Write a for loop that goes through each row of Matrix

Hi

I'm kinda stuck here, how to write a for-loop that goes through each row of matrix Z and does the following:

• If the second column value is 1, display the first column value;
• if the second column value is 2, display the negative of the first column value;
• if it is 3 then times the first column by 2

Here's what I currently have:

for i = 1:size(Z,1)
  
   if Z(i,2)==1
       value1 = Z(i,1)
       disp(value1)
   
   else Z(i,2)==2
       value2 = Z(i,1)<0
       disp(value2)
   end
   
   if Z(i,2)==3
       value3 = Z(i,1)*5
       disp(value3)
   end
   
end

The Z can be any matrix, e.g.,

    Z = [27     3;
    55     3;
    53     2;
    24     3;
    49     1;
    63     1]; 

[Walter Roberson]

What should be done if the second column is neither 1 nor 2?

[Victor Li]

if it is 3 then times the first column by 2

[Walter Roberson]

Your current code multiplies the first column by 5 instead of by 2 for the case of 3.

And remember, that for 2, you not expected to output information about whether the first column is negative: you are expected to output the negative of the first column. For example your entry [53 2] should result in -53 being displayed.

Answer 1

[Walter Roberson]

for i = 1:size(Z,1)

and refer to Z(i,1) and Z(i,2)

[Victor Li]

thanks, but how would the size function help me index into the 1st and 2nd column values in the matrix? Just updated with the newest code I wrote, is this what your are implying?

[Walter Roberson]

size(Z,1) gives you the information about the number of rows that are in Z. You will be using for i to loop over the row numbers. At any one point, i will be your current row number.

for i = 1:size(Z,1)
  
   if Z(i,2)==1
       value1 = Z(i,1)
       disp(value)
   elseif Z(i,2)==2          %<--- notice
       value2 = Z(i,1)<0
       disp(value2)

That part will test whether the first column is less than 0, and will display 1 if it is negative and 0 if it is not negative. But that is not what your assignment asks. For example if the first column is 5 then you need to display -5 and if the first column is -5 then you need to display 5.

There are a number of different ways to calculate the negative of a number. For example, the classical definition is

       negative_x = 0 - x