Convert an array of numbers into letters using logical indexing

I am working on a school project that involves reading an excel file, calculating grades for each cell, replacing number grades with letter grades, and writing the array back into another excel file. I'm hung up on the replacing number grades with letter grades part. This is what I have so far for that portion:

grades =

         6.29
         5.87
         5.28
         7.38
          7.4
         4.07
         4.23
         7.25
          3.6
         4.37
         4.74
         4.79
         7.73
         9.15
         8.83
         5.23
         0.66
         6.18
         7.71
         8.16
         6.33
         5.46
         6.63
         0.07
         
grades(grades>=9.0 & grades<=10.0) = 'A';
grades(grades>=8.5 & grades<9.0) = 'AMinus';
grades(grades>=8.0 & grades<8.5)= 'BPlus';
grades(grades>=7.5 & grades<8.0) = 'B';
grades(grades>=7.0 & grades<7.5) = 'BMinus';
grades(grades>=6.5 & grades<7.0) = 'CPlus';
grades(grades>=6.0 & grades<6.5) = 'C';
grades(grades>=5.5 & grades<6.0) = 'CMinus';
grades(grades>=5.0 & grades<5.5) = 'D';
studentAvg(studentAvg>=0 & studentAvg<5.0) = 'F';

So I've indexed the array that I've been given, and my logical indexing code determines which grades in the array are A's,B's,C's, and so on.

The problem I've run into, though, is that after logical indexing, I can't figure out how to write 'A' back into the array to the number grades which have earned A's, 'B' to those that earned B's, and so on.

I need to keep the order of my array as it is linked to a list of Student ID numbers that I don't want to resort/reindex.

Any help is much appreciated!

[madhan ravi]

Could you illustrate with an example of your desired output?

[Weston Way]

This is what I'd like it to look like, with a few more values where the '...' is.

gradesSolved = 
        91180         C
        91737         D
        92024         C+
        '...'
        99902         F

Answer 1

[per isakson]

Another approach

>> grades = reshape([ 6.29, 5.87, 5.28, 7.38, 7.40, 4.07, 4.23, 7.25, 3.60 ...
                  ,   4.37, 4.74, 4.79, 7.73, 9.15, 8.83, 5.23, 0.66, 6.18 ...
                  ,   7.71, 8.16, 6.33, 5.46, 6.63, 0.07 ], [],1 );
>> out = cssm( grades )
out =
  1×24 cell array
  Columns 1 through 9
    {'C'}    {'C-'}    {'D'}    {'B-'}    {'B-'}    {'F'}    {'F'}    {'B-'}    {'F'}
  Columns 10 through 19
    {'F'}    {'F'}    {'F'}    {'B'}    {'A'}    {'A-'}    {'D'}    {'F'}    {'C'}    {'B'}
  Columns 20 through 24
    {'B+'}    {'C'}    {'D'}    {'C+'}    {'F'}

where

function    out = cssm( grades )
    %%
    grade_edges   = [ 0.0, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 10.0 ];
    letter_grades = {'F', 'D', 'C-','C', 'C+','B-','B', 'B+','A-','A' };
    %%
    ix  = discretize( grades, grade_edges );
    out = letter_grades( 1, ix );
end

The five digits numbers, e.g. 91737, are they student IDs and where do they come from?

Answer 2

[Andrei Bobrov]

v = [0, 5:.5:10]';
S = string(cellstr(('F':-1:'A')'));
K = S(4:6)' +  {'-','','+'}';
K = [S(1:3);K(1:end-1)'];
out = discretize(grades,v,K);