how do you convert and array of doubles into floats?

[SpreadTooThin]

I have some code...

import array

a = array.array('d')
f = open('file.raw')
a.fromfile(f, 10)

now I need to convert them into floats (32 bit...) what do i do?

[Diez B. Roggisch]

SpreadTooThin schrieb:

I have some code...

import array

a = array.array('d')
f = open('file.raw')
a.fromfile(f, 10)

now I need to convert them into floats (32 bit...) what do i do?

I guess module struct is your friend.

Something like this:

struct.pack("f" * len(a), *a)

Diez

[Marc 'BlackJack' Rintsch]

In <11**********************@i3g2000cwc.googlegroups. com>, SpreadTooThin
wrote:

I have some code...

import array

a = array.array('d')
f = open('file.raw')
a.fromfile(f, 10)

now I need to convert them into floats (32 bit...) what do i do?

What about:

b = array.array('f', a)

Ciao,
Marc 'BlackJack' Rintsch

[Diez B. Roggisch]

Marc 'BlackJack' Rintsch schrieb:

In <11**********************@i3g2000cwc.googlegroups. com>, SpreadTooThin
wrote:

>I have some code...

import array

a = array.array('d')
f = open('file.raw')
a.fromfile(f, 10)

now I need to convert them into floats (32 bit...) what do i do?

What about:

b = array.array('f', a)

AFAIK d and f are synonym for arrays, as python doesn't distinguish
between these two on a type-level. And double it is in the end.

Diez

[Marc 'BlackJack' Rintsch]

In <4n************@uni-berlin.de>, Diez B. Roggisch wrote:

Marc 'BlackJack' Rintsch schrieb:

>In <11**********************@i3g2000cwc.googlegroups. com>, SpreadTooThin
wrote:

>>I have some code...

import array

a = array.array('d')
f = open('file.raw')
a.fromfile(f, 10)

now I need to convert them into floats (32 bit...) what do i do?

What about:

b = array.array('f', a)

AFAIK d and f are synonym for arrays, as python doesn't distinguish
between these two on a type-level. And double it is in the end.

No `array.array` is really about "C compiler types". You get C doubles in
form of Python's `float` type if you read from the `array.array` but it's
stored as C float and you can get the binary representation with the
`tostring()` method.

Ciao,
Marc 'BlackJack' Rintsch

[Tim Peters]

[Marc 'BlackJack' Rintsch]

>What about:

b = array.array('f', a)

[Diez B. Roggisch]

AFAIK d and f are synonym for arrays, as python doesn't distinguish
between these two on a type-level. And double it is in the end.

While Python has no type of its own corresponding to the native C
`float`, the `array` and `struct` modules do understand the native C
`float` . A Python float (same as a C `double`) gets converted to a C
`float` when stored into one of those, and a C `float` is converted to
a Python float (C `double`) when a value is extracted.

>>from array import array
x = 1.0000000001
x

1.0000000001

>>array('d', [x])[0] # full precision is preserved

1.0000000001

>>array('d', [x])[0] == x

True

>>array('f', [x])[0] # trailing bits are lost

1.0

>>array('f', [x])[0] == x

False

[SpreadTooThin]

Tim Peters wrote:

[Marc 'BlackJack' Rintsch]

What about:

b = array.array('f', a)

[Diez B. Roggisch]

AFAIK d and f are synonym for arrays, as python doesn't distinguish
between these two on a type-level. And double it is in the end.

While Python has no type of its own corresponding to the native C
`float`, the `array` and `struct` modules do understand the native C
`float` . A Python float (same as a C `double`) gets converted to a C
`float` when stored into one of those, and a C `float` is converted to
a Python float (C `double`) when a value is extracted.

>from array import array
x = 1.0000000001
x

1.0000000001

>array('d', [x])[0] # full precision is preserved

1.0000000001

>array('d', [x])[0] == x

True

>array('f', [x])[0] # trailing bits are lost

1.0

>array('f', [x])[0] == x

False

The point being that when I say
b.tofile(f) I expect the write to write 32 bit floats.

[Diez B. Roggisch]

>AFAIK d and f are synonym for arrays, as python doesn't distinguish

>between these two on a type-level. And double it is in the end.

No `array.array` is really about "C compiler types". You get C doubles in
form of Python's `float` type if you read from the `array.array` but it's
stored as C float and you can get the binary representation with the
`tostring()` method.

I skimmed the docs of array and saw 'floating point' in both, assuming
they were the same. Rereading them I actually recognized the "minimum
size in bytes" column - and that is 4 for "f" and 8 for "double".

If I were in "pardoning" mode, I'd say "but minimum doesn't exclude the
possibility of floats having 8 bytes" - but I'm not :) So, I stand
corrected.

Thanks,

Diez