Reverse coding in matlab

Question

I have a survey data in matlab and I have some reverse coded items. All I need is to do reverse coding in certain questions.I was using this code reversecoding = myStruct.mysurvey(:,4) == 5 ; mySturct.mysurvey(reversecoding,4) = 3 ; but I realized I will probably have some issues by using this.

For example when I recode the value of 3 to 5;

 myStruct.mysurvey=
  3     3     3     3
  6     6     6     6
  3     3     3     3
  7     6     6     7
  4     3     5     5 <--
  5     5     4     5 <--
       ...

reversecoding = myStruct.mysurvey(:,4) == 5 ;
   mySturct.mysurvey(reversecoding,4) = 3 

%after (the last 2 values has recoded to 3)


 myStruct.mysurvey=
  3     3     3     3
  6     6     6     6
  3     3     3     3
  7     6     6     7
  4     3     5     3 <--
  5     5     4     3 <--
       ...

But after that, when I am recoding the 3 to 5 it recodes my all 3s (including the ones that I just recoded from 5) to 5.

To exemplify;

    %after I recode 3s to 5

 myStruct.mysurvey=
  3     3     3     3
  6     6     6     6
  3     3     3     3
  7     6     6     7
  4     3     5     3 <--
  5     5     4     3 <--
       ...

     reversecoding = myStruct.mysurvey(:,4) == 3 ;
   mySturct.mysurvey(reversecoding,4) = 5 ;

myStruct.mysurvey= 3 3 3 5 6 6 6 6 3 3 3 5 7 6 6 7 4 3 5 5 <-- 5 5 4 5 <-- %they are 5 again ... %again the last two values of column 4 goes back to 5...

How can I get rid of this problem? This is the code;

reversecoding = myStruct.mysurvey(:,4) == 5 ;
   mySturct.mysurvey(reversecoding,4) = 3 ;
% after
 reversecoding = myStruct.mysurvey(:,4) == 3 ;
   mySturct.mysurvey(reversecoding,4) = 5 ;
 % all other values will be transformed.
 %old 1 will be 7
 %old 2 will be 6
      ...
 %old 6 will be 2
 %old 7 will be 1

What is the best way to recode them? I will be recoding from 1 to 7.

Create some copy of the original, then reference from that static copy to change the data instead of referencing the live, changing version. This way you wont have the 3 to 5 / 5 to 3 situation you describe. — Wolfie, Jan 31 '18 at 12:33
Didn't I see this exact same question yesterday? It had exactly the same typo too. If you are going to delete a question and repost it, at least correct the typos that were pointed out. Equality and assignment are different operators. The 2nd and 4th `==` should be `=`. — Cris Luengo, Jan 31 '18 at 15:03
@CrisLuengo It's not the exact same question. I changed it (edited 2 hours ago) also I was not aware of that typo because I am not a expert at Matlab, that's why I am asking. If you scroll down you can see that I corrected my typos :) — australopithecus, Jan 31 '18 at 15:13

score 1 · Accepted Answer · answered Jan 31 '18 at 12:42

You can use a container map to create the mapping from old to new values. This is quite simple:

map = containers.Map(old_values, new_values);

or in your case

map = containers.Map(1:10, 10:-1:1);

you can then use map to map an old value to its corresponding new value like this:

>> map(1)
ans =
    10
>> map(4)
ans =
     7

Note that you can only "call" map on single values, and not on arrays. But you can use e.g. arrayfun to simplify this to one call:

>> myStruct.mysurvey(:, 4) = [9, 2, 1, 10, 4];    % Example data
>> myStruct.mysurvey(:, 4)
ans =
     9
     2
     1
    10
     4

>> myStruct.mysurvey(:, 4) = arrayfun(@(x) map(x), myStruct.mysurvey(:, 4));
>> myStruct.mysurvey(:, 4)
ans =
     2
     9
    10
     1
     7

Thanks for your response! – australopithecus Jan 31 '18 at 12:44 — australopithecus, Jan 31 '18 at 12:44

score 0 · Answer 2 · answered Jan 31 '18 at 12:50

I found another solution to this problem but it is kind of a pain in the neck.

I created different variables for each value by using logical indexing.

%find elements to recode

fiveto3 = myStruct.mySurvey(:,4) == 5 ;
threeto5 = myStruct.mySurvey(:,4) == 3 ;
...
twoto6= myStruct.mySurvey(:,4)== 2;
sixto2= myStruct.mySurvey(:,4)== 6;
 ...
oneto7= myStruct.mySurvey(:,4) == 1;
sevento1 = myStruct.mySurvey(:,4)==7;
% time to recode
 myStruct.mySurvey(fiveto3 , 4) = 3 ;
 myStruct.mySurvey(threeto5,4) = 5 ;
 myStruct.mySurvey(twoto6 , 4) = 6 ;
 myStruct.mySurvey(sixto2 , 4) = 2 ;
 myStruct.mySurvey(oneto7 , 4) = 7 ;
 myStruct.mySurvey(sevento1 , 4) = 1 ;

 clear fiveto3 threeto5 twoto6 sixto2 oneto7 sevento1 % to get rid of these variables.

This works too but it needs more effort and more lines if you have more than 10 values to recode. You can both try container map or this one!

Reverse coding in matlab

2 Answers2