Check array for NULL elements
Char_Type[] = _isnull (a[])
This function may be used to test for the presence of NULL elements
of an array. Specifically, it returns a Char_Type
array of
with the same number of elements and dimensionality of the input
array. If an element of the input array is NULL
, then the
corresponding element of the output array will be set to 1
,
otherwise it will be set to 0
.
Set all NULL
elements of a string array A
to the empty
string ""
:
A[where(_isnull(A))] = "";
It is important to understand the difference between A==NULL
and _isnull(A)
. The latter tests all elements of A
against NULL
, whereas the former only tests A
itself.
where, array_map
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]);
In this example, the original array was no longer needed. Hence, it
is preferable to make use of the __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];
}
For 1-d arrays, this information is more easily obtained from the
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);
This is equivalent to:
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);
Alternatively, one may use
variable I = array_sort (A);
to use the built-in comparison function.
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");
creates an character array and initializes its elements to the
characters in the string "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
Get the maximum value of an array
result = max (Array_Type a [,Int_Type dim])
The max
function examines the elements of a numeric array and
returns the value of the largest element. If a second argument is
given, then it specifies the dimension of the array to be searched.
In this case, an array of dimension one less than that of the input array
will be returned with the corresponding elements in the specified
dimension replaced by the minimum value in that dimension.
Consider the 2-d array
1 2 3 4 5
6 7 8 9 10
generated by
a = _reshape ([1:10], [2, 5]);
Then max(a)
will return 10
, and max(a,0)
will return
a 1-d array with elements
6 7 8 9 10
max, sum, reshape
Get the minimum value of an array
result = min (Array_Type a [,Int_Type dim])
The min
function examines the elements of a numeric array and
returns the value of the smallest element. If a second argument is
given, then it specifies the dimension of the array to be searched.
In this case, an array of dimension one less than that of the input array
will be returned with the corresponding elements in the specified
dimension replaced by the minimum value in that dimension.
Consider the 2-d array
1 2 3 4 5
6 7 8 9 10
generated by
a = _reshape ([1:10], [2, 5]);
Then min(a)
will return 1
, and min(a,0)
will return
a 1-d array with elements
1 2 3 4 5
max, sum, reshape
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]);
However, 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
Sum over the elements of an array
result = sum (Array_Type a [, Int_Type dim])
The sum
function sums over the elements of a numeric array and
returns its result. If a second argument is given, then it
specifies the dimension of the array to be summed over. In this
case, an array of dimension one less than that of the input array
will be returned.
If the input array is an integer type, then the resulting value will
be a Double_Type
. If the input array is a Float_Type
,
then the result will be a Float_Type
.
The mean of an array a
of numbers is
sum(a)/length(a)
transpose, reshape
Transpose an 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, sum, array_info
Get indices where an integer array is non-zero
Array_Type where (Array_Type a)
The where
function examines an numeric array a
and
returns an integer array giving the indices of a
where the corresponding element of a
is non-zero.
Although this function may appear to be simple or even trivial, it is arguably one of the most important and powerful functions for manipulating arrays.
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];
Here the variable 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