Formatting numbers with same width using f-strings python

Question

I want to format array of numbers with same width using f-strings. Numbers can be both positive or negative.

Minimum working example

import numpy as np  
arr = np.random.rand(10) - 0.5 
for num in arr:
    print(f"{num:0.4f}")

The result is

0.0647
-0.2608
-0.2724
0.2642
0.0429
0.1461
-0.3285
-0.3914

Due to the negative sign, the numbers are not printed off with the same width, which is annoying. How can I get the same width using f-strings?

One way that I can think of is converting number to strings and print string. But is there a better way than that?

for num in a: 
    str_ = f"{num:0.4f}" 
    print(f"{str_:>10}")

ForceBru · Accepted Answer · 2022-05-05 16:09:16Z

17

Use a space before the format specification:

#        v-- here
>>> f"{5: 0.4f}"
' 5.0000'
>>> f"{-5: 0.4f}"
'-5.0000'

Or a plus (+) sign to force all signs to be displayed:

>>> f"{5:+0.4f}"
'+5.0000'

edited May 5, 2022 at 16:09

answered May 4, 2019 at 17:48

ForceBru

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Comments

Eugene Yarmash · Accepted Answer · 2019-05-04 19:02:06Z

6

You can use the sign formatting option:

>>> import numpy as np
>>> arr = np.random.rand(10) - 0.5
>>> for num in arr:
...     print(f'{num: .4f}')  # note the leading space in the format specifier
...
 0.1715
 0.2838
-0.4955
 0.4053
-0.3658
-0.2097
 0.4535
-0.3285
-0.2264
-0.0057

To quote the documentation:

The sign option is only valid for number types, and can be one of the following:

Option    Meaning
'+'       indicates that a sign should be used for both positive as well as
          negative numbers.
'-'       indicates that a sign should be used only for negative numbers (this
          is the default behavior).
space     indicates that a leading space should be used on positive numbers,
          and a minus sign on negative numbers.

edited May 4, 2019 at 19:02

answered May 4, 2019 at 17:51

Eugene Yarmash

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Comments

runlevel0 · Accepted Answer · 2023-09-04 19:48:17Z

In Python3 (at least 3.11.2) the format is attained by just adding enough space:

from numpy import arange

def f1(x):
     return((x**2)-(4*x)+9)

for i in arange(-1,1,0.1):
    print( print(f"{i:5.2f}\t{f1(i):7.2f}")

Being the result:

-1.00 14.00 -0.90 13.41 -0.80 12.84 -0.70 12.29 -0.60 11.76 -0.50 11.25 -0.40 10.76 -0.30 10.29 -0.20 9.84 -0.10 9.41 -0.00 9.00 0.10 8.61 0.20 8.24 0.30 7.89 0.40 7.56 0.50 7.25 0.60 6.96 0.70 6.69 0.80 6.44 0.90 6.21

The first digit before the point defines the spaces, thus 5 would be enough to hold 2 decimals, the minus or a space and the dot. For more decimals you would need to adjust the first number correspondingly.

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