how to grab the last value in a loop?

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Fabian Moreno 2021-4-11

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评论： Fabian Moreno 2021-4-13

Hi there, I would like to know how to get the last value of my loop. I am intersecting a matrix of 35 years of data with a matrix of 420 (420 months, from jan-1985 to dez-2019), where (B) is the intersect position. I am getting the mean value of the sine each 12 month, but in the last value (month 420) the result is NaN. How they are month, it has 31, 30, 29 and 28 days. So I also try to do and If statment, but it didn´t work. Some body could help me.

With the first example I got from 1 to 419 the nanmean values, but the 420 value don´t.

Thak you in advance.

%% first example
seno_m = NaN(length(Date_wd_monthly),1);
for i = 1:420
    seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i)-1,9)));
end
%% Example of the If statment
seno_m = NaN(length(Date_wd_monthly),1);
for i = 1:420
    if numel(Date_wd_daily(B(i):B(i+1)-1,9))==31 
        seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i,1)+31,9)));
    elseif numel(Date_wd_daily(B(i):B(i+1)-1,9))== 30
        seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i,1)+30,9)));
    elseif numel(Date_wd_daily(B(i):B(i+1)-1,9))== 29
        seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i,1)+29,9)));
    else numel(Date_wd_daily(B(i):B(i+1)-1,9))== 28
        seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i,1)+28,9)));
    end
end

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Answer 1

Jan 2021-4-11

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/799197-how-to-grab-the-last-value-in-a-loop#answer_672812

在 MATLAB Online 中打开

In your first code:

seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i)-1,9)));
%                                      ^^^^ ^^^^^^

This is the empty matrix in all cases. B(i):B(i)-1 does not contain any elements.

Teh 2nd code can be simplified:

seno_m = NaN(length(Date_wd_monthly),1);
for i = 1:420
    n = numel(Date_wd_daily(B(i):B(i+1)-1,9));
    seno_m(i,1)= nanmean(sin(Date_wd_daily(B(i):B(i,1) + n, 9)));
end

This can be expressed easier also, because numel(Date_wd_daily(B(i):B(i+1)-1,9)) is the same as:

numel(B(i):B(i+1)-1)

which is the same as:

B(i+1) - B(i);

If this equals 31, remember that B(i):B(i,1)+31 contains 32 elements (not 31).

To understand, what your code does and what it should do, the readers need to know what B contains.

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Fabian Moreno 2021-4-12

在 MATLAB Online 中打开

Hi Jan, thank you for help me, but it still didn´t grab the last value. Any way I found another solution and it´s worked right. This is the code:

k=1
for i=1985:2019                                     %Years Looping
      for j=1:12                                      %Months Looping
          idx=find((Date_wd_daily(:,2)==i) & (Date_wd_daily(:,3)==j));  %Find all indeces for the specific month
          meandata1= nanmean(sin(Date_wd_daily(idx,9)));
          seno_m(k,:)=[i,j,meandata1];          %Store in a new matrix the mean values per month
          k=k+1;
      end
end 

Now I would like to make a matrix (years (35), month (12)) with the mean values. I already try different loops, but I can´t. Do you know how can I group the values in a matrix. I already did it, but I would like to do in a loop.

QD_month = NaN (35,12);
QD_month (1,1:12) = seno_m (1:12,2);
QD_month (2,1:12) = seno_m (13:24,2);
QD_month (3,1:12) = seno_m (25:36,2);
QD_month (4,1:12) = seno_m (37:48,2);
QD_month (5,1:12) = seno_m (49:60,2);
QD_month (6,1:12) = seno_m (61:72,2);
QD_month (7,1:12) = seno_m (73:84,2);
QD_month (8,1:12) = seno_m (85:96,2);
QD_month (9,1:12) = seno_m (97:108,2);
QD_month (10,1:12) = seno_m (109:120,2);
QD_month (11,1:12) = seno_m (121:132,2);
QD_month (12,1:12) = seno_m (133:144,2);
QD_month (13,1:12) = seno_m (145:156,2);
QD_month (14,1:12) = seno_m (157:168,2);
QD_month (15,1:12) = seno_m (169:180,2);
QD_month (16,1:12) = seno_m (181:192,2);
QD_month (17,1:12) = seno_m (193:204,2);
QD_month (18,1:12) = seno_m (205:216,2);
QD_month (19,1:12) = seno_m (217:228,2);
QD_month (20,1:12) = seno_m (229:240,2);
QD_month (21,1:12) = seno_m (241:252,2);
QD_month (22,1:12) = seno_m (253:264,2);
QD_month (23,1:12) = seno_m (265:276,2);
QD_month (24,1:12) = seno_m (277:288,2);
QD_month (25,1:12) = seno_m (289:300,2);
QD_month (26,1:12) = seno_m (301:312,2);
QD_month (27,1:12) = seno_m (313:324,2);
QD_month (28,1:12) = seno_m (325:336,2);
QD_month (29,1:12) = seno_m (337:348,2);
QD_month (30,1:12) = seno_m (349:360,2);
QD_month (31,1:12) = seno_m (361:372,2);
QD_month (32,1:12) = seno_m (373:384,2);
QD_month (33,1:12) = seno_m (385:396,2);
QD_month (34,1:12) = seno_m (397:408,2);
QD_month (35,1:12) = seno_m (409:420,2);

Jan 2021-4-13

在 MATLAB Online 中打开

You can omit the find() for a faster logical indexing:

idx = ((Date_wd_daily(:,2) == i) & (Date_wd_daily(:,3) == j));

You do not need a loop for the conversion:

QD_month = reshape(seno_m(:, 2), 12, []).';

Fabian Moreno 2021-4-13

wow, Jan you´re awesome, I´m feel so happy doing that. Thank you, you taught me a lot. I would like to learn more! I´m pretty sure that matlab should has amazing things.

Gracias!

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