I am plotting the derivative of a curve. Getting the y value for maximum peak height of the biggest peak. But when I try to get the x value corresponding to that y value its not giving me any value. Following is the code. Any help would be appreciate

   a = load('ra1.dat');
    x = a(:,1);
    y = a(:,2);
    y_mov_average = filter(ones(1,50)/50,1,y);
    plot(x,y);
    hold on
    dy = diff(y)./diff(x);
    dy_mov_average = filter(ones(1,10)/10,1,dy);
    plot(x(2:end),dy_mov_average);
    PEAKSID = findpeaks(dy_mov_average);
    ID = class(PEAKSID);
    PEAKIDCAL = double(max(PEAKSID));
    VALUE = find(y==PEAKIDCAL);
    xaty = x(VALUE);

A portion of the ra1.dat is pasted below. Its a force distance plot by atomic force microscope.
817  -1.602783
815.8824  -1.604004
814.7649  -1.603394
813.6473  -1.602783
812.5298  -1.602478
811.4122  -1.602173
810.2946  -1.601563
809.1771  -1.601868
808.0595  -1.602478
806.942  -1.601868
805.8244  -1.602478
804.7068  -1.602478
803.5893  -1.602173
802.4717  -1.602478
801.3542  -1.603088
800.2366  -1.603699
799.1191  -1.601868
798.0015  -1.602783
796.8839  -1.603088
795.7664  -1.603394
794.6488  -1.604004
793.5313  -1.603088
792.4137  -1.604309
791.2961  -1.604004
790.1786  -1.603394
789.061  -1.604614
787.9435  -1.604004
786.8259  -1.604919
785.7084  -1.604004
784.5908  -1.603394
783.4732  -1.604309
782.3557  -1.604614
781.2381  -1.604614
780.1205  -1.604004
779.003  -1.60553
777.8854  -1.604309
776.7679  -1.604309
775.6503  -1.604614
774.5328  -1.604004
773.4152  -1.604004
772.2977  -1.604614
771.1801  -1.604919
770.0625  -1.604614
768.9449  -1.60553
767.8274  -1.604309
766.7098  -1.605225
765.5923  -1.606445
764.4747  -1.60675
763.3572  -1.60614
762.2396  -1.60614
761.1221  -1.60553
760.0045  -1.604614
758.887  -1.603394
757.7694  -1.603699
756.6518  -1.604919
755.5342  -1.60553
754.4167  -1.60553
753.2991  -1.604614
752.1816  -1.604919
751.064  -1.605835
749.9465  -1.60553
748.8289  -1.60553
747.7114  -1.60553
746.5938  -1.606445
745.4763  -1.605835
744.3587  -1.60614
743.2411  -1.604919
742.1235  -1.605835
741.006  -1.60553
739.8884  -1.60553
738.7709  -1.60675
737.6533  -1.607056
736.5358  -1.607666
735.4182  -1.607666
734.3007  -1.607056
733.1831  -1.607666
732.0656  -1.606445
730.948  -1.607971
729.8304  -1.607666
728.7128  -1.607056
727.5953  -1.607361
726.4777  -1.608276
725.3602  -1.607056
724.2426  -1.60614
723.1251  -1.608582
722.0075  -1.608887
720.89  -1.609802
719.7724  -1.609192
718.6548  -1.608887
717.5373  -1.608276
716.4197  -1.608582
715.3021  -1.609802
714.1846  -1.608276
713.067  -1.608276
711.9495  -1.609802
710.8319  -1.609497
709.7144  -1.609497
708.5968  -1.609802
707.4792  -1.610107
706.3617  -1.608276
705.2441  -1.608276
704.1266  -1.607666
703.009  -1.608887
701.8914  -1.608276
700.7739  -1.608582
699.6563  -1.608887
698.5388  -1.608582
697.4212  -1.609802
696.3036  -1.609192
695.1861  -1.610107
694.0685  -1.609802
692.951  -1.610107
691.8334  -1.609192
690.7159  -1.609192
689.5983  -1.610107
688.4807  -1.609802
687.3632  -1.609802
686.2456  -1.611023
685.1281  -1.609802
684.0105  -1.610718
682.8929  -1.610107
681.7754  -1.610413
680.6578  -1.609802
679.5403  -1.611633
678.4227  -1.611328
677.3052  -1.610718
676.1876  -1.610718
675.07  -1.611023
673.9525  -1.611328
672.8349  -1.611328
671.7173  -1.613159
670.5998  -1.612244
669.4822  -1.612244
668.3647  -1.612244
667.2471  -1.611633
666.1296  -1.611023
665.012  -1.611938
663.8944  -1.612854
662.7769  -1.613464
661.6593  -1.613159
660.5417  -1.612854
659.4242  -1.61377
658.3066  -1.61377
657.1891  -1.61377
656.0715  -1.613464
654.954  -1.613464
653.8364  -1.611328
652.7189  -1.611938
651.6013  -1.611938
650.4837  -1.611633
649.3661  -1.612549
648.2486  -1.61438
647.131  -1.612854
646.0135  -1.612549
644.8959  -1.613159
643.7784  -1.614075
642.6608  -1.613464
641.5433  -1.612854
640.4257  -1.613464
639.3082  -1.613464
638.1906  -1.612549
637.073  -1.612244
635.9554  -1.613464
634.8379  -1.613159
633.7203  -1.613464
632.6028  -1.61377
631.4852  -1.614075
630.3677  -1.61377
629.2501  -1.614075
628.1326  -1.61438
627.015  -1.615295
625.8975  -1.615906
624.7799  -1.615906
623.6623  -1.61499
622.5447  -1.616211
621.4272  -1.61499
620.3096  -1.61499
619.1921  -1.616211
618.0745  -1.615295
616.957  -1.615601
615.8394  -1.616211
614.7219  -1.615601
613.6043  -1.61499
612.4868  -1.615601
611.3692  -1.617432
610.2516  -1.615906
609.134  -1.615601
608.0165  -1.617126
606.8989  -1.617737
605.7814  -1.617126
604.6638  -1.616821
603.5463  -1.617432
602.4287  -1.617432
601.3112  -1.617737
600.1936  -1.618347
599.076  -1.618347
597.9585  -1.618347
596.8409  -1.618958
595.7233  -1.618347
594.6058  -1.618958
593.4882  -1.618042
592.3707  -1.618652
591.2531  -1.618652
590.1356  -1.618652
589.018  -1.618958
587.9005  -1.618958
586.7829  -1.618958
585.6653  -1.619873
584.5478  -1.620483

Answer 1

[KSSV]

You can get the position/index of the maximum element from array using max itself. You need not to find the index later.

a = load('ra1.dat');
x = a(:,1);
y = a(:,2);
y_mov_average = filter(ones(1,50)/50,1,y);

plot(x,y);
hold on
dy = diff(y)./diff(x);
dy_mov_average = filter(ones(1,10)/10,1,dy);
plot(x(2:end),dy_mov_average);
PEAKSID = findpeaks(dy_mov_average);
ID = class(PEAKSID);
[PEAKIDCAL,VALUE] = (max(PEAKSID));  % get max and it's index 
% VALUE = find(y==PEAKIDCAL);
xaty = x(VALUE);

[Anuradha Bhattacharya]

Thank you so much. Any ideas as to why the earlier code was not working?

[Anuradha Bhattacharya]

When I run the code I am getting max as the coordinates of the first peak only when however the first peak is smaller than the next subsequent peak. How can I fix this error?

[KSSV]

max gives the maximum..the second cannot be greater. Your first code was not working because, you were equating floating numbers, which you should not.

[Anuradha Bhattacharya]

The max x value is around 500. The upper plot is dy_mov_average, the lower plot is y. xaty is coming 723.1251