Checking String with is_numeric in C

Question

I am a newbie in stack exchange code review. I just wrote a C function where the function checks if the given string is numeric or not. What do you think about my way of doing this? And could this be done in other ways?

#include <stdio.h>
#include <stdbool.h>

bool is_numeric(const char* str)
{
    while (isspace(*str))
       str++;
    
    if (*str == '-' || *str == '+')
       str++;

    while (*str)
    {
        if (!isdigit(*str) && !isspace(*str))
            return false;
        str++;
    }

    return true;
}

int main(int argc, char** argv)
{
  printf("%sn", is_numeric("123436")  ? "true" : "false"); // should be true
  printf("%sn", is_numeric("123.436") ? "true" : "false"); // should be false
  printf("%sn", is_numeric("    567") ? "true" : "false"); // should be true
  printf("%sn", is_numeric("235   ")  ? "true" : "false"); // should be true
  printf("%sn", is_numeric("794,347") ? "true" : "false"); // should be false
  printf("%sn", is_numeric("hello")   ? "true" : "false"); // should be false
  printf("%sn", is_numeric("-3423")   ? "true" : "false"); // should be true
}

Thanks in advance!

c++

chux - Reinstate Monica · Answer

Bug: is_numeric("") returns true. Bug: is...(negative_values) is UB. (Aside from is...(EOF)). Possible when *str < 0. Bugs: as reported by user3629249 Consider allowing hex such as "0xAbC". Standard library string functions operate as if char is unsigned, even if char is signed. Recommend to do the same here. What do you think about my way of doing this? I like the idea of allowing of trailing white-space when leading white-space allowed. #include #include // missing in OP code. bool is_numeric_integer(const char *str) { const unsigned char *ustr = (const unsigned char *) str; while (isspace(*ustr)) { ustr++; } if (*ustr == '-' || *ustr == '+') { ustr++; } const unsigned char *begin = ustr; while (isdigit(*ustr)) { ustr++; } if (begin == ustr) { return false; // no digits. } // If you want to allow trailing white-space while (isspace(*ustr)) { ustr++; } return *ustr == ''; // fail with trailing junk }

user3629249 · Answer

regarding:
if (!isdigit(*str) && !isspace(*str))

the isdigit() handles 0...9, so catches if any of the passed in char array is not numeric.
The && !isspace(*str))  has nothing to do with numeric values

Checking String with is_numeric in C

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