7.6 Foreach Loops
A second form of for loops allows iteration over elements of
containers. If ys
is an expression denoting a container (vector,
row vector, matrix, or array) with elements of type T
, then the
following is a well-formed foreach statement.
for (y in ys) {
... do something with y ...
}
The order in which elements of ys
are visited is defined for container types as follows.
vector
,row_vector
: elements visited in order,y
is of typedouble
matrix
: elements visited in column-major order,y
is of typedouble
T[]
: elements visited in order,y
is of typeT
.
Consequently, if ys
is a two dimensional array real[ , ]
, y
will
be a one-dimensional array of real values (type real[]
). If ’ysis a matrix, then
ywill be a real value (type
real`). To loop over
all values of a two-dimensional array using foreach statements would
require a doubly-nested loop,
real yss[2, 3];
for (ys in yss)
for (y in ys)
... do something with y ...
whereas a matrix can be looped over in one foreach statement
matrix[2, 3] yss;
for (y in yss)
... do something with y...
In both cases, the loop variable y
is of type real. The elements of the matrix are visited in column-major order (e.g.,
y[1, 1],
y[2, 1],
y[1, 2], ...,
y[2, 3]), whereas the elements of the two-dimensional array are visited in row-major order (e.g.,
y[1, 1],
y[1, 2],
y[1, 3],
y[2, 1], ...,
y[2, 3]`).