Matlab: Numerical array index into a string array (without loops)

Question

I'm doing a set of problems from the MATLAB's introductory course at MIT OCW. You can see it here, it's problem number 9, part g.iii.

I have one matrix with the final grades of a course, all of them range from 1 to 5. And I have another array with only letters from 'F' to 'A' (in a 'decreasing' order).

I know how to change elements in a matrix, I suppose I could do something like this for each number:

totalGrades(find(totalGrades==1)) = 'F';
totalGrades(find(totalGrades==2)) = 'E';
totalGrades(find(totalGrades==3)) = 'C';
totalGrades(find(totalGrades==4)) = 'B';
totalGrades(find(totalGrades==5)) = 'A';

But then, what's the purpose of creating the string array "letters"?

I thought about using a loop, but we're supposed to solve the problem without one at that point of the course.

Is there a way? I'll be glad to know. Here's my code for the whole problem, but I got stuck in that last question.

load('classGrades.mat');
disp(namesAndGrades(1:5,1:8));
grades = namesAndGrades(1:15,2:size(namesAndGrades,2));
mean(grades);
meanGrades = nanmean(grades);
meanMatrix = ones(15,1)*meanGrades;
curvedGrades = 3.5*(grades./meanMatrix);

% Verifying 
nanmean(curvedGrades)
mean(curvedGrades)

curvedGrades(curvedGrades>=5) = 5;

totalGrades = nanmean(curvedGrades,2);

letters = 'FDCBA';

Thanks a lot!

Maybe something like totalGrades(find(totalGrades==1))=letters(1); and then totalGrades(find(totalGrades==2))=letters(2); and so on... — AlessioX
– AlessioX, Commented Feb 10, 2016 at 12:26
This would be a great question to ask your professor. See also: Matrix Indexing — sco1
– sco1, Commented Feb 10, 2016 at 12:48
It did change something but I don't know if I require some extra line to convert a number to string. Take a look at my outcome: totalGrades(totalGrades==5)=letters(5) y = 4 3 4 4 4 65 3 3 4 3 3 3 3 65 4 — Aquiles Páez
– Aquiles Páez, Commented Feb 10, 2016 at 13:02

R. Schifini · Accepted Answer · 2016-02-10 13:31:15Z

1

Try:

letters=['F','D','C','B','A'];
tg = [1 2 1 3 3 1];
letters(tg)

Result:

ans = FDFCCF

This works even when tg (total grade) is a matrix:

letters=['F','D','C','B','A'];
tg = [1 2 1 ; 3 3 1];
result = letters(tg);
result


result =                                                                                                                                                                             

FDF                                                                                                                                                                             
CCF

Edit (brief explanation):
It is easy to understand that when you do letters(2) you get the second element of letters (D).

But you can also select several elements from letters by giving it an array: letters([1 2]) will return the first and second elements (FD).

So, letters(indexesArray) will result in a new array that has the same length of indexesArray. But, this array has to contain numbers from 1 to the length of letters (or an error will pop up).

edited Feb 10, 2016 at 13:31

answered Feb 10, 2016 at 12:36

R. Schifini

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3 Comments

sco1 Over a year ago

It would be useful to explain why this is the case, since it's the part OP is not understanding.

Aquiles Páez Over a year ago

Thanks a lot R. Schifini, nailed it. And as always, it was a pretty simple solution. Need to study more.

R. Schifini Over a year ago

You were trying to use logical indexing to change the numbers to letters, but this solution is more a lookup on letters by using total grades as the array of indexes.

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