How to check if a string contains only numbers?

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Rahul Pillai
Rahul Pillai 2017-12-14
评论： Steven Lord 2025-4-16
Is there any expression for this ? For example :
-100 should return 1
a100b should return 0
0.1256 should return 1 and so on

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Walter Roberson
Walter Roberson 2017-12-14
The easiest way is to check if str2double() returns nan. Otherwise you need to parse the string and there are a number of odd cases to account for in scientific notation.
Note: if you do use str2double() you should give some consideration as to what result you want for complex numbers, whether those are to be treated as numeric or as invalid because of the 'i' or 'j' present.
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Walter Roberson
Walter Roberson 2025-4-16
Ran in:
@Charles Kluepfel
Hah, yes,
nan == nan
ans = logical
0
nan >= nan
ans = logical
0
nan < nan
ans = logical
0
all returning false is an oddity of floating point numbers. Oddly enough,
nan ~= nan
ans = logical
1
returns true, so an alternate way to test for nan is
x ~= x
Steven Lord
Steven Lord 2025-4-16
I would not describe the behavior of the relational operators with one of the inputs being NaN an "oddity". It's how NaN is defined to work in the IEEE 754 specification, as summarized on the NaN Wikipedia page. In IEEE 754-2019 it's section 5.11. A short excerpt:
"Four mutually exclusive relations are possible: less than, equal, greater than, and unordered; unordered arises when at least one operand is a NaN. Every NaN shall compare unordered with everything, including itself."
If you want to tell if something is a NaN and don't want to use isnan or ismissing for whatever reason, you could use isequaln instead of isequal. isequaln explicitly treats NaN as equal to NaN.

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