@Markiyan Hirnyk Thanks. Could any positive semi-definite function be a pdf? (Anyway seems i figured out a way to generate points)

@Carl Love Thank you very much! It works now!

Thank you very much! It works now!

@Markiyan Hirnyk Thank you very much!

@Carl Love Thank you so much for your explanation! @acer provides a solution and it works now!

@acer Thank you! It works now!

@Thomas Richard 

Excuse me, I got one more problem.

Suppose I have

***

str:="hello world";
str2:="hello world2";
WriteFile[APPEND]("D:\\output3.txt", str);
WriteFile[APPEND]("D:\\output3.txt", str2);

***

The output file does not contain both hello world and hello world 2. Why [APPEND] does not work and how to make it work?

Thank you

@acer Thank you so much!

@Carl Love The plan is to generate lists from program, then sort them. In the whole procedure, I would like to avoid manually operation, such as [ [] [] [].... ] (Hopefully I have expresssed clearly). Anyway there are already solutions provided by tomleslie, Kitonum, and you ... 

@Carl Love Thank you very much for your advice. The size of task will be ~10^4 items, hence I would like to have a method as 

robust as possible.

@Carl Love Thank you for your advice. In general, I may create a table like

[k1,k2,k3]

[k2,k1,k3]

....

  It would work as 

f:= [[1,3,2], [2,1,3], [1,2,3], [2,3,1], [3,2,1], [3,1,2]]:

  But for general ones, manually written as [ [] [] []... ] would be rather inconvient. Is there any systemmaical input for doing that? 

Thank you very much

Cong

@Carl Love Thank you! It works now!

@Carl Love Thank you! It works now!

@Carl Love Thank you so much! It works now!

@Carl Love Thank you very much. 

 Excuse me, how should I define a list systematically, like

for i from 1 to 3 do

  f[i]:=i;

end do;

  how to let f be a list?

Thank you!