How to find a value in column 2 which corresponds to particular value in column 1?

Let say I have a 2 colomn matrix A=[1 3 7 9 4 5 3 2 4; 2 3 1 3 4 5 9 2 4], Based on the value particular vaue in colomn 1 how can I find corresponding value in B for example when I have a value of 9 in then how can I look for corresponding value in colomn 2?

Answer 1

[Image Analyst]

I think it's ambiguously worded but I tried to follow your directions as precisely as I could:

format short g
% Define A with 2 rows and 9 columns
A=[1 3 7 9 4 5 3 2 4; 2 3 1 3 4 5 9 2 4]

A = 2×9

1 3 7 9 4 5 3 2 4 2 3 1 3 4 5 9 2 4

% Define B
B = rand(size(A))

B = 2×9

0.15297 0.37938 0.683 0.88068 0.96106 0.65517 0.19194 0.51023 0.1633 0.55587 0.12864 0.91176 0.37441 0.20156 0.88471 0.36068 0.58696 0.97853

% Poster says "Based on the value particular vaue in colomn 1 
% how can I find corresponding value in B?
% For example when I have a value of 9 in then how can I look for corresponding value
% in colomn 2?"

% Find a particular value in A's column 1. 
% The location of that value was not specified by the poster.
% Let's say that you want the value in the first row of column 1 of A.
value = A(1,1) % = 1 for this example.

value = 
     1

% Now assume that value we want that value from the corresponding row (1)
% of matrix B but in column 2.  In other words B(1, 2)
output = B(value, 2)

output = 
      0.37938

[Muhammad Qaisar Fahim]

I think the question is unclear. Let me rephrase it. I have a big matrix but for sake of understanding I am considering a small matrix here. A=[1 3 7 9 4 5 3 2 4; 2 3 1 3 4 5 9 2 4]. It has 2 rows and 9 colomns. Now let say the value 1st row of A is selected as 3 which is the 2nd value. I want its corresponding value in row two. I know it can be done by indexing the row and colomn numbers but I want a way which is based on value. such that when I cal value three it will give the corresponding entries.

[Torsten]

A=[1 3 7 9 4 5 3 2 4; 2 3 1 3 4 5 9 2 4]

A = 2×9

1 3 7 9 4 5 3 2 4 2 3 1 3 4 5 9 2 4

value = A(2,A(1,3))

value = 9

if I understood correctly what you want.

[Muhammad Qaisar Fahim]

Why this is giving 9?

[Muhammad Qaisar Fahim]

All of the above answers are talking about ndexing. Actually I dont want simple indexing. I want value in row two which correspond to the value in row one. Not the index of row 1

[Torsten]

So what do you expect as result if 3 is selected for the first row ? 3 is not only 2nd element, but also 7th element of the first row.

Your explanation is still unclear what you expect as result.

Just give an example with solution.

[Muhammad Qaisar Fahim]

I am sorry If my explanation is unclear to you let me explain with example.

Say A=[1 2 3 4 5 6 7 8 9 10; 11 12 13 14 15 16 17 18 19 20]

Now for I want value in row two which is corresponding to value "6" in row 1. One method is simply indexing like A(2,6) gives 16. But I dont want this method of indexing. I want a function which based on the value of variable x give the correponding value in row. if x=6 it gives 16 if x=1 it gives 11 and if x=10 it gives 20 etc. I hope it will help to understand the question.

[Image Analyst]

Try this:

A=[1 2 3 4 5 6 7 8 9 10; 11 12 13 14 15 16 17 18 19 20]

A = 2×10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

x = 6;
% Find columns where A is 6 in the first row
columns = find(A(1,:) == x);
% Get the values of A in the second row of those locations.
output = A(2, columns)

output = 16

[Muhammad Qaisar Fahim]

Thanks it helped but a follow up question on my dtat set NOw I want to save the values by running this command and I am using this for loop but it is giving error. How can I fix this? I have attached screen shots of the data along with the dimenstions so that you can understand what I want to extract. So actually I am extracting the value of x from the connections matrix (in screen shot) and then finding the corresponding value in Slope_Indices matrix. I want to save all the values in order in Elevation_Node1.

for k=1:78153

Node=Connections(k,1)

Node1 = Slope_Indices(:,1) == Node;

Elevation_Node1(k) = Slope_Indices(Node1, 2)

end

[Image Analyst]

Node1 is either true or false, not an index like you'd get from find()

Node1 = find(Slope_Indices(:,1) == Node);

I think you can invest 2 hours in this

MATLAB Academy - Free 2 hour training

and learn how to do it yourself. It's just basic indexing. If you can't do this then you're going to have trouble with every other step beyond this. So learn the basics and you'll be self sufficient.

[Muhammad Qaisar Fahim]

Thanks that works

Answer 2

[KSSV]

A=[1 3 7 9 4 5 3 2 4; 2 3 1 3 4 5 9 2 4] ;
idx =  A(1,:)==9 

idx = 1×9 logical array
   0   0   0   1   0   0   0   0   0

iwant = A(2,idx)

iwant = 3

[Muhammad Qaisar Fahim]

Thanks @KSSV. Actually the above answer is talking about ndexing. Actually I dont want simple indexing. I want value in row two which correspond to the value in row one. Not corresponding to the index of row 1