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Finding the maximum value without using built-in functions

Asked by Abdullah on 13 May 2011
My instructor assigned me a problem to solve, but unfortunately I couldn't, though I know it's an easy trick. I will write the program first, then I will ask the question:
n=input('Enter the number of the student: ');
for a=1:n
B(a)=input('Enter the grades as numerical values: ');
end
It's clear that variable B is a matrix containing the grades as numerical values. The problem that's assigned to me, is to find a way to show the biggest grade in the matrix without using MAX function or any other built-in functions, but rather by comparing the grades using for loop. It's like comparing the first grade with the other grades, and then the loop will keep assigning the bigger value each time the condition is verified.
I don't know how to do it although I have tried numerous times, so I would be thankful for your help.

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4 Answers

Answer by Leah
on 13 May 2011
 Accepted Answer

I think sort counts as a built-in function. I assume all of your grades will be positive
n=input('Enter the number of the student: '); maxgrade=0; for a=1:n
B(a)=input('Enter the grades as numerical values: ');
if B(a) > maxgrade
maxgrade=B(a);
end
end
sprintf('Max Grade: %0.5g',maxgrade)

  1 Comment

That was the point. Typically the question is "How can I find the max without using MAX"; the two examples I provided were cheap workarounds. However, this professor has also eliminated other built-ins, making them wise up to our awesomeness.

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Answer by Andy
on 13 May 2011

Before the loop, create a variable called currMax, and set it equal to 0. Within the loop, with each grade that's entered, compare it to currMax. If it is bigger than currMax, then you can replace currMax with that new grade. Otherwise, do not replace currMax. There's not much more to say without doing your assignment for you.

  3 Comments

Slight improvement (IMHO): set currMax to the first element, then loop over the rest. (In this case we can assume all the values are positive, but not in general.)
Fair enough, but if the class is receiving negative grades, then there are bigger problems than our mistaken assumption. :)
Initialize currMax to -inf, for Andy's method.

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Answer by Sean de Wolski
on 13 May 2011

Those professors are wising up to the:
the_max = min(-x);
%or
s = sort(x);
the_max = s(end);
tricks...

  1 Comment

Fight back! This assignment, as stated ("any other built-in function"), is impossible! Proof:
which gt
edit gt

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Answer by Abdullah on 13 May 2011

Thank you all for your contributions, but Leah has given me the right answer. I just don't believe that I haven't though in such a solution.
I found another problem that I think can't be solved without using a built-in function, but let's see if you have it. The problem is that I need to find the rank of the biggest grade in the array B. The traditional way by using MAX function is to write for example, [y,c]=max(B), where y is the biggest value, and c is the position of that value in the matrix, hence it's the rank in our question. Is there a way to get the rank shown up without using a built-in function?

  1 Comment

just record a inside the if statement.
if B(a) > maxgrade
maxgrade=B(a);
idx = a;
end

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