如何确定字符串中是否有浮点数

Question

I aim to print a list of every element inside a user-inputted list. The outputted list identifies symbols as strings, and numbers as floats, to basically compare elements as floats and strings. If the number in the list is already a float, the output is not printed properly.

expression_list = []
expression = "(3.1+2)*5"

for index in expression:
    try: 
        float_index = float(index)
        expression_list.append(float_index)
    except ValueError:
        expression_list.append(index)
print(expression_list)     

I expect the output to be ['(', 3.1, '+', 2.0, ')', '*', 5.0] instead I get ['(', 3.0, '.', 1.0, '+', 2.0, ')', '*', 5.0]

Answer 1

I would use re.findall here:

expression = "(3.1+2)*5"
output = re.findall(r'[()]|[^\w]+|(?:\d+(?:\.\d+)?)', expression)
print(output)

['(', '3.1', '+', '2', ')', '*', '5']

The pattern used is:

[()] | [^\w]+ | (?:\d+(?:\.\d+)?)

This matches:

[()]              an opening or closing parenthesis, as a separate match
[^\w]             any other non word character
(?:\d+(?:\.\d+)?  a number, with or without a decimal component

Answer 2

This is happening because your for loop for index in expression: iterates through every character of the string, so it is not checking if 3.1 is a float, it checks if 3 is a float and 1 is a float by converting it, and float('3')=3.0 , hence you see the result.

In [8]: expression_list = [] 
   ...: expression = "(3.1+2)*5" 
   ...:  
   ...:  
   ...: for index in expression: 
   ...:     print(index) 
   ...:                                                                                                                                                                           
(
3
.
1
+
2
)
*
5

Perhaps you can split the strings in a manner which extract out the floats and the operators and brackets, which Tim's answer has provided to you.

Assuming you have that output, your code will work as expected

import re

expression_list = []
expression = "(3.1+2)*5"

literals = re.findall(r'[()]|[^\w]+|(?:\d+(?:\.\d+)?)', expression)

for index in literals:
    try:
        float_index = float(index)
        expression_list.append(float_index)
    except ValueError:
        expression_list.append(index)
print(expression_list)

The output will be ['(', 3.1, '+', 2.0, ')', '*', 5.0] as you expected!