How to index and replace

Asked by ubaid haroon on 12 Jan 2017

Latest activity Edited by Stephen Cobeldick on 12 Jan 2017

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Hello,

I am trying to learn MATLAB and decided to use the MIT course to look at the slides and I figure the homework would be challenging so I decided to do that as well. I am stuck on problem 9.G.iii. Can someone please assist me.

https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-094-introduction-to-matlab-january-iap-2010/assignments/MIT6_094IAP10_assn01.pdf

Here is my code

clear all, close all, clc
%b
load classGrades
%c
namesAndGrades
%d
grades = namesAndGrades(:,2:end)
%e
meanGrades = nanmean(grades(:,1))
for i = 1:7
   meanGrades(i,:) = nanmean(grades(:,i))
end
[gradesRows, gradesCol] = size(grades)
%f
for i = 1:15
    meanMatrix(i,:) = meanGrades
end
curvedGrades = 3.5*(grades./meanMatrix)
checkCurvedGrades = nanmean(curvedGrades);
%f.iv
curvedGrades5 = find(curvedGrades > 5)
for i = curvedGrades5
curvedGrades(i) = 5
end
checkCurve = find(curvedGrades == 5)
tf = isequal(curvedGrades5,checkCurve)
%g 
totalGrade = nanmean(curvedGrades')
totalGrade = ceil(totalGrade)
letters = ['F','D','C','B','A'];
findF = find(totalGrade ==1);
findD = find(totalGrade ==2);
findC = find(totalGrade ==3);
findB = find(totalGrade ==4);
findA = find(totalGrade ==5);

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2 Answers

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Answer by Stephen Cobeldick on 12 Jan 2017

Edited by Stephen Cobeldick on 12 Jan 2017

Accepted Answer

totalGrade is already the index (values from one to five) that can be used directly into letters, therefore you do not need find or the equivalence operation, and I also simplified your concatenating characters into one string:

 letters = 'FDCBA';
 letterGrades = letters(totalGrades)

This simply follows exactly what the instructions state: "make the final letter grades ... by using totalGrades... to index into letters". The instructions do not tell you to use find, or equivalence operators.

In fact all of your code uses unnecessary find operations, and very inefficient loops. Loops are not the point of this homework. The very first line states that the point is to think about using MATLAB "in terms of matrices and vectors because that is how MATLAB organizes data". You are still thinking in terms of loops, which are slow, ugly, and are definitely not thinking about data in terms of matrices and vectors.

For example, this slow find and ugly loop:

 curvedGrades5 = find(curvedGrades > 5)
 for i = curvedGrades5
   curvedGrades(i) = 5;
 end

should be just one line using logical indexing:

curvedGrades(curvedGrades>5) = 5;

If you want to learn how to use MATLAB efficiently then you need to revise your work, and especially learn about logical indexing.

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Answer by ubaid haroon on 12 Jan 2017

I see it now. Wow that was easy.

Thank you!

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