How do we get an average associated with an input of string arrays?

So we have a function made already as letter_grade_num

function letter_grade = letter_grade_num(grade)

        switch grade
               case 'A' 
               letter_grade = 7
               case 'B'
               letter_grade = 6
               case 'C' 
               letter_grade = 5
               case 'D'
               letter_grade = 4
               case 'E' 
               letter_grade = 3
               case 'F'
               letter_grade = 2
               case 'G'
               letter_grade = 1
  end
  end

This above function will take one letter grade and will return a number associated to it like:

letter_grade_num('A')
= 7

But now we want a function that uses this as a sub-function and takes an array of strings as an input and gives an average in numbers as an output. How would we do that?

Answer 1

[Walter Roberson]

mean( cellfun(@letter_grade_num, ArrayOfStrings) )

[Tab for Lab]

this is not doing anything. We are basically just adding a mean before that whole thing

[Walter Roberson]

Check again.

mean(cellfun(@letter_grade_num, {'A', 'C', 'D', 'A', 'B', 'B'}))

letter_grade =

       7

letter_grade =

       5

letter_grade =

       4

letter_grade =

       7

letter_grade =

       6

letter_grade =

       6

ans =

            5.83333333333333

[Tab for Lab]

I get that but its not really using our main function. It is just adding curly brackets in the bracket. Can you please explain it a bit?

[Tab for Lab]

Also, what does array of string mean?

[Walter Roberson]

function g_avg = Our_Main_Function( Array_Of_Strings)
  g_avg = mean( cellfun( @letter_grade_num, Array_Of_Strings) );
end

Now it uses Our_Main_Function. A test case is

MyArrayOfStrings = {'A', 'C', 'D', 'A', 'B', 'B'};
OurMainFunction( MyArrayOfStrings )

For array of strings see http://www.mathworks.com/help/matlab/matlab_prog/cell-arrays-of-strings.html

Note: an array of strings is different than an array of characters! An array of characters would be (for example) 'ACDABB' . It would be completely valid for the assignment to have required an array of characters instead of an array of strings.

Imagine, though, that the letter to number conversion had to take into account '+' and '-', like grades of 'A+' and 'B-'. If you were to put those in as arrays of strings, no change would be required to cellfun version: you could just have

    MyArrayOfStrings = {'A-', 'C+', 'D+', 'A', 'B', 'B+'};

and nothing about OurMainFunction would have to change. But if you were using an array of characters instead, 'A-C+D+ABB+' then OurMainFunction would have to be changed to figure out where the boundaries were, to break this apart into individual grades, taking into account that a grade is not necessarily followed by a + or -. This would make the task of OurMainFunction harder.

When you are coding, you usually want to make the task of your routines easier -- it makes it easier to write the routines correctly and makes it easier to write code that executes quickly. Saving memory by using 'A-C+D+ABB+' is something you prefer to only have to do at I/O interfaces (reading from file or user, writing to file) or if you need to work with pretty big arrays.