Is there an elegant way in C to check if a given string is a "double"? Not if the type of the variable is double, but if the string contains a real number. For example:
char input[50];
printf("please enter a real number: \n");
scanf("%s", input);
if ( is_double(input) ) {
//user entered "2"
return true;
//user entered "2.5"
return true;
//user entered "83.5321"
return true;
//user entered "w"
return false;
//user entered "hello world"
return false;
}
You need to define if 12 or 1e23 are doubles for you. And what about -4z or 12.3, ? So specify what are acceptable and forbidden inputs (hint: using EBNF on paper could be helpful).
Notice that strtod can be used and can give the pointer to the last parsed character.
So (after adding a #include <stdlib.h> near the beginning of your file ....)
char* endp=NULL;
double x = strtod(input, &endp);
if (*endp == 0) { // parsed a number
Also sscanf (you need to include <stdio.h>) returns the number of scanned items, and accepts %n to give current byte offset.
int pos= 0;
double x = 0.0;
if (sscanf(input, "%f%n", &x, &pos)>=1 && pos>0) { // parsed a number
You could also use regexp (regcomp(3) & regexec(3)...) or parse your string manually
Left as an exercise.
PS. Please read carefully the linked documentations.
strtod, but for some reason I get x=-var-create: unable to create variable object in the debugger..var or create ?strtod is not set properly. I get this message from the debugger.printf or read the documentation of your debugger or lower the optimization level. I still don't understand what is var or create. Your question (and my answer) don't mention it.strtod seems like a good solution, I will check why the variable isn't set properlyAs long as you aren't allowing scientific notation:
#include <ctype.h>
#include <string.h>
#include <stdbool.h>
bool is_double(const char *input)
{
unsigned long length = strlen(input);
int num_periods = 0;
int num_digits = 0;
for (unsigned int i = 0; i < length; i++)
{
if ( i == 0 )
{
if ( input[i] == '-' || input[i] == '+' )
continue;
}
if ( input[i] == '.' )
{
if ( ++num_periods > 1 ) return false;
}
else
{
if (isdigit(input[i]))
{
num_digits++;
}
else
return false;
}
} /* end for loop */
if ( num_digits == 0 )
return false;
else
return true;
}
if ( num_periods == 0 ) { num_periods = 1; } else return false; with if (num_periods++ != 0) return false;. It's more compact — but doesn't change the functionality. At the moment, the code doesn't handle signs before the number. You explicitly say you don't handle exponents.
strtodand see if it could convert the full string.fgets(input, sizeof(input)-1, stdin);is better thanscanf("%s",input);scanfwas just for the example, I actually get the string withgetcharand reallocate memory as needed.gcc -Wall -Wextra -gwith GCC...) then use the debuggergdband don't forget to read documentation.