Non Repeated letters positions in a string

[pavank]

data _null_;
string = 'antioxidant';
length unique $200 position_list $200;
do i = 1 to length(string);
letter = substr(string, i, 1);
count = countc(string, letter);
if count = 1 then do;
unique = cats(unique, letter);
position_list = cats(position_list, i);
end;
end;
put unique;
put position_list;
run;

I want output position of non repeated letters like below

Non-Repeated_Letters	O	X	D
antioxidant	5	6	8

[PaigeMiller]

data nonduplicates;
    string='Arnold Ziffel';
    length thischar $ 1;
    do pos=1 to length(string);
        thischar=substr(string,pos,1);
        count=countc(string,thischar);
        if count=1 then output;
    end;
    keep pos thischar;
run;

This code treats capital C as a different letter than lower case c. Your problem description was silent about this issue. 

[ballardw]

A quick comment about the thinking involved with

position_list = cats(position_list, i);

Let's examine a hypothetical resulting position_list value of 12345.... (dots are other digits not germane at this point).

How do you tell position 1 for the first unique letter and 23 for the second from a position of 12 for the first and 34 for the second?

If I were contemplating creating such as list I would place a delimiter using the CATX function for example.

position_list = catx(',',position_list, i);

 Then we would see 1,23,45... or 12,34,5... and the ambiguity goes away.

Any rules about characters other than letters?