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convert element in a list to float

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  convert element in a list to float forums_mp@hotmail.com - 2015-09-13 12:55 -0700
    Re: convert element in a list to float Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-13 21:07 +0100
    Re: convert element in a list to float Laura Creighton <lac@openend.se> - 2015-09-13 23:02 +0200
      Re: convert element in a list to float Denis McMahon <denismfmcmahon@gmail.com> - 2015-09-13 23:43 +0000
    Re: convert element in a list to float Steven D'Aprano <steve@pearwood.info> - 2015-09-14 11:09 +1000

#96522 — convert element in a list to float

For starters, I googled and saw a plethora of writings on how to convert an entire list from string to float.   My interest is on select elements in the list.  The output from the print statement: print scenarioList 
  
is as follows

[ '3000000', '"N"', '11400000', '"E"' ]

I need to convert the first and third element to float.  
  lat = ( float ) scenarioList [ 0 ]
  lon = ( float ) scenarioList [ 2 ]

fails (invalid syntax).  How can I achieve my objective. 

Thanks in advance

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#96523

On 13/09/2015 20:55, forums_mp@hotmail.com wrote:
>
> For starters, I googled and saw a plethora of writings on how to convert an entire list from string to float.   My interest is on select elements in the list.  The output from the print statement: print scenarioList
>
> is as follows
>
> [ '3000000', '"N"', '11400000', '"E"' ]
>
> I need to convert the first and third element to float.
>    lat = ( float ) scenarioList [ 0 ]
>    lon = ( float ) scenarioList [ 2 ]
>
> fails (invalid syntax).  How can I achieve my objective.
>
> Thanks in advance
>

Strong hint, you do not cast the strings to floats, you call the builtin 
float() function to do the conversion.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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#96528

In a message of Sun, 13 Sep 2015 12:55:13 -0700, forums_mp@hotmail.com writes:
>
>
>For starters, I googled and saw a plethora of writings on how to convert an entire list from string to float.   My interest is on select elements in the list.  The output from the print statement: print scenarioList 
>  
>is as follows
>
>[ '3000000', '"N"', '11400000', '"E"' ]
>
>I need to convert the first and third element to float.  
>  lat = ( float ) scenarioList [ 0 ]
>  lon = ( float ) scenarioList [ 2 ]
>
>fails (invalid syntax).  How can I achieve my objective. 
>
>Thanks in advance

Does this help?

>>> l = [ '3000000', '"N"', '11400000', '"E"' ]
>>> [float(l[0]), l[1], float(l[2]), l[3]]
[3000000.0, '"N"', 11400000.0, '"E"']

Laura

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#96533

On Sun, 13 Sep 2015 23:02:55 +0200, Laura Creighton wrote:

> In a message of Sun, 13 Sep 2015 12:55:13 -0700, forums_mp@hotmail.com
> writes:
>>
>>For starters, I googled and saw a plethora of writings on how to convert
>>an entire list from string to float.   My interest is on select elements
>>in the list.  The output from the print statement: print scenarioList
>>  
>>is as follows
>>
>>[ '3000000', '"N"', '11400000', '"E"' ]
>>
>>I need to convert the first and third element to float.
>>  lat = ( float ) scenarioList [ 0 ]
>>  lon = ( float ) scenarioList [ 2 ]
>>
>>fails (invalid syntax).  How can I achieve my objective.
>>
>>Thanks in advance
> 
> Does this help?
> 
>>>> l = [ '3000000', '"N"', '11400000', '"E"' ]
>>>> [float(l[0]), l[1], float(l[2]), l[3]]
> [3000000.0, '"N"', 11400000.0, '"E"']

Here's a method that will convert any value in a list that can be made a 
float into a float, and (I think) should leave all others as they are. It 
users a helper function and a list comprehension.

>>> def tofloat(x):
...     try:
...         return float(x)
...     except ValueError:
...         return None
... 
>>> l = [ '3000000', '"N"', '11400000', '"E"' ]
>>> l = [ tofloat(x) or x for x in l ]
>>> l
[3000000.0, '"N"', 11400000.0, '"E"']

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#96538

Hi Forums_MP and welcome,


On Mon, 14 Sep 2015 05:55 am, forums_mp@hotmail.com wrote:

> For starters, I googled and saw a plethora of writings on how to convert
> an entire list from string to float.   My interest is on select elements
> in the list.  The output from the print statement: print scenarioList
>   
> is as follows
> 
> [ '3000000', '"N"', '11400000', '"E"' ]
> 
> I need to convert the first and third element to float.
>   lat = ( float ) scenarioList [ 0 ]
>   lon = ( float ) scenarioList [ 2 ]
> 
> fails (invalid syntax).  How can I achieve my objective.


By now you have hopefully discovered that the answer is to call float as a
function:

lat = float(scenarioList[0])


but I wonder what in the documentation or examples you saw suggested to you
that Python used the C type-cast syntax `(float)value`? Whatever it was
that gave you this wrong impression needs to be fixed.




-- 
Steven

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