Copy an array to a new shape
Array_Type _reshape (Array_Type A, Array_Type I)
The _reshape function creates a copy of an array A,
reshapes it to the form specified by I and returns the result.
The elements of I specify the new dimensions of the copy of
A and must be consistent with the number of elements A.
If A is a 100 element 1-d array, a new array 2-d array of
size 20 by 5 may be created from the elements of A
by
A = _reshape (A, [20, 5]);
__tmp operator to avoid the
creation of a new array, i.e.,
A = _reshape (__tmp(A), [20,5]);
The reshape function performs a similar function to
_reshape. In fact, the _reshape function could have been
implemented via:
define _reshape (a, i)
{
a = @a; % Make a new copy
reshape (a, i);
return a;
}
reshape, array_info
Returns information about an array
(Array_Type, Integer_Type, DataType_Type) array_info (Array_Type a)
The array_info function returns information about the array a.
It returns three values: an 1-d integer array array specifying the
size of each dimension of a, the number of dimensions of
a, and the data type of a.
The array_info function may be used to find the number of rows
of an array:
define num_rows (a)
{
variable dims, num_dims, data_type;
(dims, num_dims, data_type) = array_info (a);
return dims [0];
}
length function.
typeof, reshape, length, _reshape
Apply a function to each element of an array
Array_Type array_map (type, func, arg0, ...)
DataType_Type type;
Ref_Type func;
The array_map function may be used to apply a function to each
element of an array and returns the result as an array of a
specified type. The type parameter indicates what kind of
array should be returned and generally corresponds to the return
type of the function. The arg0 parameter should be an array
and is used to determine the dimensions of the resulting array. If
any subsequent arguments correspond to an array of the same size,
then those array elements will be passed in parallel with the first
arrays arguments.
The first example illustrates how to apply the strlen function
to an array of strings:
S = ["", "Train", "Subway", "Car"];
L = array_map (Integer_Type, &strlen, S);
S = ["", "Train", "Subway", "Car"];
L = Integer_Type [length (S)];
for (i = 0; i < length (S); i++) L[i] = strlen (S[i]);
Now consider an example involving the strcat function:
files = ["slang", "slstring", "slarray"];
exts = ".c";
cfiles = array_map (String_Type, &strcat, files, exts);
% ==> cfiles = ["slang.c slstring.c slarray.c"];
exts = [".a",".b",".c"];
xfiles = array_map (String_Type, &strcat, files, exts);
% ==> xfiles = ["slang.a", "slstring.b", "slarray.c"];
Many mathemetical functions already work transparantly on arrays. For example, the following two statements produce identical results:
B = sin (A);
B = array_map (Double_Type, &sin, A);
array_info, strlen, strcat, sin
Sort an array
Array_Type array_sort (Array_Type a [, String_Type or Ref_Type f])
array_sort sorts the array a into ascending order and
returns an integer array that represents the result of the sort. If
the optional second parameter f is present, the function
specified by f will be used to compare elements of a;
otherwise, a built-in sorting function will be used.
If f is present, then it must be either a string representing
the name of the comparison function, or a reference to the function.
The sort function represented by f must be a S-lang
user-defined function that takes two arguments. The function must
return an integer that is less than zero if the first parameter is
considered to be less than the second, zero if they are equal, and a
value greater than zero if the first is greater than the second.
If the comparision function is not specified, then a built-in comparison
function appropriate for the data type will be used. For example,
if a is an array of character strings, then the sort will be
preformed using strcmp.
The integer array returned by this function is simply an index that
indicates the order of the sorted array. The input array a is
not changed.
An array of strings may be sorted using the strcmp function
since it fits the specification for the sorting function described
above:
variable A = String_Type [3];
A[0] = "gamma"; A[1] = "alpha"; A[2] = "beta";
variable I = array_sort (A, &strcmp);
variable I = array_sort (A);
After the array_sort has executed, the variable I will
have the values [2, 0, 1]. This array can be used to
re-shuffle the elements of A into the sorted order via the
array index expression A = A[I].
strcmp
Initialize an array of characters
init_char_array (Array_Type a, String_Type s)
The init_char_array function may be used to initialize a
character array a by setting the elements of the array
a to the corresponding characters of the string s.
The statements
variable a = Char_Type [10];
init_char_array (a, "HelloWorld");
"HelloWorld".
The character array must be large enough to hold all the characters of the initialization string.
bstring_to_array, strlen, strcat
Get the length of an object
Integer_Type length (obj)
The length function may be used to get information about the
length of an object. For simple scalar data-types, it returns 1.
For arrays, it returns the total number of elements of the array.
If obj is a string, length returns 1 because a
String_Type object is considered to be a scalar. To get the
number of characters in a string, use the strlen function.
array_info, typeof, strlen
Reshape an array
reshape (Array_Type A, Array_Type I)
The reshape function changes the size of A to have the size
specified by the 1-d integer array I. The elements of I
specify the new dimensions of A and must be consistent with
the number of elements A.
If A is a 100 element 1-d array, it can be changed to a
2-d 20 by 5 array via
reshape (A, [20, 5]);
reshape(A, [11,5]) will result in an error because
the the [11,5] array specifies 55 elements.
Since reshape modifies the shape of an array, and arrays are
treated as references, then all references to the array will
reference the new shape. If this effect is unwanted, then use the
_reshape function instead.
_reshape, array_info
Transpose a 2d array
Array_Type transpose (Array_Type a)
The transpose function returns the transpose of a specified
array. By definition, the transpose of an array, say one with
elements a[i,j,...k] is an array whose elements are
a[k,...,j,i].
_reshape, reshape, array_info
Get indices where an integer array is non-zero
Array_Type where (Array_Type a)
The where function examines an integer array a and
returns a 2-d integer array whose rows are the indices of a
where the corresponding element of a is non-zero.
Consider the following:
variable X = [0.0:10.0:0.01];
variable A = sin (X);
variable I = where (A < 0.0);
A[I] = cos (X) [I];
X has been assigned an array of doubles
whose elements range from 0.0 through 10.0 in
increments of 0.01. The second statement assigns A to
an array whose elements are the sin of the elements of X.
The third statement uses the where function to get the indices of
the elements of A that are less than 0.0. Finally, the
last statement substitutes into A the cos of the
elements of X at the positions of A where the
corresponding sin is less than 0. The end result is
that the elements of A are a mixture of sines and cosines.
array_info, sin, cos