TypeError with map with no len()

[Paul Moore]

You're using Python 3, and I suspect that you're working from
instructions that assume Python 2. In Python 3, the result of map() is
a generator, not a list (which is what Python 2's map returned). In
order to get an actual list (which appears to be what you need for
your plot call) you just need to call the list constructor:

y1 = list(map(math.sin, math.pi*t))

Although given that you're using numpy, it may be that there's a more
idiomatic numpy way of doing this. I'm not a numpy expert though, so I
can't help on that.

Paul

On 25 September 2017 at 17:44, john polo <jpolo at mail.usf.edu> wrote:
> Python List,
>
> I am trying to make practice data for plotting purposes. I am using Python
> 3.6. The instructions I have are
>
> import matplotlib.pyplot as plt
> import math
> import numpy as np
> t = np.arange(0, 2.5, 0.1)
> y1 = map(math.sin, math.pi*t)
> plt.plot(t,y1)
>
> However, at this point, I get a TypeError that says
>
> object of type 'map' has no len()
>
> In [6]: t
> Out[6]:
> array([ 0. ,  0.1,  0.2,  0.3,  0.4,  0.5,  0.6,  0.7, 0.8,  0.9,  1. ,
>         1.1,  1.2,  1.3,  1.4,  1.5,  1.6,  1.7,  1.8, 1.9,  2. ,  2.1,
>         2.2,  2.3,  2.4])
> In [7]: y1
> Out[7]: <map at 0x6927128>
> In [8]: math.pi*t
> Out[8]:
> array([ 0.        ,  0.31415927,  0.62831853,  0.9424778 , 1.25663706,
>         1.57079633,  1.88495559,  2.19911486,  2.51327412, 2.82743339,
>         3.14159265,  3.45575192,  3.76991118,  4.08407045, 4.39822972,
>         4.71238898,  5.02654825,  5.34070751,  5.65486678, 5.96902604,
>         6.28318531,  6.59734457,  6.91150384,  7.2256631 , 7.53982237])
>
> At the start of creating y1, it appears there is an array to iterate through
> for the math.sin function used in map(), but y1 doesn't appear to be saving
> any values. I expected y1 to hold a math.sin() output for each item in
> math.pi*t, but it appears to be empty. Am I misunderstanding map()? Is there
> something else I should be doing instead to populate y1?
>
>
> John
>
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