Modifying entries of a matrix selectively
In my linear algebra class we have a small component of learning Matlab with the occasional assignment.
The question asked to modify an existing (not shown) $25 times 25$ matrix with the following conditions:
- if an entry in the matrix $B$ is $geq 0$ then multiply by $4$
- if an entry in the matrix $B$ is $<0$ then add $6$ to it
I created a function file in Matlab by doing the following:
function [A] = modify_matrix(B, n, m),
A = zeros(n,m);
for i = 1:n,
for j = 1:m,
if B >= 0,
A = B*4;
else,
A = B+6;
A(i,j) = B(i,j);
end
end
end
end
This simply reproduces the same matrix $B$.
Any suggestions?
matlab
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
asked Nov 21 '18 at 20:40
Forextrader
346
add a comment |
In my linear algebra class we have a small component of learning Matlab with the occasional assignment.
The question asked to modify an existing (not shown) $25 times 25$ matrix with the following conditions:
- if an entry in the matrix $B$ is $geq 0$ then multiply by $4$
- if an entry in the matrix $B$ is $<0$ then add $6$ to it
I created a function file in Matlab by doing the following:
function [A] = modify_matrix(B, n, m),
A = zeros(n,m);
for i = 1:n,
for j = 1:m,
if B >= 0,
A = B*4;
else,
A = B+6;
A(i,j) = B(i,j);
end
end
end
end
This simply reproduces the same matrix $B$.
Any suggestions?
matlab
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
asked Nov 21 '18 at 20:40
Forextrader
346
You said $B*4$, but all that does is return a value. You need to assign it, i.e. $A = B*4$.
– TrostAft
Nov 21 '18 at 20:44
Thanks, I changed that but it still is not modifying matrix B
– Forextrader
Nov 21 '18 at 20:48
Do you mean even if just one entry was positive we multiply whole the matrix by 4 or just do the case with that individual entry?
– Mostafa Ayaz
Nov 21 '18 at 20:56
just the individual entry
– Forextrader
Nov 21 '18 at 21:01
i modified what I posted originally. I was able to produce a matrix that only added 4 when entries were <0 but for some reason it would not multiply by 4 when entry was >=0
– Forextrader
Nov 21 '18 at 21:05
add a comment |
In my linear algebra class we have a small component of learning Matlab with the occasional assignment.
The question asked to modify an existing (not shown) $25 times 25$ matrix with the following conditions:
- if an entry in the matrix $B$ is $geq 0$ then multiply by $4$
- if an entry in the matrix $B$ is $<0$ then add $6$ to it
I created a function file in Matlab by doing the following:
function [A] = modify_matrix(B, n, m),
A = zeros(n,m);
for i = 1:n,
for j = 1:m,
if B >= 0,
A = B*4;
else,
A = B+6;
A(i,j) = B(i,j);
end
end
end
end
This simply reproduces the same matrix $B$.
Any suggestions?
matlab
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
asked Nov 21 '18 at 20:40
Forextrader
346
In my linear algebra class we have a small component of learning Matlab with the occasional assignment.
The question asked to modify an existing (not shown) $25 times 25$ matrix with the following conditions:
- if an entry in the matrix $B$ is $geq 0$ then multiply by $4$
- if an entry in the matrix $B$ is $<0$ then add $6$ to it
I created a function file in Matlab by doing the following:
function [A] = modify_matrix(B, n, m),
A = zeros(n,m);
for i = 1:n,
for j = 1:m,
if B >= 0,
A = B*4;
else,
A = B+6;
A(i,j) = B(i,j);
end
end
end
end
This simply reproduces the same matrix $B$.
Any suggestions?
matlab
matlab
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
asked Nov 21 '18 at 20:40
Forextrader
346
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
asked Nov 21 '18 at 20:40
Forextrader
346
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
edited Nov 24 '18 at 5:31
Rócherz
2,7762721
2,7762721
asked Nov 21 '18 at 20:40
Forextrader
346
asked Nov 21 '18 at 20:40
Forextrader
346
asked Nov 21 '18 at 20:40
Forextrader
346
346
You said $B*4$, but all that does is return a value. You need to assign it, i.e. $A = B*4$.
– TrostAft
Nov 21 '18 at 20:44
Thanks, I changed that but it still is not modifying matrix B
– Forextrader
Nov 21 '18 at 20:48
Do you mean even if just one entry was positive we multiply whole the matrix by 4 or just do the case with that individual entry?
– Mostafa Ayaz
Nov 21 '18 at 20:56
just the individual entry
– Forextrader
Nov 21 '18 at 21:01
i modified what I posted originally. I was able to produce a matrix that only added 4 when entries were <0 but for some reason it would not multiply by 4 when entry was >=0
– Forextrader
Nov 21 '18 at 21:05
add a comment |
You said $B*4$, but all that does is return a value. You need to assign it, i.e. $A = B*4$.
– TrostAft
Nov 21 '18 at 20:44
Thanks, I changed that but it still is not modifying matrix B
– Forextrader
Nov 21 '18 at 20:48
Do you mean even if just one entry was positive we multiply whole the matrix by 4 or just do the case with that individual entry?
– Mostafa Ayaz
Nov 21 '18 at 20:56
just the individual entry
– Forextrader
Nov 21 '18 at 21:01
i modified what I posted originally. I was able to produce a matrix that only added 4 when entries were <0 but for some reason it would not multiply by 4 when entry was >=0
– Forextrader
Nov 21 '18 at 21:05
You said $B*4$, but all that does is return a value. You need to assign it, i.e. $A = B*4$.
– TrostAft
Nov 21 '18 at 20:44
You said $B*4$, but all that does is return a value. You need to assign it, i.e. $A = B*4$.
– TrostAft
Nov 21 '18 at 20:44
Thanks, I changed that but it still is not modifying matrix B
– Forextrader
Nov 21 '18 at 20:48
Thanks, I changed that but it still is not modifying matrix B
– Forextrader
Nov 21 '18 at 20:48
Do you mean even if just one entry was positive we multiply whole the matrix by 4 or just do the case with that individual entry?
– Mostafa Ayaz
Nov 21 '18 at 20:56
Do you mean even if just one entry was positive we multiply whole the matrix by 4 or just do the case with that individual entry?
– Mostafa Ayaz
Nov 21 '18 at 20:56
just the individual entry
– Forextrader
Nov 21 '18 at 21:01
just the individual entry
– Forextrader
Nov 21 '18 at 21:01
i modified what I posted originally. I was able to produce a matrix that only added 4 when entries were <0 but for some reason it would not multiply by 4 when entry was >=0
– Forextrader
Nov 21 '18 at 21:05
i modified what I posted originally. I was able to produce a matrix that only added 4 when entries were <0 but for some reason it would not multiply by 4 when entry was >=0
– Forextrader
Nov 21 '18 at 21:05
add a comment |
2 Answers
2
active
oldest
votes
Using Matlab vectorization tricks you can do that in one line:
A=B.*(B>0)*4+(B+6).*(B<0)
answered Nov 22 '18 at 13:49
Dmitry
633517
add a comment |
This is what your if-else is missing in your code:
if B(i,j) >= 0,
A(i,j) = 4*B(i,j);
else,
A(i,j) = 6+B(i,j);
end
Those (i,j) are what enables you to access/modify an entry.
However, as Dmitry pointed out, Matlab enables us to operate over matrices directly (I'm proposing my own variation of Dmitry's one-liner):
A = 4*B.*(aux = (B>=0)) +(B+6).*(1-aux)
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
add a comment |
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2 Answers
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active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Using Matlab vectorization tricks you can do that in one line:
A=B.*(B>0)*4+(B+6).*(B<0)
answered Nov 22 '18 at 13:49
Dmitry
633517
add a comment |
Using Matlab vectorization tricks you can do that in one line:
A=B.*(B>0)*4+(B+6).*(B<0)
answered Nov 22 '18 at 13:49
Dmitry
633517
add a comment |
Using Matlab vectorization tricks you can do that in one line:
A=B.*(B>0)*4+(B+6).*(B<0)
answered Nov 22 '18 at 13:49
Dmitry
633517
Using Matlab vectorization tricks you can do that in one line:
A=B.*(B>0)*4+(B+6).*(B<0)
answered Nov 22 '18 at 13:49
Dmitry
633517
answered Nov 22 '18 at 13:49
Dmitry
633517
answered Nov 22 '18 at 13:49
Dmitry
633517
answered Nov 22 '18 at 13:49
Dmitry
633517
633517
add a comment |
add a comment |
This is what your if-else is missing in your code:
if B(i,j) >= 0,
A(i,j) = 4*B(i,j);
else,
A(i,j) = 6+B(i,j);
end
Those (i,j) are what enables you to access/modify an entry.
However, as Dmitry pointed out, Matlab enables us to operate over matrices directly (I'm proposing my own variation of Dmitry's one-liner):
A = 4*B.*(aux = (B>=0)) +(B+6).*(1-aux)
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
add a comment |
This is what your if-else is missing in your code:
if B(i,j) >= 0,
A(i,j) = 4*B(i,j);
else,
A(i,j) = 6+B(i,j);
end
Those (i,j) are what enables you to access/modify an entry.
However, as Dmitry pointed out, Matlab enables us to operate over matrices directly (I'm proposing my own variation of Dmitry's one-liner):
A = 4*B.*(aux = (B>=0)) +(B+6).*(1-aux)
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
add a comment |
This is what your if-else is missing in your code:
if B(i,j) >= 0,
A(i,j) = 4*B(i,j);
else,
A(i,j) = 6+B(i,j);
end
Those (i,j) are what enables you to access/modify an entry.
However, as Dmitry pointed out, Matlab enables us to operate over matrices directly (I'm proposing my own variation of Dmitry's one-liner):
A = 4*B.*(aux = (B>=0)) +(B+6).*(1-aux)
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
This is what your if-else is missing in your code:
if B(i,j) >= 0,
A(i,j) = 4*B(i,j);
else,
A(i,j) = 6+B(i,j);
end
Those (i,j) are what enables you to access/modify an entry.
However, as Dmitry pointed out, Matlab enables us to operate over matrices directly (I'm proposing my own variation of Dmitry's one-liner):
A = 4*B.*(aux = (B>=0)) +(B+6).*(1-aux)
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
answered Nov 24 '18 at 5:31
Rócherz
2,7762721
2,7762721
add a comment |
add a comment |
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You said $B*4$, but all that does is return a value. You need to assign it, i.e. $A = B*4$.
– TrostAft
Nov 21 '18 at 20:44
Thanks, I changed that but it still is not modifying matrix B
– Forextrader
Nov 21 '18 at 20:48
Do you mean even if just one entry was positive we multiply whole the matrix by 4 or just do the case with that individual entry?
– Mostafa Ayaz
Nov 21 '18 at 20:56
just the individual entry
– Forextrader
Nov 21 '18 at 21:01
i modified what I posted originally. I was able to produce a matrix that only added 4 when entries were <0 but for some reason it would not multiply by 4 when entry was >=0
– Forextrader
Nov 21 '18 at 21:05