how p(:,j) differs from p(j,:)?

I often see people using these,

how p(:,j) differs from p(j,:)?

Answer 1

[Azzi Abdelmalek]

 p=[1 2;3 4;5 6]

check this

p(:,2)
p(3,:)

p(:,2) means all lines, column number 2, p(3,:) means all columns, line number 3

[Hajar ALSHOUFI]

if we have n points =21

I saw a code the author wrote:

p(ny,:) where p is some matrix, how can we understad it, I know the first one refers to rows and the second for columns but how can we understand it what rows is he speaking about

Answer 2

[Wayne King]

p(:,j) takes all the rows and the j-th column of p

p(j,:) takes the j-th row and all the columns of p

So

   p = [1 2; 3 4];
   j = 1;
   p(j,:)
   p(:,j)