QVector always push back 0?

Yeoman_Li

I am using FFTW to build a music visualizer. And in my header file, I construct a struct:

struct SpectrumStruct{
    qreal frequency;
    qreal amplitude;
};

QVector<SpectrumStruct> m_spectrum;

And in my .cpp file,

void Widget::debug(QAudioBuffer buf)
{
    QAudioBuffer :: S16S * data = buf.data<QAudioBuffer :: S16S>();
    int sampleRate = buf.format().sampleRate();
//    int N = buf.frameCount();
    int N = 1024;
    fftw_complex *in, *out;
    in = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * N);
    out = (fftw_complex *)fftw_malloc(sizeof(fftw_complex) * N);
    for (int i = 0; i < N; ++i) {
        qreal hannwindow = 0.5 * (1 - qCos((2 * M_PI * i) / (N - 1)));
        in[i][0] = data[i].left * hannwindow;
        in[i][1] = 0;
    }
    fftw_plan myPlan = fftw_plan_dft_1d(N, in, out, FFTW_FORWARD, FFTW_ESTIMATE);
    fftw_execute(myPlan);
    for(int i = 0; i < N / 2; ++i) {
        struct SpectrumStruct thisSP;
        thisSP.frequency = double(i * sampleRate / N);
        qreal thisAmpt = qSqrt(out[i][0] * out[i][0] + out[i][1] * out[i][1]);
//        thisAmpt = 2.0 * thisAmpt / N;
        thisAmpt = 0.15 * log10(thisAmpt);
        thisAmpt = qMax(qreal(0.0), thisAmpt);
        thisAmpt = qMin(qreal(1.0), thisAmpt);
        thisSP.amplitude = thisAmpt;
        m_spectrum.push_back(thisSP);
        qDebug() << m_spectrum[i].frequency << "\t" << m_spectrum[i].amplitude << "\t" << thisAmpt;
    }

    fftw_destroy_plan(myPlan);
    fftw_free(in);
    fftw_free(out);

}

I tried to print each sample point's frequency and amplitude . The thisAmpt should be same with the m_spectrum[i].amplitude. But finally in the console thisAmpt is correct but m_spectrum[i].amplitude is always 0. How can it be?

Part of the console result:

20887 0 0.0475125
20930 0 0.0734866
20973 0 0.0784833
21016 0 0.156529
21059 0 0.19284
21102 0 0.168585
21145 0 0.134795
21188 0 0.119863
21231 0 0.122281
21274 0 0.138717
21317 0 0.15457
21360 0 0.139525
21404 0 0.0697819
21447 0 0.0985039
21490 0 0.153734
21533 0 0.147471
21576 0 0.0658756
21619 0 0.13765
21662 0 0.151573
21705 0 0.179327
21748 0 0.184664
21791 0 0.162867
21834 0 0.166042
21877 0 0.128155
21920 0 0.0839292
21963 0 0.0618584
22006 0 0.146992

J.Hilk

@Yeoman_Li
mmh, to be honest, I would expect m_spectrum[i].amplitude and thisAmpt to be the same value. Does this also happen, when you use append instead of push_back ? And what Qt-Version is this ?

eliseev

I don't see m_spectrum cleared in this function. Without it m_spectrum[i] is not necessarily equal to thisSP. Try m_spectrum.last().

Yeoman_Li

@J.Hilk I just make a silly mistake. Forget to clear the vector before add value to it.
See https://stackoverflow.com/questions/53833570/qvector-always-push-back-0/53834747#53834747

Yeoman_Li

@eliseev You are right, I forget to clear it.Thank you:)