Given a matrix "A", how to create a row vector of 1s that has the same number of elements as "A" has rows?

Given a matrix "A", do the following things:

1. Create a row vector of 1's that has the same number of elements as "A" has rows.
2. Create a column vector of 1's that has the same number of elements as "A" has columns.
3. Using matrix multiplication, assign the product of the row vector, the matrix "A", and the column vector (in this order) to the variable "result".

A = [1:5; 6:10; 11:15; 16:20];

[Alex Mcaulley]

See this:

Matlab tutorials

Answer 1

[Manali Gupta]

Given a matrix "A", you can find the number of rows of A by using "size(A,1)". You can find the number of columns of "A" by using "size(A,2)". You can then use "ones" to create a row or column vector of 1's. Finally, use the "mtimes" or * operator to perform matrix multiplication. 

A = [1:5; 6:10; 11:15; 16:20]; 

row_vector = ones(1,size(A,1)); 

column_vector = ones(size(A,2),1); 

result = row_vector*A*column_vector; 

For more information about learning the basics of MATLAB®, see MATLAB Onramp. 

[Ayush Khandelwal]

Result show error

[Aadesh Navade]

can you explain the syntax row and column

[Mariam Aldeepa]

Why when you calculate the result multiplied the matrix "A" with another two matrices ?

Answer 2

[Alex Mcaulley]

I think the goal of this forum is not to do anyone's homework, but to solve generic questions about Matlab that can help more users. This exercise is very simple, anyone who has ever used Matlab could do it, so, I think the best thing we can do is help @Gaurav to know how to learn the basics of Matlab.

[Carlo Romagnolo]

Hi! I know this might be a long a shot, as this thread has been inactive for quite some time, but I was wondering if you could help me with a general question on this.

Here is my code to solve this problem:

A = [1:5; 6:10; 11:15; 16:20];
format compact

row = [1,1,1,1]
column = [1;1;1;1;1]

result = row * A * column

I was wondering if there is a better/more elegant way to answer the question which does not involve hardcoding the row and column vectors but instead uses functions.

[Walter Roberson]

ones(1,size(A, 1))

Answer 3

[Khom Raj Thapa Magar]

% Given 
A = [1:5; 6:10; 11:15; 16:20];
row_vec = ones(1,size(A,1))

row_vec = 1×4

1 1 1 1

column_vec = ones(length(A),1)

column_vec = 5×1

1 1 1 1 1

result = row_vec * A * column_vec

result = 210

[DGM]

The length() function does not return the number of columns in an array. It returns the size of the longest dimension. If A has more rows than columns, this code will fail. Use size() to get the size of a specific dimension.

Answer 4

[Sudhanshu Rasal]

Simple way to do this question is

X=[1 1 1 1]
Y=[1;1;1;1;1]
result=X*A*Y

????????

[Aadesh Navade]

yeah i got the same result for this too

[el mehdi latrach]

no, it does not work ,

it shows error

[DGM]

The point of writing a program is to have the computer do the work. Literally writing out the vectors manually makes as much sense as calculating the inner product on paper and writing

result = 210; % this is my entire program

Both examples will fail for obvious reasons if the size of A changes.

Answer 5

[Tushar Parmar]

A = [1:5; 6:10; 11:15; 16:20]
B(1:4)=1;
f=B*A
C(1:5)=1;
C=C'
result=f*C

[Ayush Khandelwal]

Thanks bhai

[Ayush Khandelwal]

Give me the answer of this question as systematic way

[ASWATHI V]

THANK YOU

[Jeeva G]

Thank you!!!!

[Tharindu Priyankara]

THANKS

[DGM]

The point of writing a program is to have the computer do the work. Use size() to find the size of A; use ones() to generate the vectors programmatically.

[JIA XI]

this code doesn't work,now MATLAB requires .* for multiplication.

[DGM]

It doesn't need .* for the matrix product. It just needs the vectors to be compatible with the size of A. Nothing in the given example responds to the actual size of A. it just presumes A is always a 4x5 matrix. Any other matrix will make the code fail.

The code produces the correct answer only for a 4x5 matrix

A = [1:5; 6:10; 11:15; 16:20]; % a 4x5 matrix
B(1:4) = 1; % a row vector (1x4)
C(1:5) = 1; % a row vector (1x5)
C = C'; % now it's a column vector

f = B*A; % partial solution 
result = f*C % the rest of the product

result = 210

Any other input will fail.

clear
A = rand(5,6); % anything other than a 4x5 matrix
B(1:4) = 1; % this is still 1x4
C(1:5) = 1; % this is still 1x5
C = C'; % this is still 5x1

f = B*A; % this will fail if A does not have 4 rows

Error using *
Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To operate on each element of the matrix individually, use TIMES (.*) for elementwise multiplication.

result = f*C % this will fail if A does not have 5 columns

Remember, we're not trying to do elementwise multiplication; the error message is just suggesting that maybe that's one motivation that would lead us to using incompatible array sizes.

If A can change size, then the code needs to generate the vectors that correspond to the actual size of A, not just some fixed presumed size. Otherwise, it will break.

Answer 6

[Arakala Gautham]

R_vector = ones(1,size(A,1));
C_vector=ones(size(A,2),1);
result = R_vector*A*C_vector;

[Sudhanshu Rasal]

It will not show the result in command window??? You used colon at the end of result?

[Ayush Khandelwal]

Kya Kare error aa raha hai

[Aadesh Navade]

remove that colon(;) from the last line

[DGM]

Note that the accepted answer was copy-pasted verbatim from this answer.

[Abhishek]

A = [1:5; 6:10; 11:15; 16:20];
R = (A(:,1))';
R(:)=1;
C = (A(1,:))';
C(:)=1;
result = R*A*C
%@DGM bro is this code ookay?

Answer 7

[Rishabh Nirala]

A = [1:5; 6:10; 11:15; 16:20];
C = [1;1;1;1;1]
R = [1 1 1 1 ]
P = R * A
result = P*C

ANSWER = 210

[DGM]

Again, the point of writing a program is to have the computer do the work. Use size() to find the size of A; use ones() to generate the vectors programmatically.

Answer 8

[Sneham Shrikant Vaidya]

A = [1:5; 6:10; 11:15; 16:20];
A
x = [1 1 1 1 ]
y = [1;1;1;1;1]
z = A*y
result =x*z

you can also perform this way as we know z =(lxm)*(mxn) so we first multiply A*y as their inner dimension ara same

then we obtain result matrix z that has inner dimension equal to x so now we can multiply x*z to get final ans

[DGM]

Use size() to find the size of A; use ones() to generate the vectors programmatically. Otherwise, this fails if the size of A changes.

Adding explanations to your answers is good though.

Answer 9

[VISHWA D]

A = [1:5; 6:10; 11:15; 16:20];
row_vector=[1 1 1 1 1]
col_vector=[1; 1; 1; 1]
result=(row_vector*(A'))*(col_vector)

[lijuan wang]

I thought the assignment has a wrong requirement, right?

we should create a vector which has the some rows as A. So it should be row_vector=[1 1 1 1]'?

[DGM]

@lijuan wang is correct. This answer creates the vectors incorrectly. Not only are they created manually in a way that would break if the size of A changes, they do not correspond to the specified dimensions of A. The result is correct, but the vectors are wrong, and the method is poor practice.

Answer 10

[Chintan]

row_vector=ones(size(A(:,1)))'
coloumn_vector=ones(size(A(1,:)))'
result=row_vector*A*coloumn_vector

[DGM]

I suppose that's one way, but size() supports dimension specification, which would avoid the need to address vectors of A or to transpose anything.

Also note that the ctranspose, ' operator is the complex conjugate transpose. If you just want to reorient a vector or matrix, use transpose, .' instead.

Answer 11

[Waseem Zafar]

A = [1:5; 6:10, 11:15; 16:20]

R = [1 1 1 1]

C = [1;1;1;1;1]

Z = A*C

result = R*Z

[DGM]

This example is identical to at least two other bad examples in this thread, differing only in the variable names.

You are told to use the size of A to construct two vectors. This example, and at least four others define the vectors literally instead of programmatically creating them as told. Let me be clear. You literally wrote two of the intermediate results into the script instead of making your script calculate them. If I were grading this, I would accept it no more than if you had just written

result = 210;

... because it neglects the entire point of the exercise to presuppose the input and bake the results into the script as literals.

An answer can earn its place if it demonstrates something either good or bad, but there is no value in duplicate, unformatted, unexplained answers with obvious, openly-discussed flaws. It doesn't add anything to the discussion and only serves to confuse readers who are looking for help.

Is there always room left in a thread for another answer that's not a near-duplicate? No. Sometimes it's not. Is this question completely exhausted? Not sure, but with extremely simple direct questions, it doesn't take long. If you want to answer questions, pick a thread where there is still room for a new answer.