How to detect if a string begins with a number?

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In order to detect that a string is a number it is usually propossed this:

bool isParam(string line)
{
    char* p;
    strtol(line.c_str(), &p, 10);
    return *p == 0;
}

Unfortunately for me *p points to first character if no number is inside, that is no 0

How I have to use the strtol()?

What I have tried:

Then I tried this that seems to work unless somebody writes decimals beginning by a point:

bool isNumber(string str1) 
{ 	
    char p = str1.c_str()[0]; return (p >= '0' && p <= '9'); 
}

After Richard MacCutchan post this worked for me:

bool isnumber(string str1) { return (isdigit(str1.c_str()[0])>0);}

Posted 3-Jul-17 23:04pm

Javier Luis Lopez

Updated 4-Jul-17 23:11pm

v3

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[no name] 4-Jul-17 6:41am

And you tested also the case where one starts a number with "." or "+" or "-"?
:-)

3 solutions

Solution 2

If you need to detect also floating point values, then use strtod[^] (and afterwards check the value of the str_end parameter).

Posted 4-Jul-17 0:23am

CPallini

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Richard MacCutchan · Accepted Answer · 2017-07-04T00:02:00

Solution 1

Study the C library functions, and you will find isdigit().

Posted 4-Jul-17 0:02am

Richard MacCutchan

Javier Luis Lopez · Accepted Answer · 2017-07-04T23:11:00

Because an unknown reason, I had an Assertion error when using isdigit() on character -96 ' ', so I should recommend using:

if (line[0] >= '0' && line[0] <= '9')

instead of isdigit()

Here is the function used (the line[] has tabulators):

//char line[]="N 2	32.1dC	28.7dC	25.2dC	20.7dC	19.6dC	18.5dC	74 %	67 %	52 %	4 kph	2 kph	0 kph";
double string_to_double2(char *line, int num)
{
	int i1 = 0, max = (int) strlen(line);
	for (int i = 0; i < max; i++)
	{
		//if (isdigit(line[0])>0)
		if (line[0] >= '0' && line[0] <= '9')
		{
			if (i1 == num) return strtod(line,&line);
			strtod(line, &line);i1++;
		}
		else line++;
	}
	return -99999999;
}