MATLAB: Conditionals and character arrays to assign grade based on score

Question

How would I set up a statement to assign a letter grade based on an inputed score. eg. if score is between 90 and 100 then grade = a.

This is what I have so far:

function grade = assignGrade(score)
%Enter the commands for your function here.  
if (score >= 90) && (score <= 100) 
    grade = 'A'   
end

This is my code to call my function:

score = 75;
grade = assignGrade(score)
assignGrade(91)=grade

none of this works but I have included it to show you what I have tried

Answer 1

score = 75;
grade = assignGrade(score);
fprintf('The grade for score %d was %s\n', score, grade);
The grade for score 75 was undefined
function grade = assignGrade(score)
%Enter the commands for your function here.  
    if (score >= 90) && (score <= 100) 
        grade = 'A';
    else
        grade = 'undefined';
    end
end