i
0.001317964739399 + 0.000682460501571i
0.000349058948051 + 0.002674201272202i
-0.001684884158707 - 0.000640259092086i
-0.000878046846166 + 0.002090322061183i
0.000017848403353 - 0.000495263516297i
0.000802884469623 - 0.001769241259875i
0.001803796928052 + 0.000319702526871i
-0.001507985869583 + 0.001123830171823i
-0.000133656504791 + 0.000054353863241i
0.000443377947227 - 0.002073096558817i
0.002625543849526 + 0.001037104423102i
0.001283562042216 - 0.000855341907948i
0.001690063348324 + 0.000139630162640i
0.002221809217307 + 0.000687101621501i
0.000850110098716 - 0.001294448469563i
0.000678876401114 + 0.002190333520453i
-0.000122452068500 + 0.001590327883673i
0.001911608261996 + 0.002132432317272i
-0.001255667660083 - 0.000254113854892i
0.000502580943466 + 0.000828435382671i
0.002503276464045 - 0.001201049865073i
-0.000607238680120 + 0.000256915753116i
0.000491769705110 + 0.001428164102981i
-0.002191858687055 - 0.000974062580574i
0.002975662968139 + 0.000719539330124i
-0.001861635715701 + 0.000525536225213i
0.002944493706193 + 0.003170105190536i
-0.002102718086437 + 0.000349736278996i
-0.001556944764524 + 0.000427023062860i
0.001274493397202 - 0.000127337940853i
-0.000112906707825 - 0.001805996486923i
0.005578657992319 + 0.000926823744157i
-0.005791634751852 - 0.002526085886894i
0.012856303304599 + 0.001080645604734i
-0.005850698801703 - 0.000216651950420i
0.003853153787452 + 0.002023631130418i
-0.000443185935397 - 0.000308035524811i
-0.004684409887853 - 0.003175414468610i
0.013515213478997 + 0.002166798336642i
-0.014305192477317 + 0.000474115939438i
0.017735324252322 + 0.003028416146602i
-0.021811429158957 - 0.003207842695919i
0.021577859002924 + 0.002456894682518i
-0.001061055121786 - 0.001998424724582i
-0.005797878506124 + 0.000492192018536i
0.027192760859384 + 0.000603671006620i
-0.061656447655613 - 0.003328454544711i
0.133108569308995 + 0.003373860065190i
-0.147813810099868 - 0.000951173782191i
0.300430732843556 + 0.005783095388433i

But results from C++ code are as follows:

   72422.083779590306     -21175.927029026894       
   11125.992351540337     3106.1219836735977       
   6843.8805862924828     3554.4005362181247       
   4356.1650364638035     2570.7688328224426       
   3420.3547292047629     0.31787260351634089       
   4700.1741689654591     -2577.2663742471132       
   7910.0731580255633     -3855.9217257203227       
   13419.434500006109     -4115.2819077652211       
   -13282.696522193037     -30704.471141475420       
   7971.4850412689193     3265.5656871257138       
    5862.2579664486675     4181.4237102004063       
    3696.9860231590023     2941.4488545757381       
    3051.9471877097153     129.42359763279092       
    4852.0985869896149     -2863.1910672093150       
    9114.7261030580539     -5404.2257888381619       
    48675.808343791687     -29709.095883131220       
    -34398.075003605700     -18366.992612829283       
    5138.7854450924133     4996.7355815242800       
    4654.2058745860568     4864.4564801580163       
    1543.8705522082814     4071.7857881112859       
    1819.7875797722779     457.90605746160725       
    2830.8298912636742     -2275.1392367139283       
    9716.4329594613992     -7783.0865968464159       
    -3630.7897718976387     9142.7833067804131       
    -8845.6457668685016     414.34287820314398       
    4992.1599224602105     4868.9723525938753       
    3229.5822994476157     4878.3164657138086       
    736.67044680706999     3430.1969587796079       
    957.35710922778412     -1031.5653556290436       
    2480.5289862458612     -5208.5851937693360       
    25700.781958828022     -39844.385866482604       
    -394.49267492773350     7591.7737507502407       
    -20665.267900238770     27776.650484342088       
    -1473.5210380044171     4785.8222036708621       
    60.051900129337277     7119.4630475685044       
    -830.62218704141992     3490.4524725087017       
    -1074.6118697358522     -875.00645669774576       
    2877.6149243907121     -9498.2795773337984       
    -9229.4478008981059     18304.491631238121       
    308.96239530547393     4249.9275609692568       
    -2404.7107375548189     8985.3059745066676       
    2132.7840149464282     4447.5810777565584       
    -477.38110141003563     4978.7152523136738       
    -2549.9004910493386     2030.8234215058337       
    -3073.6612221284904     -4054.7995961807842       
    5432.9358533986097     -38643.332288489612       
    -3605.4539005091733     9775.7604208193752       
    45.552641515049103     4113.9140568298253       
    221.05280647733633     12207.662006532393       
    -278.76886434015228     7710.8290184780490       
    -1890.2508641515733     3916.7204912494567       
    -4536.8419186779274     2104.3932757967714       
    -5059.1810510076248     -8576.6255176689756       
    -6110.5398786129972     29323.824846673553       
    -2794.5666980626256     4779.6064599904876       
    -198.13055532286543     1728.5801399849456       
    1797.5246407960512     3609.7437062598865       
    -576.50490459068919     3309.6370867571586       
    -3325.1497702358815     1971.7321719294284       
    -6523.0645758753290     -1698.9721656553158       
    -12240.582997565247     -30058.889007986600       
    -2717.1305815325768     11518.450610493081       
    -1454.2548795338439     4192.0167255739652       
    52.786090168379495     1026.7177630023957       
    -10338.642087783273     13642.323636409885       
    -6812.9003180914069     2866.9799795768654       
    -3792.8505170487479     -230.78805726075026       
    -7213.3223499798705     -7679.6190964047646       
    321.28615576961795     36259.999009415274       
    -4383.7395961326183     3415.7340629150949       
    -2309.0277783778693     -1088.5516338423911       
    -180.79148605519302     -1836.5173221714188       
    743.63999390828008     -6101.2152527950202       
    -1134.2989609607107     -524.23819381373767       
    -4792.5955569409125     -2348.0148101847226       
    -14766.177531123119     -21182.107722402230       
    -439.08920376328257     10344.607622734815       
    -2542.4461870583073     599.14559236859304       
    -845.19793586519643     -2599.3865384547471       
    1187.5292526634566     -2785.6446093624072       
    -9038.0486912758315     -7065.9521600213020       
    80.625318829726467     10.714344970158622       
    -5653.8415272376815     -4732.4052208226767       
    21190.907021163606     37007.587554493541       
    -852.94447933068420     3637.7765490408310       
    -2098.0122985887024     -2686.0869132157095       
    261.25956450206797     -4322.5060757986057       
    1970.2539612731198     -4518.7374882269842       
    -7274.3941270798468     -4518.5287317769698       
    -330.11620341411918     -2225.0447706025757       
    -11852.288982194095     -10043.221245743609       
    6679.5226265230413     7775.3855646021684       
    1377.9236897985454     1025.0031917079912       
    113.67914951110266     -3664.2480443867912       
    3090.0901443598441     -4740.7616094401865       
    4285.8937306032358     -4849.8787958145267       
    -24356.495771448343     14480.960943266133       
    -5295.8195730284415     -3506.5828383575276       
    40818.185859963669     30465.471866954791       
    3565.6822875299658     3109.8019153641180       
      997.99930535255316     -1037.8513161753970       
      1596.4431752630942     -4058.2212596692852       
      2371.8530430438618     -6235.0706981591920       
      5157.0953715735923     -5258.0134317301263       
      -5483.7061459089282     6393.6879372332833       
      -2472.6887179699484     -4024.0136908333516       
      11191.498476511286     4243.0178602305305       
      4326.6288886882658     1068.9429182750932       
      2523.3147519425652     -1914.9842759916771       
      4750.3542365514413     -4293.9197562996178       
      4884.6046593881738     -6045.4671383659079       
      7723.2247447638074     -4995.1376654136848       
      -8572.1840474269666     38548.652251609477       
      57941.834946725779     12129.156253256031       
      8352.6956653903162     2187.0454560172848       
      3106.6583415751998     386.09826727500115       
      3429.7058523298415     -1988.1225111991816       
      1628.0516246254488     -4714.1787317279168       
      5902.3957729349340     -5738.5628963074414       
      8049.8073106827114     -4652.4255896710165       
      9145.3967079252434     32125.876528317396       
      12010.057611526385     766.20021100067629       
      8174.8653008944102     1558.3663819684973       
      4622.4202209866007     570.29637943909381       
      5678.5596863576793     -2317.1898630674232       
      3994.7403360780213     -4371.9898884749864       
      8688.5972400898154     -5958.6490558767273       
      10928.004975388818     -5130.2471041175613       
      2965.8936925455027     8488.5318855891073       
      1091.4535707318753     -1704.1399129821730       
      -3792.8505170487479     -230.78805726075026       
      -7213.3223499798705     -7679.6190964047646       
      0.00000000000000000     0.00000000000000000       
      0.00000000000000000     0.00000000000000000       
      0.00000000000000000     0.00000000000000000       
      0.00000000000000000     0.00000000000000000

C++ codes generated by matlab coder are:

/// ifft.c

/* * ifft.c * * Code generation for function 'ifft' * * C source code generated on: Thu Aug 01 16:42:35 2013 * */

/* Include files */

#include "rt_nonfinite.h"

#include "Dissociation.h"

#include "ifft.h"

#include "Dissociation_emxutil.h"

#include "Dissociation_rtwutil.h"

/* Function Definitions */

/* * */ void b_ifft(const emxArray_creal_T *x, emxArray_creal_T *y)

{

uint32_T sz[2];

int32_T ju;

int32_T minval;

emxArray_real_T *costab1q;

int32_T nd2;

int32_T ixDelta;

int32_T nRowsD2;

int32_T nRowsD4;

int32_T lastChan;

real_T e;

int32_T k;
emxArray_real_T *costab;
emxArray_real_T *sintab;
int32_T ix;
int32_T chanStart;
int32_T i;
boolean_T tst;
real_T temp_re;
real_T temp_im;
int32_T iDelta;
int32_T iDelta2;
int32_T iheight;
int32_T ihi;
real_T twid_im;
for (ju = 0; ju < 2; ju++) {
  sz[ju] = (uint32_T)x->size[ju];
}

ju = y->size[0] * y->size[1];
y->size[0] = x->size[0];
y->size[1] = (int32_T)sz[1];
emxEnsureCapacity((emxArray__common *)y, ju, (int32_T)sizeof(creal_T));
if ((x->size[0] == 0) || (x->size[1] == 0)) {
} else {
  minval = x->size[0];
  b_emxInit_real_T(&costab1q, 2);
  nd2 = (x->size[0] - minval) + 1;
  if (1 >= nd2) {
    ixDelta = 1;
  } else {
    ixDelta = nd2;
  }

    ju = x->size[0];
    nRowsD2 = ju / 2;
    nRowsD4 = nRowsD2 / 2;
    lastChan = x->size[0] * (div_s32(x->size[0] * x->size[1], x->size[0]) - 1);
    e = 6.2831853071795862 / (real_T)x->size[0];
    ju = costab1q->size[0] * costab1q->size[1];
    costab1q->size[0] = 1;
    costab1q->size[1] = nRowsD4 + 1;
    emxEnsureCapacity((emxArray__common *)costab1q, ju, (int32_T)sizeof(real_T));
    costab1q->data[0] = 1.0;
    nd2 = nRowsD4 / 2;
    for (k = 1; k <= nd2; k++) {
      costab1q->data[k] = cos(e * (real_T)k);
    }

    for (k = nd2 + 1; k < nRowsD4; k++) {
      costab1q->data[k] = sin(e * (real_T)(nRowsD4 - k));
    }

    b_emxInit_real_T(&costab, 2);
    b_emxInit_real_T(&sintab, 2);
    costab1q->data[nRowsD4] = 0.0;
    nd2 = (costab1q->size[1] - 1) << 1;
    ju = costab->size[0] * costab->size[1];
    costab->size[0] = 1;
    costab->size[1] = nd2 + 1;
    emxEnsureCapacity((emxArray__common *)costab, ju, (int32_T)sizeof(real_T));
    ju = sintab->size[0] * sintab->size[1];
    sintab->size[0] = 1;
    sintab->size[1] = nd2 + 1;
    emxEnsureCapacity((emxArray__common *)sintab, ju, (int32_T)sizeof(real_T));
    costab->data[0] = 1.0;
    sintab->data[0] = 0.0;
    for (k = 1; k < costab1q->size[1]; k++) {
      costab->data[k] = costab1q->data[k];
      sintab->data[k] = costab1q->data[(costab1q->size[1] - k) - 1];
    }

    for (k = costab1q->size[1]; k <= nd2; k++) {
      costab->data[k] = -costab1q->data[nd2 - k];
      sintab->data[k] = costab1q->data[(k - costab1q->size[1]) + 1];
    }

    emxFree_real_T(&costab1q);
    ix = 0;
    for (chanStart = 0; chanStart <= lastChan; chanStart += x->size[0]) {
      ju = 0;
      nd2 = chanStart;
      for (i = 1; i < minval; i++) {
        y->data[nd2] = x->data[ix];
        nd2 = x->size[0];
        tst = TRUE;
        while (tst) {
          nd2 >>= 1;
          ju ^= nd2;
          tst = ((ju & nd2) == 0);
        }

        nd2 = chanStart + ju;
        ix++;
      }

      y->data[nd2] = x->data[ix];
      ix += ixDelta;
      ju = (chanStart + x->size[0]) - 2;
      if (x->size[0] > 1) {
        for (i = chanStart; i <= ju; i += 2) {
          temp_re = y->data[i + 1].re;
          temp_im = y->data[i + 1].im;
          y->data[i + 1].re = y->data[i].re - y->data[i + 1].re;
          y->data[i + 1].im = y->data[i].im - y->data[i + 1].im;
          y->data[i].re += temp_re;
          y->data[i].im += temp_im;
        }
      }

      iDelta = 2;
      iDelta2 = 4;
      k = nRowsD4;
      iheight = 1 + ((nRowsD4 - 1) << 2);
      while (k > 0) {
        i = chanStart;
        ihi = chanStart + iheight;
        while (i < ihi) {
          nd2 = i + iDelta;
          temp_re = y->data[nd2].re;
          temp_im = y->data[nd2].im;
          y->data[i + iDelta].re = y->data[i].re - y->data[nd2].re;
          y->data[i + iDelta].im = y->data[i].im - y->data[nd2].im;
          y->data[i].re += temp_re;
          y->data[i].im += temp_im;
          i += iDelta2;
        }

        nd2 = chanStart + 1;
        for (ju = k; ju < nRowsD2; ju += k) {
          e = costab->data[ju];
          twid_im = sintab->data[ju];
          i = nd2;
          ihi = nd2 + iheight;
          while (i < ihi) {
            temp_re = e * y->data[i + iDelta].re - twid_im * y->data[i + iDelta]
              .im;
            temp_im = e * y->data[i + iDelta].im + twid_im * y->data[i + iDelta]
              .re;
            y->data[i + iDelta].re = y->data[i].re - temp_re;
            y->data[i + iDelta].im = y->data[i].im - temp_im;
            y->data[i].re += temp_re;
            y->data[i].im += temp_im;
            i += iDelta2;
          }

          nd2++;
        }

        k /= 2;
        iDelta = iDelta2;
        iDelta2 <<= 1;
        iheight -= iDelta;
      }
    }

    emxFree_real_T(&sintab);
    emxFree_real_T(&costab);
    if (y->size[0] > 1) {
      e = 1.0 / (real_T)y->size[0];
      ju = y->size[0] * y->size[1];
      emxEnsureCapacity((emxArray__common *)y, ju, (int32_T)sizeof(creal_T));
      nd2 = y->size[0];
      ju = y->size[1];
      nd2 *= ju;
      for (ju = 0; ju < nd2; ju++) {
        y->data[ju].re *= e;
        y->data[ju].im *= e;
      }
    }
  }
}

/* * */

void ifft(const emxArray_real_T *x, emxArray_creal_T *y)

{

int32_T ju;

int32_T minval;

emxArray_real_T *costab1q;

int32_T nd2;

int32_T ixDelta;

int32_T nRowsD2;

int32_T nRowsD4;
int32_T lastChan;

real_T e;
int32_T k;
emxArray_real_T *costab;
emxArray_real_T *sintab;
int32_T ix;
int32_T chanStart;
int32_T i;
boolean_T tst;
real_T temp_re;
real_T temp_im;
int32_T iDelta;
int32_T iDelta2;
int32_T iheight;
int32_T ihi;
real_T twid_im;
ju = y->size[0];
y->size[0] = x->size[0];
emxEnsureCapacity((emxArray__common *)y, ju, (int32_T)sizeof(creal_T));
if (x->size[0] == 0) {
} else {
  minval = x->size[0];
  b_emxInit_real_T(&costab1q, 2);
  nd2 = (x->size[0] - minval) + 1;
  if (1 >= nd2) {
    ixDelta = 1;
  } else {
    ixDelta = nd2;
  }

    ju = x->size[0];
    nRowsD2 = ju / 2;
    nRowsD4 = nRowsD2 / 2;
    lastChan = x->size[0] * (div_s32(x->size[0], x->size[0]) - 1);
    e = 6.2831853071795862 / (real_T)x->size[0];
    ju = costab1q->size[0] * costab1q->size[1];
    costab1q->size[0] = 1;
    costab1q->size[1] = nRowsD4 + 1;
    emxEnsureCapacity((emxArray__common *)costab1q, ju, (int32_T)sizeof(real_T));
    costab1q->data[0] = 1.0;
    nd2 = nRowsD4 / 2;
    for (k = 1; k <= nd2; k++) {
      costab1q->data[k] = cos(e * (real_T)k);
    }

    for (k = nd2 + 1; k < nRowsD4; k++) {
      costab1q->data[k] = sin(e * (real_T)(nRowsD4 - k));
    }

    b_emxInit_real_T(&costab, 2);
    b_emxInit_real_T(&sintab, 2);
    costab1q->data[nRowsD4] = 0.0;
    nd2 = (costab1q->size[1] - 1) << 1;
    ju = costab->size[0] * costab->size[1];
    costab->size[0] = 1;
    costab->size[1] = nd2 + 1;
    emxEnsureCapacity((emxArray__common *)costab, ju, (int32_T)sizeof(real_T));
    ju = sintab->size[0] * sintab->size[1];
    sintab->size[0] = 1;
    sintab->size[1] = nd2 + 1;
    emxEnsureCapacity((emxArray__common *)sintab, ju, (int32_T)sizeof(real_T));
    costab->data[0] = 1.0;
    sintab->data[0] = 0.0;
    for (k = 1; k < costab1q->size[1]; k++) {
      costab->data[k] = costab1q->data[k];
      sintab->data[k] = costab1q->data[(costab1q->size[1] - k) - 1];
    }

    for (k = costab1q->size[1]; k <= nd2; k++) {
      costab->data[k] = -costab1q->data[nd2 - k];
      sintab->data[k] = costab1q->data[(k - costab1q->size[1]) + 1];
    }

    emxFree_real_T(&costab1q);
    ix = 0;
    for (chanStart = 0; chanStart <= lastChan; chanStart += x->size[0]) {
      ju = 0;
      nd2 = chanStart;
      for (i = 1; i < minval; i++) {
        y->data[nd2].re = x->data[ix];
        y->data[nd2].im = 0.0;
        nd2 = x->size[0];
        tst = TRUE;
        while (tst) {
          nd2 >>= 1;
          ju ^= nd2;
          tst = ((ju & nd2) == 0);
        }

        nd2 = chanStart + ju;
        ix++;
      }

      y->data[nd2].re = x->data[ix];
      y->data[nd2].im = 0.0;
      ix += ixDelta;
      nd2 = (chanStart + x->size[0]) - 2;
      if (x->size[0] > 1) {
        for (i = chanStart; i <= nd2; i += 2) {
          temp_re = y->data[i + 1].re;
          temp_im = y->data[i + 1].im;
          y->data[i + 1].re = y->data[i].re - y->data[i + 1].re;
          y->data[i + 1].im = y->data[i].im - y->data[i + 1].im;
          y->data[i].re += temp_re;
          y->data[i].im += temp_im;
        }
      }

      iDelta = 2;
      iDelta2 = 4;
      k = nRowsD4;
      iheight = 1 + ((nRowsD4 - 1) << 2);
      while (k > 0) {
        i = chanStart;
        ihi = chanStart + iheight;
        while (i < ihi) {
          nd2 = i + iDelta;
          temp_re = y->data[nd2].re;
          temp_im = y->data[nd2].im;
          y->data[i + iDelta].re = y->data[i].re - y->data[nd2].re;
          y->data[i + iDelta].im = y->data[i].im - y->data[nd2].im;
          y->data[i].re += temp_re;
          y->data[i].im += temp_im;
          i += iDelta2;
        }

        nd2 = chanStart + 1;
        for (ju = k; ju < nRowsD2; ju += k) {
          e = costab->data[ju];
          twid_im = sintab->data[ju];
          i = nd2;
          ihi = nd2 + iheight;
          while (i < ihi) {
            temp_re = e * y->data[i + iDelta].re - twid_im * y->data[i + iDelta]
              .im;
            temp_im = e * y->data[i + iDelta].im + twid_im * y->data[i + iDelta]
              .re;
            y->data[i + iDelta].re = y->data[i].re - temp_re;
            y->data[i + iDelta].im = y->data[i].im - temp_im;
            y->data[i].re += temp_re;
            y->data[i].im += temp_im;
            i += iDelta2;
          }

          nd2++;
        }

        k /= 2;
        iDelta = iDelta2;
        iDelta2 <<= 1;
        iheight -= iDelta;
      }
    }

    emxFree_real_T(&sintab);
    emxFree_real_T(&costab);
    if (y->size[0] > 1) {
      e = 1.0 / (real_T)y->size[0];
      ju = y->size[0];
      emxEnsureCapacity((emxArray__common *)y, ju, (int32_T)sizeof(creal_T));
      nd2 = y->size[0];
      for (ju = 0; ju < nd2; ju++) {
        y->data[ju].re *= e;
        y->data[ju].im *= e;
      }
    }
  }
}

/* End of code generation (ifft.c) */

Thanks, Tang Laoya

[Sean de Wolski]

Did you build my example?

[laoya]

Hi Sean de Wolski,

Thank you very much for your kindly reply.

I noticed that you have added 'nextpow2' in calling IFFT, however, the results are different from just placing [] at there. This function is from here:

http://www.biomecardio.com/matlab/dctn.m

so I can't modify the code.

Thanks, Tang Laoya

[Sean de Wolski]

If you download the same file from here:

http://www.mathworks.com/matlabcentral/fileexchange/26385-resize-any-arrays-and-images

Then you can make changes to it since it's covered under BSD.

[laoya]

Dear Sean de Wolski,

Thanks again for your kindly reply. Could yoy please give me more suggestion about how to resize the array to get almost the same result as the matlab code?

y = ifft(x,[],1);

What I mean that I can't modify the code is that that's a classical function and I have got reasonable results by matlab code using that function. However, if I modify the code to calculate the IFFT, the final results are totally different from what I got currently.

In fact, I have found another FFT source code by by Jens Joergen Nielsen (<http://www.jjj.de/fft/mixfft03.zip>) from here:

http://www.jjj.de/fft/fftpage.html

and write my own code as follows:

function y=ifft2(x)

x=conj(x);

% y=fft(x,[],1);

y=myfft(x,length(x)); % from mixfft03

y=conj(y)/length(x);

The results are the same as that from matlab. So it shows that it is a bug of matlab coder.

Thanks,

Tang Laoya

Answer 3

[laoya]

Dear all,

I found another bug of matlab (2013a) coder. When 'a' is a vector with size 1*N, the generated C code will crash for the following matlab code:

b=diff(a,2);

however, if I change the code to the following, the C code works:

b=diff(a',2);

Could anyone help me to take a look at it?

Thanks, Tang Laoya