|MadSci Network: Computer Science|
The ALLOCATE statement and ALLOCATABLE type qualifier are features introduced in Fortran 90. They allow arrays whose size is determined at run time rather than at compile time. In a nutshell, ALLOCATABLE is used as part of a type declaration to tell the compiler that the size of the array will be determined later, as the program runs. The compiler keeps track of the array name, but no space is actually allocated until you use the ALLOCATE statement, which reserves memory for the array. It is similar to leaving the last subscript of an array pointer unspecified in C, except that Fortran allows you to leave the size of multiple subscripts unspecified until run time. For example, you can easily create a square matrix whose size is not specified in FORTRAN using this declaration: REAL, ALLOCATABLE :: matrix(:,:) There is no direct equivalent in C, since it is limited to leaving the last subscript unspecified. The ALLOCATE statement sets aside space for the array itself. It is usually coded with one or two parameters, although you can actually allocate multiple arrays with a single ALLOCATE statement. The first parameter specifies the size of the array by giving the array name and the number of elements in the array. For a multidimensional array, separate the size of the subscripts by commas. The second parameter is optional. If you use it, an error code is stored in the variable you list. The error code is zero if the program was able to allocate enough memory for the array, and greater than zero if the program could not allocate the memory--in which case you canít use the array, of course. If an error occurs and you have used an error parameter, the program keeps going, allowing you to handle the error on your own. If an error occurs and you did not give the error parameter, the program will stop with some sort of error. To allocate a matrix for our previous example based on a parameter passed to a subroutine, we could use the statement ALLOCATE (matrix(n,n), STAT = errorCode) Compared to C, ALLOCATE does the same job as malloc(), but it is more specialized. malloc() allocates any kind of memory, while ALLOCATE is only used to allocate memory for variable size arrays. There is also a DEALLOCATE statement that frees the memory used by the array. You may not see it often in real code, since the array is also deallocated automatically when the program block it is in finishes. For example, if you ALLOCATE and array in a SUBROUTINE, the array is automatically deallocated when you return from that subroutine. Itís parameters are the names of one or more arrays separated by commas, with an optional STAT error parameter. Mike Westerfield Byte Works, Inc. References: http://www.pcc.qub.ac.uk/tec/courses/f90/ohp/header_ohMIF_11.html http://www.nsc.liu.se/~boein/f77to90/a3.html#section10
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