fortran中的一些函数

函数:SIGN(A,B)

Description:

SIGN(A,B)

returns the value of A with the sign of B.

Standard:Fortran 77 and later

Class:Elemental function

Syntax:

RESULT = SIGN(A, B)

Arguments:

A	Shall be of type INTEGER or REAL
B	Shall be of the same type and kind as A

SIGN(A，B）以B的符号返回A的值。返回值的符号位B的符号，返回值的绝对值为A的绝对值

print *,sign(1,-8)

print *,sign(9,-5)

print *,sign(-3,2)

输出分别为-1，-9,3

MAXLOC

— Location of the maximum value within an array

MAXLOC函数返回最大值在数组中的位置
当没有指定位维数参数的时候，返回值的维数等于数组的rank

Description:
Determines the location of the element in the array with the maximum value, or, if the DIM argument is supplied, determines the locations of the maximum element along each row of the array in the DIM direction.
If MASK is present, only the elements for which MASK is

.TRUE.

are considered. If more than one element in the array has the maximum value, the location returned is that of the first such element in array element order. If
the array has zero size, or all of the elements of MASK are

.FALSE.

, then the result is an array of zeroes. Similarly, if DIM is supplied and all of the elements of MASK along a given row are zero, the result value
for that row is zero.

Standard:Fortran 95 and later

Class:Transformational function

Syntax:

RESULT = MAXLOC(ARRAY, DIM [, MASK])

RESULT = MAXLOC(ARRAY [, MASK])

Arguments:

ARRAY	Shall be an array of type INTEGER or REAL .
DIM	(Optional) Shall be a scalar of type INTEGER , with a value between one and the rank of ARRAY, inclusive. It may not be an optional dummy argument.
MASK	Shall be an array of type LOGICAL , and conformable with ARRAY.

Return value:If DIM is absent, the result is a rank-one array with a length equal to the rank of ARRAY. If DIM is present, the result is an array with a rank one less than the rank of ARRAY, and a size corresponding to the
size of ARRAY with theDIM dimension removed. If DIM is present and ARRAY has a rank of one, the result is a scalar. In all cases, the result is of default

INTEGER

type.

program main
implicit none
integer ::i,j
integer,dimension(2) ::pos
integer,dimension(2:4,3:6) ::arr

data((arr(i,j),i=2,4),j=3,6)/1,3,4,5,6,7,8,9,10,25,76,89/
pos=maxloc(arr)
write(*,*)'i=',pos(1),'j=',pos(2)
pos=minloc(arr)
write(*,*)'i=',pos(1),'j=',pos(2)

end

程序输出为
i= 3 j= 4

i= 1 j= 1
输出的最大值的位置为相对于第一个元素的位置，是相对位置，而不是该元素的实际索引
比如第一个元素的位置是(2,3) maxloc对应元素的实际位置需(3,4)+(2,3)-(1,1)=(4,6)
minloc函数也相同

EQUIVALENCE

cite from （http://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vn9b/index.html）
equivalence表达式

The EQUIVALENCE statement specifies that two or more variables or arrays in a program unit share
the same memory.

EQUIVALENCE (nlist)
[, (nlist)] ...

Parameter	Description
nlist	List of variable names, array element names, array names, and character substring names separated by commas

An EQUIVALENCE statement stipulates that the storage sequence of the entities whose names appear
in the list nlist must have the same first memory location.

EQUIVALENCE语句规定了名字出现在nlist中的实体的存储序列必须具有相同的首个内存位置

An array name, if present, refers to the first element of the array.
如果数组名出现在参数中，那么它代表数组的第一个元素

program main
implicit none
integer,dimension(2) ::arr
integer ::i
equivalence(i,arr(1))
arr(1)=5
write(*,*) i  !输出5
i=100
write(*,*) arr(1)   !输出100
end

equivalence语句有点像C++的引用，equivalence的参数中的变量指向了内存中的同一个位置，修改其中一个参数，其他参数的值也就跟着改变

获取cpu的时间，可以用于测试某段代码所花的时间
cite from http://gcc.gnu.org/onlinedocs/gcc-4.0.4/gfortran/CPU_005fTIME.html

7.34

CPU_TIME

— CPU elapsed time in seconds

Description:Returns a

REAL

value representing the elapsed CPU time in seconds. This is useful for testing segments of code to determine execution time.

Option:f95, gnu

Class:subroutine

Syntax:

CPU_TIME(X)

Arguments:

X	The type shall be REAL with intent out.

Return value:None

Example:

program test_cpu_time
real :: start, finish
call cpu_time(start)
! put code to test here
call cpu_time(finish)
print '("Time = ",f6.3," seconds.")',finish-start
end program test_cpu_time

13. Common blocks -f77 common block

cite from http://www.stanford.edu/class/me200c/tutorial_77/13_common.html
Fortran 77 has no global variables, i.e. variables that are shared among several program units (subroutines). The only way to pass information between subroutines we have seen so far is to use the subroutine
parameter list. Sometimes this is inconvenient, e.g., when many subroutines share a large set of parameters. In such cases one can use a common block. This is a way to specify that certain variables should be shared among certain subroutines. But
in general, the use of common blocks should be minimized.

Syntax

common / name / list-of-variables

Example

Suppose you have two parameters alpha and beta that many of your subroutines need. The following example shows how it can be done using common blocks.

program main
some declarations
real alpha, beta
common /coeff/ alpha, beta

statements
stop
end

subroutine sub1 (some arguments)
declarations of arguments
real alpha, beta
common /coeff/ alpha, beta

statements
return
end

subroutine sub2 (some arguments)
declarations of arguments
real alpha, beta
common /coeff/ alpha, beta

statements
return
end

fortran77没有全局变量，在子程序之间传递信息的方法就是通过子程序的参数列表。
common block
当子程序之间共享大集合的变量的时候这就不方便了.这种方式指定特定的变量在特定的子程序之间共享

比如变量alpha与beta需要在不同子程序之间共享，可以定义如下
这里斜杠里面的coeff是common block的名字
common /coeff/ alpha, beta

The common statement should appear together with the variable declarations, before the executable statements.
Different common blocks must have different names (just like variables).
A variable cannot belong to more than one common block.
The variables in a common block do not need to have the same names each place they occur (although it is a good idea to do so), but they must be listed in the same order and have the same type and size.

common语句应该与变量声明放在一起，位于执行语句之前（不然通不过编译）

common block中的变量不需要在每一次出现的时候使用相同的变量名（尽管用相同变量名比较好），但是他们必须被以同样的顺序排列，具有相同的类型与大小

To illustrate this, look at the following continuation of our example:

subroutine sub3 (some arguments)
declarations of arguments
real a, b
common /coeff/ a, b

statements
return
end

This declaration is equivalent to the previous version that used alpha and beta.

the following paragraph cites from f90 standard
5.5.2.1 Common block storage sequence

For each common block, a common block storage sequence is formed as follows:

(1) A storage sequence is formed consisting of the sequence of storage units contained in the storage

sequences (14.6.3.1) of all data objects in the common block object lists for the common block. The

order of the storage sequences is the same as the order of the appearance of the common block

object lists in the scoping unit.

(2) The storage sequence formed in (1) is extended to include all storage units of any storage sequence

associated with it by equivalence association. The sequence may be extended only by adding

storage units beyond the last storage unit. Data objects associated with an entity in a common block

are considered to be in that common block.

common block对象列表中的数据对象在内存中的布局是连续的，顺序与common block对象列表中出现的顺序相同

ibset

ibset将参数i的二进制表示中位置pos上的位设置为1并且返回第一个参数i的值

cite from

http://gcc.gnu.org/onlinedocs/gfortran/IBSET.html

IBSET

— Set bit

Description:

IBSET

returns the value of I with the bit at position POS set to one.

Standard:Fortran 95 and later

Class:Elemental function

Syntax:

RESULT = IBSET(I, POS)

Arguments:

I	The type shall be INTEGER .
POS	The type shall be INTEGER .

Return value:
The return value is of type

INTEGER

and of the same kind as I.

ibclr返回第一个参数i的值并将二进制表示中pos位上的位设为0

IBCLR

— Clear bit

Description:

IBCLR

returns the value of I with the bit at position POS set to zero.

Standard:Fortran 95 and later

Class:Elemental function

Syntax:

RESULT = IBCLR(I, POS)

Arguments:

I	The type shall be INTEGER .
POS	The type shall be INTEGER .

Return value:The return value is of type

INTEGER

and of the same kind as I.

如果参数i的二进制表示中pos位是1，函数返回true

BTEST

— Bit test function

Description:

BTEST(I,POS)

returns logical

.TRUE.

if the bit at POS in I is set. The counting of the bits starts at 0.

Standard:Fortran 95 and later

Class:Elemental function

Syntax:

RESULT = BTEST(I, POS)

Arguments:

I	The type shall be INTEGER .
POS	The type shall be INTEGER .

Return value:The return value is of type

LOGICAL

ishft参数中shift>0为左移，shift等于0不移位，shift<0为右移

8.135

ISHFT

— Shift bits

Description:

ISHFT

returns a value corresponding to I with all of the bits shifted SHIFT places. A value of SHIFT greater than zero corresponds to a left shift, a value of zero corresponds to no shift, and a value less than
zero corresponds to a right shift. If the absolute value of SHIFT is greater than

BIT_SIZE(I)

, the value is undefined. Bits shifted out from the left end or right end are lost; zeros are shifted in from the opposite end.

Standard:Fortran 95 and later

Class:Elemental function

Syntax:

RESULT = ISHFT(I, SHIFT)

Arguments:

I	The type shall be INTEGER .
SHIFT	The type shall be INTEGER .

Return value:
The return value is of type

INTEGER

and of the same kind as I.

8.180

MVBITS

— Move bits from one integer to another

Description:Moves LEN bits from positions FROMPOS through

FROMPOS+LEN-1

of FROM to positions TOPOS through

TOPOS+LEN-1

of TO. The portion of argument TO not affected by the
movement of bits is unchanged. The values of

FROMPOS+LEN-1

and

TOPOS+LEN-1

must be less than

BIT_SIZE(FROM)

.

Standard:Fortran 95 and later

Class:Elemental subroutine

Syntax:

CALL MVBITS(FROM, FROMPOS, LEN, TO, TOPOS)

Arguments:

FROM	The type shall be INTEGER .
FROMPOS	The type shall be INTEGER .
LEN	The type shall be INTEGER .
TO	The type shall be INTEGER , of the same kind as FROM.
TOPOS	The type shall be INTEGER .

8.35

BIT_SIZE

— Bit size inquiry function

Description:

BIT_SIZE(I)

returns the number of bits (integer precision plus sign bit) represented by the type of I. The result of

BIT_SIZE(I)

is independent of the actual value of I.

Standard:Fortran 95 and later

Class:Inquiry function

Syntax:

RESULT = BIT_SIZE(I)

Arguments:

I	The type shall be INTEGER .