# I need some help in seeing where I am going wrong and how to proceed with writing a particular funciton for a MATLAB course I am taking please.

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Alexander Villanueva il 26 Giu 2019
Commentato: Mukti Awad il 18 Ago 2019
The question is this:
Write a function called max_sum that takes v, a row vector of numbers, and n, a positive integer as inputs. The function needs to find the n consecutive elements of v whose sum is the largest possible. In other words, if v is [1 2 3 4 5 4 3 2 1] and n is 3, it will find 4 5 and 4 because their sum of 13 is the largest of any 3 consecutive elements of v. If multiple such sequences exist in v, max_sum returns the first one. The function returns summa, the sum as the first output argument and index, the index of the first element of the n consecutive ones as the second output. If the input n is larger than the number of elements of v, the function returns 0 as the sum and -1 as the index. Here are a few example runs:
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],3)
summa = 13
index = 4
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],2)
summa = 9
index = 4
THe code I have written so far is as follows. My problem is that I am stuck on obtaining the largest possible sum. I wrote code and have been able to generate different sums in a vector but I am unable to put those answers in a vector themselves, by putting the answers in a vector I want to be able to use the 'max' function to obtain the largest sum. I haven't even touched finding the 'index' portion yet. Here's my code:
function [summa,index] = max_sum(v,n)
r = 0;
summa = zeros(1,length(v)); %% created a vector of 0's to length of v
if n > length(v)
summa = 0;
index = -1;
else
while n <= length(v)
summa = sum(v(r+1:n)) % here need to replace 0's with the sum (with logical index) so i can choose max
r = r+1;
n = n+1;
end
summa = max(summa);
index = n;
end
%summa = v;
%index = n;
end
Again, I need some help because I am unsure how to proceed from here. Thank you.
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### Risposta accettata

James Tursa il 27 Giu 2019
Modificato: James Tursa il 27 Giu 2019
Some issues:
n is a fixed input ... you should not be changing n inside your function. Get rid of that n = n + 1 statement.
The values you are summing v(r+1:n) isn't correct. You need to be summing n consecutive numbers. So, assuming the first index of these numbers is r, the subset would be v(r:r+n-1)
The loop needs to run as long as that last index of r+n-1 isn't greater than numel(v).
Prior to the loop, you should initialize summa and index to default values (e.g., maybe summa = -inf, or perhaps the sum of the first sequence of numbers in the vector). The summa variable is a scalar value (see the examples), not a vector, so you shouldn't be initializing it to a vector of zeroes.
Inside the loop you should have something like this logic:
if( the sum of the v(r:r+n-1) elements is greater than the current value of summa )
save the sum in summa and save the r in index
end
Give a try at making these changes and then post replies on this thread if you have more problems.
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Alexander Villanueva il 27 Giu 2019
Thank you for the clarity, I have updated the code and had another issue regarding giving me the first index if there are consecutive sums but I was able to figure that part out. However another issue popped up. The problem is that if 'n' is greater than numel(v), the answer is supposed to be summa = 0 and index = -1. However, when I do a test run where n is greater than the length(v), that answer does not appear, rather it does give summa = 0 and index = -1 but there is also an error that appears alongside it that says "Index exceeds the number of array elements". I thought the reason for having my first "if' statement was to take care of this issue yet it does not appear to be working. Here's the updated code:
function [summa,index] = max_sum(v,n)
r = 1;
summa = 0;
index = 0;
if n >= length(v)
summa = 0
index = -1
else
while n < numel(v)
sum(v(r:r+n-1));
r = r+1;
if sum(v(r:r+n-1)) == sum(v(r-1:r-1+n-1))
r=r-1;
break
elseif sum(v(r:r+n-1)) > sum(v(r+1:r+1+n-1))
break
end
end
end
index = r;
summa = sum(v(r:r+n-1));
end
James Tursa il 27 Giu 2019
Modificato: James Tursa il 27 Giu 2019
This:
if n >= length(v)
should be this:
if n > length(v)
This:
while n < numel(v)
doesn't match my suggestion:
while( that last index of r+n-1 isn't greater than numel(v) )
And this:
if sum(v(r:r+n-1)) == sum(v(r-1:r-1+n-1))
doesn't match my suggestion either:
if( the sum of the v(r:r+n-1) elements is greater than the current value of summa )
Also, your entire if-elseif-end block is more complicated than my suggestion of a simple if-end block, so look at that again.
So, take a look at fixing these up and then get back to us with your progress.

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### Più risposte (1)

Mukti Awad il 16 Ago 2019
I am getting this error while executing code
• Assessment result: incorrect[1 2 3 4 5 4 3 2 1]max_sum([ 1 2 3 4 5 4 3 2 1 ], 10) returned the following MATLAB error: Index exceeds the number of array elements (9).
• Assessment result: incorrectrandom vectorsVariable summa has an incorrect value. max_sum([ 71 -54 17 60 -51 -3 98 -65 -61 35 90 94 -11 -69 82 -86 -12 ], 5) returned sum = 121 and index = 3 which is incorrect...
##### 2 CommentiMostra NessunoNascondi Nessuno
James Tursa il 16 Ago 2019
Mukti Awad il 18 Ago 2019
Code is here:
function [summa,index] = max_sum(v,n)
r = 1;
summa = 0;
index = 0;
if n > length(v)
summa = 0;
index = -1;
else
while n < numel(v)
sum(v(r:r+n-1));
r = r+1;
if sum(v(r:r+n-1)) == sum(v(r-1:r-1+n-1))
r=r-1;
break
elseif sum(v(r:r+n-1)) > sum(v(r+1:r+1+n-1))
break
end
end
end
index = r;
summa = sum(v(r:r+n-1));
end
Getting error:
Assessment: 0 of 2 Tests Passed
Assessment result: incorrect[1 2 3 4 5 4 3 2 1]
max_sum([ 1 2 3 4 5 4 3 2 1 ], 10) returned the following MATLAB error:
Index exceeds the number of array elements (9).
Assessment result: incorrectrandom vectors
Variable summa has an incorrect value.
max_sum([ -81 -77 -45 -68 7 85 -9 30 89 62 83 83 94 -77 52 43 -62 -76 -2 26 -80 ], 2) returned sum = 92 and index = 5 which is incorrect...

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