How to convert float notation to power of 10 scientific notation in Python?

Question

In Python, I have values given by 0.000001,0.00001,0.0001,....,1.0 stored in my_Array (np.array). For each of these values I need to label a curve in a plot and store it in the legend as val = 10e-6 instead of val = 0.000001. The second version is automatically stored if I use (for the i'th value):

matplolib.pyplot.plot(...., label = 'val = ' + str(my_Array[i]))

Is there a function converting the float notation to the scientific power of 10 notation?

This is built into the basic string formatting, try e.g. `format(0.00001, '.0e')`. — jonrsharpe, Jul 24 '18 at 15:48

score 12 · Accepted Answer · answered Jul 24 '18 at 16:09

You may use a combination of a ScalarFormatter and a FuncFormatter to format your values as mathtext. I.e. instead of 0.01 or 1e-2 it would look like .

import matplotlib.pyplot as plt
import matplotlib.ticker as mticker

vals = [0.000001,0.00001,0.0001,0.01,1.0]

f = mticker.ScalarFormatter(useOffset=False, useMathText=True)
g = lambda x,pos : "${}$".format(f._formatSciNotation('%1.10e' % x))
fmt = mticker.FuncFormatter(g)

fig, ax = plt.subplots()
for i,val in enumerate(vals):
    ax.plot([0,1],[i,i], label="val = {}".format(fmt(val)))

ax.legend()    
plt.show()

This is essentially the same concept as in Can I show decimal places and scientific notation on the axis of a matplotlib plot?, just not for the axes, but the legend.

AttributeError: 'ScalarFormatter' object has no attribute '_formatSciNotation' — zxdawn, Jun 02 '22 at 11:58

Lodi · Answer 2 · 2018-07-24T15:58:34.767

1

You can just get the round between '%.2E%' and the desired number.

'%.2E' % Decimal('40800000000.00000000000000')

# returns '4.08E+10'

as seen on Display a decimal in scientific notation

The '%.2E%' rounds to the 2nd decimal point. To round to just the first one, use '%.1E%'.

-

To achieve what you want, just:

x = 0.0001
y = '%.2E' % x

print (y)

# prints 1.00E-04

(EDITED after jonrsharpe's tip)

edited Jul 24 '18 at 15:58

answered Jul 24 '18 at 15:44

Lodi

565
4
16

2

Conversion to `Decimal` is completely unnecessary here, that string format works on any numerical types. – jonrsharpe Jul 24 '18 at 15:48
can you please edit your answer explaining that the '2' is rounding to the second decimal? – Giacomo Jul 24 '18 at 15:49
Also *"You can just get the modulus"* is totally wrong, that's **not** what is happening. That's a different usage of the `%` symbol than modulus. – jonrsharpe Jul 24 '18 at 15:55
I'll try to gather some more info and edit my answer, then – Lodi Jul 24 '18 at 15:58
1

you can add the "format()" method mentioned by @jonrsharpe in your answer. – Giacomo Jul 24 '18 at 16:00

score 1 · Answer 3 · answered Sep 06 '19 at 08:37

In theory the below should work without the need to call private functions. However, something is broken in version 3.11 at least such that no check to the power limits is actually done inside ScalarFormatter.

import matplotlib.ticker as mticker
f = mticker.ScalarFormatter(useMathText=True)
f.set_powerlimits((-3,3))
"${}$".format(f.format_data(0.0001))

How to convert float notation to power of 10 scientific notation in Python?

3 Answers3