Need help finding out if I wrote this code correctly

Question

Kevin Nelson 2022-9-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1795370-need-help-finding-out-if-i-wrote-this-code-correctly

编辑： Star Strider 2022-9-5

N = 365;
n = (1:80);
for M = 1:1000
    j = (1:n);
    pn = 1-(1-((n-1)/N));
end
%%
fig = figure(1); clf
plot(pn)
xlabel('Value')
ylabel('Probability')
ylim(x,0.3)

What I'm trying to do is perform an experiment on (pn) 1000 times, with a value (n) from 1 through 80. I'm supposed to be getting 0.5 at a value greater than 22 but I'm not getting that. First time using matlab so help would be very much appreciated.

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Kevin Nelson 2022-9-5

What I'm trying to do is perform an experiment on (pn) 1000 times, with a value (n) from 1 through 80. I'm supposed to be getting 0.5 at a value greater than 22 but I'm not getting that. First time using matlab so help would be very much appreciated.

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Answer 1

Star Strider 2022-9-5

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编辑：Star Strider 2022-9-5

If you want to save the values of ‘pn’ in the loop, subscript it —

pn(M) = 1-(1-((n-1)/N));

That will create a row vector that you can then plot.

What is the ‘j’ calculation supposed to do? The code calculates it in each iteration and never uses it (at least not that I can see).

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Walter Roberson 2022-9-5

n is a vector, so you would have to assign to either pn(M,:) or pn(:,M)

Star Strider 2022-9-5

编辑：Star Strider 2022-9-5

在 MATLAB Online 中打开

I just now noticed that.

EDIT — (5 Sep 2022 at 13:16)

I believe this is the ‘Birthday Problem’ the solution of which is simply:

N = 365;
V = 1:N;
pn = cumprod((N:-1:1) / N);
Result = [V; pn]
Result = 2×365
    1.0000    2.0000    3.0000    4.0000    5.0000    6.0000    7.0000    8.0000    9.0000   10.0000   11.0000   12.0000   13.0000   14.0000   15.0000   16.0000   17.0000   18.0000   19.0000   20.0000   21.0000   22.0000   23.0000   24.0000   25.0000   26.0000   27.0000   28.0000   29.0000   30.0000
    1.0000    0.9973    0.9918    0.9836    0.9729    0.9595    0.9438    0.9257    0.9054    0.8831    0.8589    0.8330    0.8056    0.7769    0.7471    0.7164    0.6850    0.6531    0.6209    0.5886    0.5563    0.5243    0.4927    0.4617    0.4313    0.4018    0.3731    0.3455    0.3190    0.2937
Eq = find(pn >= 0.5, 1, 'last')
Eq = 22

I don’t understand the reason that it needs to be simulated 1000 times unless some different sort of calculation is involved.

.

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Answer 2

Walter Roberson 2022-9-5

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n = (1:80);

n is a vector.

for M = 1:1000
    j = (1:n);

n is a vector, so j is 1:VECTOR instead of being 1:SCALAR or 1:SCALAR:SCALAR . You have a logic error there. What MATLAB does in this particular case is to define the output as if you had written 1:VECTOR(1) . Since n = 1:80 then n(1) is 1, so you are effectively defining j = 1:1 which is the same as j = 1

pn = 1-(1-((n-1)/N));

but you never use j in the calculation or afterwards, so there is no point calculating it.

You overwrite all of pn in the calculation. The end result is the same if you had only done the last iteration, when M = 1000.

You did not include any comments, so we cannot tell what you are trying to calculate, so we cannot suggest what correct code might be. (But I get the impression that possibly you are testing out the "Birthday Paradox" ?)