how to check if a command line argument is there in c code example

Example 1: c check if char is an operator

char o = '+';	//Or some other character

if (o == '%' || o == '/' || o == '*' || o == '+' || o == '-') {
	//Do something
    //...
}

Example 2: check command line input is a number in c

bool isNumber(char number[])
{
    int i = 0;

    //checking for negative numbers
    if (number[0] == '-')
        i = 1;
    for (; number[i] != 0; i++)
    {
        //if (number[i] > '9' || number[i] < '0')
        if (!isdigit(number[i]))
            return false;
    }
    return true;
}