Formatting Numbers So They Align On Decimal Point

Question

In Python, I need to format numbers so they align on the decimal point, like so:

  4.8
 49.723
456.781
-72.18
  5
 13

Is there a straighforward way to do this?

Answer 1

If you know the precision (digits after the decimal point) that you need, and you don't mind having some trailing zeros when whole numbers are used, you could use the new f-string in Python 3.6 (PEP498):

numbers = [4.8, 49.723, 456.781, -72.18, 5, 13]

for number in numbers:
    print(f'{number:9.4f}')

Prints:

  4.8000
 49.7230
456.7810
-72.1800
  5.0000
 13.0000

Answer 2

I don't think there's a straight-forward way to do it, since you need to know the position of the decimal point in all the numbers before you start printing them. (I just had a look at Caramiriel's link, and some of the links from that page, but I couldn't find anything particularly applicable to this case).

So it looks like you have to do some string-based inspection & manipulation of the numbers in the list. Eg,

def dot_aligned(seq):
    snums = [str(n) for n in seq]
    dots = [s.find('.') for s in snums]
    m = max(dots)
    return [' '*(m - d) + s for s, d in zip(snums, dots)]

nums = [4.8, 49.723, 456.781, -72.18]

for s in dot_aligned(nums):
    print(s)

output

  4.8
 49.723
456.781
-72.18

---

If you want to handle a list of floats with some plain ints mixed in, then this approach gets a bit messier.

def dot_aligned(seq):
    snums = [str(n) for n in seq]
    dots = []
    for s in snums:
        p = s.find('.')
        if p == -1:
            p = len(s)
        dots.append(p)
    m = max(dots)
    return [' '*(m - d) + s for s, d in zip(snums, dots)]

nums = [4.8, 49.723, 456.781, -72.18, 5, 13]

for s in dot_aligned(nums):
    print(s)
    

output

  4.8
 49.723
456.781
-72.18
  5
 13

---

As Mark Ransom notes in the comments, we can simplify handling ints by using .split:

def dot_aligned(seq):
    snums = [str(n) for n in seq]
    dots = [len(s.split('.', 1)[0]) for s in snums]
    m = max(dots)
    return [' '*(m - d) + s for s, d in zip(snums, dots)]

---

Masher mentions in a comment that it can be useful to add padding on the right so that the numbers can be printed in aligned columns. However, we don't need to compute the size of that padding for each string, we can use the str.ljust method.

def dot_aligned(seq):
    snums = [str(n) for n in seq]
    dots = [len(s.split('.', 1)[0]) for s in snums]
    m = max(dots)
    left_pad = [' '*(m - d) + s for s, d in zip(snums, dots)]
    ms = max(map(len, left_pad))
    return [s.ljust(ms) for s in left_pad]

nums = [4.8, 49.723, 456.781, -72.18, 5, 13, 1.2345] * 3
cols = 4
# Get number of cells in the output grid, using ceiling division
size = len(nums) // -cols * -cols

padded = dot_aligned(nums)
for i in range(0, size, cols):
    print(*padded[i:i+cols])

output

  4.8     49.723  456.781  -72.18  
  5       13        1.2345   4.8   
 49.723  456.781  -72.18     5     
 13        1.2345   4.8     49.723 
456.781  -72.18     5       13     
  1.2345