How can I detect either numbers or letters in a string? I am aware you use the ASCII codes, but what functions take advantage of them?
You may use str.isdigit()
and str.isalpha()
to check whether given string is positive integer and alphabet respectively.
Sample Results:
# For alphabet
>>> 'A'.isdigit()
False
>>> 'A'.isalpha()
True
# For digit
>>> '1'.isdigit()
True
>>> '1'.isalpha()
False
str.isdigit()
returns False
if the string is a negative number or a float number. For example:
# returns `False` for float
>>> '123.3'.isdigit()
False
# returns `False` for negative number
>>> '-123'.isdigit()
False
If you want to also check for the negative integers and float
, then you may write a custom function to check for it as:
def is_number(n):
try:
float(n) # Type-casting the string to `float`.
# If string is not a valid `float`,
# it'll raise `ValueError` exception
except ValueError:
return False
return True
Sample Run:
>>> is_number('123') # positive integer number
True
>>> is_number('123.4') # positive float number
True
>>> is_number('-123') # negative integer number
True
>>> is_number('-123.4') # negative `float` number
True
>>> is_number('abc') # `False` for "some random" string
False
The above functions will return True
for the "NAN" (Not a number) string because for Python it is valid float representing it is not a number. For example:
>>> is_number('NaN')
True
In order to check whether the number is "NaN", you may use math.isnan()
as:
>>> import math
>>> nan_num = float('nan')
>>> math.isnan(nan_num)
True
Or if you don't want to import additional library to check this, then you may simply check it via comparing it with itself using ==
. Python returns False
when nan
float is compared with itself. For example:
# `nan_num` variable is taken from above example
>>> nan_num == nan_num
False
Hence, above function is_number
can be updated to return False
for "NaN"
as:
def is_number(n):
is_number = True
try:
num = float(n)
# check for "nan" floats
is_number = num == num # or use `math.isnan(num)`
except ValueError:
is_number = False
return is_number
Sample Run:
>>> is_number('Nan') # not a number "Nan" string
False
>>> is_number('nan') # not a number string "nan" with all lower cased
False
>>> is_number('123') # positive integer
True
>>> is_number('-123') # negative integer
True
>>> is_number('-1.12') # negative `float`
True
>>> is_number('abc') # "some random" string
False
The above function will still return you False
for the complex numbers. If you want your is_number
function to treat complex numbers as valid number, then you need to type cast your passed string to complex()
instead of float()
. Then your is_number
function will look like:
def is_number(n):
is_number = True
try:
# v type-casting the number here as `complex`, instead of `float`
num = complex(n)
is_number = num == num
except ValueError:
is_number = False
return is_number
Sample Run:
>>> is_number('1+2j') # Valid
True # : complex number
>>> is_number('1+ 2j') # Invalid
False # : string with space in complex number represetantion
# is treated as invalid complex number
>>> is_number('123') # Valid
True # : positive integer
>>> is_number('-123') # Valid
True # : negative integer
>>> is_number('abc') # Invalid
False # : some random string, not a valid number
>>> is_number('nan') # Invalid
False # : not a number "nan" string
PS: Each operation for each check depending on the type of number comes with additional overhead. Choose the version of is_number
function which fits your requirement.