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Diffstat (limited to 'Doc/library/cmath.rst')
| -rw-r--r-- | Doc/library/cmath.rst | 120 |
1 files changed, 107 insertions, 13 deletions
diff --git a/Doc/library/cmath.rst b/Doc/library/cmath.rst index 5a9ae05..f78f69c 100644 --- a/Doc/library/cmath.rst +++ b/Doc/library/cmath.rst @@ -14,8 +14,81 @@ method: these methods are used to convert the object to a complex or floating-point number, respectively, and the function is then applied to the result of the conversion. -The functions are: +.. note:: + On platforms with hardware and system-level support for signed + zeros, functions involving branch cuts are continuous on *both* + sides of the branch cut: the sign of the zero distinguishes one + side of the branch cut from the other. On platforms that do not + support signed zeros the continuity is as specified below. + + +Complex coordinates +------------------- + +Complex numbers can be expressed by two important coordinate systems. +Python's :class:`complex` type uses rectangular coordinates where a number +on the complex plain is defined by two floats, the real part and the imaginary +part. + +Definition:: + + z = x + 1j * y + + x := real(z) + y := imag(z) + +In engineering the polar coordinate system is popular for complex numbers. In +polar coordinates a complex number is defined by the radius *r* and the phase +angle *φ*. The radius *r* is the absolute value of the complex, which can be +viewed as distance from (0, 0). The radius *r* is always 0 or a positive float. +The phase angle *φ* is the counter clockwise angle from the positive x axis, +e.g. *1* has the angle *0*, *1j* has the angle *π/2* and *-1* the angle *-π*. + +.. note:: + While :func:`phase` and func:`polar` return *+π* for a negative real they + may return *-π* for a complex with a very small negative imaginary + part, e.g. *-1-1E-300j*. + + +Definition:: + + z = r * exp(1j * φ) + z = r * cis(φ) + + r := abs(z) := sqrt(real(z)**2 + imag(z)**2) + phi := phase(z) := atan2(imag(z), real(z)) + cis(φ) := cos(φ) + 1j * sin(φ) + + +.. function:: phase(x) + + Return phase, also known as the argument, of a complex. + + .. versionadded:: 2.6 + + +.. function:: polar(x) + + Convert a :class:`complex` from rectangular coordinates to polar + coordinates. The function returns a tuple with the two elements + *r* and *phi*. *r* is the distance from 0 and *phi* the phase + angle. + + .. versionadded:: 2.6 + + +.. function:: rect(r, phi) + + Convert from polar coordinates to rectangular coordinates and return + a :class:`complex`. + + .. versionadded:: 2.6 + + + +cmath functions +--------------- .. function:: acos(x) @@ -37,30 +110,35 @@ The functions are: .. function:: asinh(x) - Return the hyperbolic arc sine of *x*. There are two branch cuts, extending - left from ``±1j`` to ``±∞j``, both continuous from above. These branch cuts - should be considered a bug to be corrected in a future release. The correct - branch cuts should extend along the imaginary axis, one from ``1j`` up to - ``∞j`` and continuous from the right, and one from ``-1j`` down to ``-∞j`` - and continuous from the left. + Return the hyperbolic arc sine of *x*. There are two branch cuts: + One extends from ``1j`` along the imaginary axis to ``∞j``, + continuous from the right. The other extends from ``-1j`` along + the imaginary axis to ``-∞j``, continuous from the left. + + .. versionchanged:: 2.6 + branch cuts moved to match those recommended by the C99 standard .. function:: atan(x) Return the arc tangent of *x*. There are two branch cuts: One extends from - ``1j`` along the imaginary axis to ``∞j``, continuous from the left. The + ``1j`` along the imaginary axis to ``∞j``, continuous from the right. The other extends from ``-1j`` along the imaginary axis to ``-∞j``, continuous - from the left. (This should probably be changed so the upper cut becomes - continuous from the other side.) + from the left. + + .. versionchanged:: 2.6 + direction of continuity of upper cut reversed .. function:: atanh(x) Return the hyperbolic arc tangent of *x*. There are two branch cuts: One - extends from ``1`` along the real axis to ``∞``, continuous from above. The + extends from ``1`` along the real axis to ``∞``, continuous from below. The other extends from ``-1`` along the real axis to ``-∞``, continuous from - above. (This should probably be changed so the right cut becomes continuous - from the other side.) + above. + + .. versionchanged:: 2.6 + direction of continuity of right cut reversed .. function:: cos(x) @@ -78,6 +156,21 @@ The functions are: Return the exponential value ``e**x``. +.. function:: isinf(x) + + Return *True* if the real or the imaginary part of x is positive + or negative infinity. + + .. versionadded:: 2.6 + + +.. function:: isnan(x) + + Return *True* if the real or imaginary part of x is not a number (NaN). + + .. versionadded:: 2.6 + + .. function:: log(x[, base]) Returns the logarithm of *x* to the given *base*. If the *base* is not @@ -151,3 +244,4 @@ cuts for numerical purposes, a good reference should be the following: nothing's sign bit. In Iserles, A., and Powell, M. (eds.), The state of the art in numerical analysis. Clarendon Press (1987) pp165-211. + |