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Write a function called halfsum that takes as input an at most two-dimensional array A and computes the sum of the elements of A that are in the lower right triangular part of A

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Will Gotcher
Will Gotcher 2016-9-7
关闭： Image Analyst 2020-6-13
Write a function called halfsum that takes as input an at most two-dimensional array A and computes the sum of the elements of A that are in the lower right triangular part of A, that is, elements in the counter-diagonal (going from the bottom left corner, up and to the right) and elements that are to the right of it. For example, if the input is [1 2 3; 4 5 6; 7 8 9], then the function would return 38.
This is all I have so far:
function sum=halfsum(v)
sum=0;
[row col]=size(v);
for i= 1:row;
for j=1:col;
I have spent hours on trying to figure out how to sum the reverse diagonal starting at the bottom left element and everything to the right of it and cannot seem to figure it out. Any help is greatly appreciated!
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Image Analyst
Image Analyst 2020-6-13
Do not post up here. If you think that none of the 18 answers below and 2 answers above answer the question and that your answer offers something unique, then you can reopen the question to post your answer that is different than the prior 20 solutions.

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采纳的回答

James Tursa
James Tursa 2016-9-7
编辑：James Tursa 2016-9-7
Ask yourself, "what does it mean to be in the lower right triangle" in terms of i and j? I.e., what test could you make on i and j to determine if you were in the lower right triangle? Then code that up inside your for loops. E.g.,
mysum = 0; % <-- Changed sum to mysum since sum is the name of a MATLAB function
[row,col] = size(v);
for i= 1:row
for j=1:col
if( in lower right triangle ) % <-- you put test code inside parentheses
mysum = mysum + v(i,j);
end
end
end
So, you just need to figure out a test you can make on i and j to determine if you are in the lower right triangle, and insert that code inside the parentheses as noted above. If you can't figure this out immediately, write down all of the i,j pairs for a small matrix on a piece of paper and then look for a pattern that you can exploit and code up that isolates the lower right triangle. Do an example for both a square and non-square matrix to make sure the code you come up with works in both cases.
SIDE NOTE: There are ways to do this without a double for loop, but you should first learn how to do this with for loops.
  2 个评论
Will Gotcher
Will Gotcher 2016-9-20
Hey James,
Late response I know. Thanks for the reply. That is roughly the code I had at the time of posting the question. Do you have any tips how I can single out the bottom right triangle? I have spent several full days trying to figure this out and just cannot get it. Thanks!

更多回答（17 个）

Vikrant Prasad
Vikrant Prasad 2019-3-22
function summa = halfsum(A)
[row, col]=size(A);
sum=0;
for i=1:row
for j=i:col
if i<=j
sum= sum + A(i,j);
end
end
end
summa =sum;
end
  3 个评论
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Image Analyst
Image Analyst 2019-3-31
编辑：Image Analyst 2019-3-31
Did you try
if i>=j
and it didn't work??? Or did you do
summa = sum(A(triu(A)))
You didn't use sum for the name of the variable, contrary to what I said in my answer, did you??
Please attach the code so we can check why it's not working.
mayank ghugretkar
mayank ghugretkar 2019-6-10
another similar way
1. with for loop
function summa=halfsum(A)
[row col]=size(A);
summa=0;
j=0;
for i=1:row
for j=1:col
if j>=i
summa= A(i,j) +summa;
end
end
end
end
2. with While loop
function summa=halfsum(A)
[row col]=size(A);
summa=0;
for i=1:row
j=i;
while (j>=i && j<=col)
summa= A(i,j)+ summa;
j=j+1;
end
end
summa;
end
i love trying this things... simplifying n solving
Prashanth Reddy Penta Reddy
function u = halfsum(P)
u1 = P(end:-1:1, 1:end);
u2 = triu(u1);
u = sum(u2(:));
end
  3 个评论
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William Gallego
William Gallego 2018-11-3
编辑：William Gallego 2018-11-3
Great, I understood both ways. Actually, I spent most of my time trying to figure out the loop for ii=m:-1:1. I forgot to count backward :(. Thanks for the help!
Nijita Kesavan Namboothiri
function summa=halfsum(m)
summa=0
[r c]=size(m)
for i=1:r
for j=1:c
if i<=j
summa=summa+m(i,j)
end
end
end
Image Analyst
Image Analyst 2016-9-7
Try
theSum = 0; % DON'T USE sum as a variable name!!!
[rows, columns] = size(v);
for row = 1 : rows
for col = (columns - row + 1) : columns
theSum = theSum + .......
Justin Whetten
Justin Whetten 2016-12-13
For anyone who might be struggling still with this
function sum=halfsum(A);
sum = 0;
[row,col] = size(A);
for i= 1:row
for j=1:col
if (i+j)>=(row+1)
sum = sum + A(i,j);
end
end
end
  2 个评论
Steven Lord
Steven Lord 2016-12-13
There's no need for a test inside the loops.
In row 1, how many elements will be included in the sum and in which column are those elements located?
In row 2, how many elements will be included in the sum and in which column are those elements located?
Continue asking this question and see if there is a pattern to the answers that you can exploit in the definition of the second for loop to force only those elements that should be included in the sum to be included.
Image Analyst
Image Analyst 2016-12-14
And of course, NEVER use "sum" as the name of your variable (as I mentioned in my answer) since that's the name of a very important built-in function that you won't be able to use anymore if you redefine "sum" to be your own personal variable.
And if you're puzzled by what Steve said, just see my answer.
MANAV MALHOTRA
MANAV MALHOTRA 2019-6-25
function summa=halfsum(m)
summa=0;
for i=1:size(m,1)
for j=1:size(m,2)
if(i<=j)
summa=summa+sum(m(i,j));
end
end
end
for those who are less familiar with matlab
Roshan Singh
Roshan Singh 2019-9-15
function summa=halfsum(a)
[r,c]=size(a);
summa=0;
for x=1:r
for y=1:c
if (x==y)||(x<y)
summa=a(x,y)+summa;
end
end
end
fprintf('summation of right hand matrix is...%d\n',summa)
end
Sebastián Añazco
Sebastián Añazco 2019-9-21
function summa = halfsum(A)
summa = 0;
for n = 1 : size (A,1)
for m = 1:size(A,2)
if n >m
a = 0;
else
a = A(n, m);
end
summa = summa +a;
end
end
Shubham Pandey
Shubham Pandey 2020-4-3
function sum = halfsum(a)
sum = 0;
[row,col] = size(a);
for i=1:row
for j=1:col
if i<=j
sum=sum+a(i,j);
end
end
end
end
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Image Analyst
Image Analyst 2020-4-3
But every MATLAB expert would advise anyone to never use the name of a built-in function like sum() as the name of a variable.
Nitin Kumar soni
Nitin Kumar soni 2020-4-27
function summa=halfsum(n)
[row,col]=size(n);
summa=0;
for i=1:row
for j=1:col
if i<=j
summa=summa+n(i,j);
end
end
end
Ashay Nagdive
Ashay Nagdive 2020-5-1
function a = halfsum(A)
[row col]= size(A);
a = 0;
for r= 1: row
for c= 1: col
if c >= r
a= a + A(r,c)
else
A(r,c)= 0;
end
end
end
SANTOSH KAMBLE
SANTOSH KAMBLE 2020-5-4
function summa=halfsum(x)
[row col]=size(x);
summa=0;
for r=1:row
for c=1:col
y=x(r,c);
if r<=c
summa=summa+y;
end
end
end
end
ADARSH VISAJI
ADARSH VISAJI 2020-5-9
function summa = halfsum(A)
[row, col]=size(A);
sum=0;
for i=1:row
for j=i:col
if i<=j
sum= sum + A(i,j);
end
end
end
summa =sum;
end
if u guys have any dobts on matlab problem u can contact me through instagram
my insta id:- visaji_adarsh
  1 个评论
Image Analyst
Image Analyst 2020-5-9
This is similar to most of the other answers however you used the function name "sum" as a variable. It is not recommended to overwrite built-in functions with variables because then you can never use the built in function name anymore within that scope. For example if you wanted to do
theSum = sum(1:100);
you would not be able to because you made sum a scalar number -- it's not the built-in function anymore.
Taif Ahmed BIpul
Taif Ahmed BIpul 2020-5-15
function summa=halfsum(A)
[m,n]=size(A);
summ=0;
for i=1:m
for ii=i:n
summ=summ+A(i,ii);
end
end
summa=summ;
vighnesh rana
vighnesh rana 2020-5-19
function summa = halfsum(A)
[row col] = size(A);
total = 0;
for r = 1:row
for c= r:col
total = total + A(r,c);
end
end
summa = total;
end
Geetanjali Alle
Geetanjali Alle 2020-6-5
function summa = halfsum(M)
[row column]=size(M);
summa=0;
for i=1:row
for j=1:column
if i<j||i==j
summa=summa + sum(M(i,j));
end
end
end
Hope this helps!
  1 个评论
Walter Roberson
Walter Roberson 2020-6-8
i<j||i==j
would be more efficient as
i<=j
There is also an easy change to prevent you from needing the if at all.
sum(M(i,j))
M(i,j) is a scalar. sum() of a scalar is the same scalar. You do not need sum() there.
AYUSH MISHRA
AYUSH MISHRA 2020-6-11
function summa=halfsum(A)
[m,n]=size(A);
summa=0;
for i =1:m
fprintf('active row :%d \n',i);
for j=1:n
fprintf('active collumn :%d \n',j);
if m<=n
summa=summa+A(i,j);
end
fprintf('summa aeter every loop complition : %d \n',summa);
end
end
run it and understand every step working in loop

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