fortran之complex精度类型与cmplx函数精度类型大坑

cmplx(x,y)出来的结果为complex的默认精度，不管x，y的精度是多少

 

从fortran wiki （http://fortranwiki.org/fortran/show/cmplx）上看cmplx函数有三种形式：

 

1.cmplx(x). x可以是interger、real、complex， 结果为complex的默认精度

 

2.cmplx(x,y). x可以是interger、real，y为integer、real，结果为complex的默认精度

 

3.cmplx(x,y,kind). x可以是interger、real，y为integer、real，kind为interger，结果精度为kind

 

complex(kind=n) 等价于complex*2n。例如：

complex(4) 等价于 complex*8

complex(8) 等价于 complex*16

complex(16) 等价于 complex*32

 

complex默认为complex(real, real)类型，real受编译器的real影响。

 

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from: http://fortranwiki.org/fortran/show/cmplx

Description

cmplx(x [, y [, kind]]) returns a complex number where x is converted to the real component. If y is present it is converted to the imaginary component. If y is not present then the imaginary component is set to 0.0. If x is complex then y must not be present.

Standard

FORTRAN 77 and later

Class

Elemental function

Syntax

result = cmplx(x [, y [, kind]])

Arguments

• x - The type may be integer, real, or complex.
• y - (Optional; only allowed if x is not complex.) May be integer or real.
• kind - (Optional) An integer initialization expression indicating the kind parameter of the result.

Return value

The return value is of complex type, with a kind equal to kind if it is specified. If kind is not specified, the result is of the default complex kind, regardless of the kinds of x and y.

Example

program test_cmplx
    integer :: i = 42
    real :: x = 3.14
    complex :: z
    z = cmplx(i, x)
    print *, z, cmplx(x)
end program test_cmplx

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from:https://software.intel.com/content/www/us/en/develop/documentation/fortran-compiler-developer-guide-and-reference/top/language-reference/data-types-constants-and-variables/intrinsic-data-types/complex-data-types.html

Complex data types can be specified as follows:

COMPLEX

COMPLEX([KIND=]n)

COMPLEX*s

DOUBLE COMPLEX

 

n 

Is a constant expression that evaluates to kind 4, 8, or 16.

 

s

Is 8, 16, or 32. COMPLEX(4) is specified as COMPLEX*8; COMPLEX(8) is specified as COMPLEX*16; COMPLEX(16) is specified as COMPLEX*32.

 

 

If a kind parameter is specified, the complex constant has the kind specified. If no kind parameter is specified, the kind of both parts is default real, and the constant is of type default complex.

 

Default real is affected by compiler option real-size and by the REAL directive.

 

The default KIND for DOUBLE COMPLEX is affected by compiler option double-size. If the compiler option is not specified, default DOUBLE COMPLEX is COMPLEX(8). No kind parameter is permitted for data declared with type DOUBLE COMPLEX.

 

A complex number of any kind is made up of a real part and an imaginary part. The REAL and AIMAG intrinsic functions return the real and imaginary parts of a complex number respectively. The CMPLX intrinsic constructs a complex number from two real numbers. The %re and %im complex part designators access the real and imaginary parts of a complex number respectively.