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QVector always push back 0?



  • 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


  • Moderators

    @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 ?



  • 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().



  • @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



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


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