Matplotlib: TypeError: can't multiply sequence by non-int of type 'numpy.float64'

Question

I am trying to fit a linear line of best fit to my matplotlib graph. I keep getting the error that x and y do not have the same first dimension. But they both have lengths of 15. What am I doing wrong?

import matplotlib.pyplot as plt
from scipy import stats
import numpy as np

x = [0.46,0.59,0.68,0.99,0.39,0.31,1.09,0.77,0.72,0.49,0.55,0.62,0.58,0.88,0.78]
y = [0.315,0.383,0.452,0.650,0.279,0.215,0.727,0.512,0.478,0.335,0.365,0.424,0.390,0.585,0.511]
xerr = [0.01]*15
yerr = [0.001]*15

plt.rc('font', family='serif', size=13)
m, b = np.polyfit(x, y, 1)
plt.plot(x,y,'s',color='#0066FF')
plt.plot(x, m*x + b, 'r-') #BREAKS ON THIS LINE
plt.errorbar(x,y,xerr=xerr,yerr=0,linestyle="None",color='black')
plt.xlabel('$\Delta t$ $(s)$',fontsize=20)
plt.ylabel('$\Delta p$ $(hPa)$',fontsize=20)
plt.autoscale(enable=True, axis=u'both', tight=False)
plt.grid(False)
plt.xlim(0.2,1.2)
plt.ylim(0,0.8)
plt.show()

Error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~\AppData\Local\Temp/ipykernel_34116/1820029981.py in <module>
      7 m, b = np.polyfit(x, y, 1)
      8 plt.plot(x,y,'s',color='#0066FF')
----> 9 plt.plot(x, m*x + b, 'r-') #BREAKS ON THIS LINE
     10 plt.errorbar(x,y,xerr=xerr,yerr=0,linestyle="None",color='black')
     11 plt.xlabel('$\Delta t$ $(s)$',fontsize=20)

TypeError: can't multiply sequence by non-int of type 'numpy.float64'

Bas Swinckels · Accepted Answer · 2021-11-07T12:12:14.147

You should make x and y numpy arrays, not lists:

x = np.array([0.46,0.59,0.68,0.99,0.39,0.31,1.09,
              0.77,0.72,0.49,0.55,0.62,0.58,0.88,0.78])
y = np.array([0.315,0.383,0.452,0.650,0.279,0.215,0.727,0.512,
              0.478,0.335,0.365,0.424,0.390,0.585,0.511])

With this change, it produces the expected plot. If they are lists, m * x will not produce the result you expect, but an empty list. Note that m is anumpy.float64 scalar, not a standard Python float.

I actually consider this a bit dubious behavior of Numpy. In normal Python, multiplying a list with an integer just repeats the list:

In [42]: 2 * [1, 2, 3]
Out[42]: [1, 2, 3, 1, 2, 3]

while multiplying a list with a float gives an error (as I think it should):

In [43]: 1.5 * [1, 2, 3]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-43-d710bb467cdd> in <module>()
----> 1 1.5 * [1, 2, 3]
TypeError: can't multiply sequence by non-int of type 'float'

The weird thing is that multiplying a Python list with a Numpy scalar apparently works:

In [45]: np.float64(0.5) * [1, 2, 3]
Out[45]: []

In [46]: np.float64(1.5) * [1, 2, 3]
Out[46]: [1, 2, 3]

In [47]: np.float64(2.5) * [1, 2, 3]
Out[47]: [1, 2, 3, 1, 2, 3]

So it seems that the float gets truncated to an int, after which you get the standard Python behavior of repeating the list, which is quite unexpected behavior. The best thing would have been to raise an error (so that you would have spotted the problem yourself instead of having to ask your question on Stackoverflow) or to just show the expected element-wise multiplication (in which your code would have just worked). Interestingly, addition between a list and a Numpy scalar does work:

In [69]: np.float64(0.123) + [1, 2, 3]
Out[69]: array([ 1.123,  2.123,  3.123])

wait I don't get it, what was the problem? Just converting to `np.array`'s fixes things? — Charlie Parker, Sep 28 '17 at 19:31
@CharlieParker: Yes it works. Nothing strange here, let me explain. When the OP did `m,b=np.polyfit(x,y,1)`, here you get `m` to be a float64 value, for example `m=0.642` in the above case. Now when you try to multiply a float `m` with a list `x`, you end up getting an empty list `[]`. It is because to multiply each element of a list with a value, you need to either convert the list to a `numpy array` or use the built in `map` function. See [this answer](https://stackoverflow.com/questions/8194959/how-to-multiply-individual-elements-of-a-list-with-a-number) for more info. — Srivatsan, Sep 29 '17 at 07:02

Srivatsan · Answer 2 · 2015-04-24T12:21:48.960

Changing your lists to numpy arrays will do the job!!

import matplotlib.pyplot as plt
from scipy import stats
import numpy as np 

x = np.array([0.46,0.59,0.68,0.99,0.39,0.31,1.09,0.77,0.72,0.49,0.55,0.62,0.58,0.88,0.78]) # x is a numpy array now
y = np.array([0.315,0.383,0.452,0.650,0.279,0.215,0.727,0.512,0.478,0.335,0.365,0.424,0.390,0.585,0.511]) # y is a numpy array now
xerr = [0.01]*15
yerr = [0.001]*15

plt.rc('font', family='serif', size=13)
m, b = np.polyfit(x, y, 1)
plt.plot(x,y,'s',color='#0066FF')
plt.plot(x, m*x + b, 'r-') #BREAKS ON THIS LINE
plt.errorbar(x,y,xerr=xerr,yerr=0,linestyle="None",color='black')
plt.xlabel('$\Delta t$ $(s)$',fontsize=20)
plt.ylabel('$\Delta p$ $(hPa)$',fontsize=20)
plt.autoscale(enable=True, axis=u'both', tight=False)
plt.grid(False)
plt.xlim(0.2,1.2)
plt.ylim(0,0.8)
plt.show()

enter image description here

@CharlieParker: Yes it works. Nothing strange here, let me explain. When the OP did `m,b=np.polyfit(x,y,1)`, here you get `m` to be a float64 value, for example `m=0.642` in the above case. Now when you try to multiply a float `m` with a list `x`, you end up getting an empty list `[]`. It is because to multiply each element of a list with a value, you need to either convert the list to a `numpy array` or use the built in `map` function. See [this answer](https://stackoverflow.com/questions/8194959/how-to-multiply-individual-elements-of-a-list-with-a-number) for more info. — Srivatsan, Sep 29 '17 at 07:00

Matplotlib: TypeError: can't multiply sequence by non-int of type 'numpy.float64'

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