I have a code where I convert celsius to fahrenheit. And I need to check what user writes char or int.
I've tried isalpha and isdigit, but they do not work.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main()
{
char t[] = "";
scanf("%s", &t);
if(isalpha(t))
{
printf("It's char\n");
}
else if (isdigit(t))
{
printf("It's int\n");
}
return 0;
}
isalpha
and isdigit
are applied to objects of the type int that contain a char.
You are trying to apply these functions to an object of the type char * (an array type is implicitly converted to a pointer type in expressions).
Moreover the array t
declared like
char t[] = "";
is not enough large to store even one character gotten from scanf because it also need to store the terminating zero. Otherwise a call of scanf will have undefined behavior. And the call of scanf is also incorrect.
scanf("%s", &t);
^^^
It should be written at least like
scanf("%s", t);
You could declare an object of the type char like
char t;
and then use scanf like
scanf(" %c", &t);
and at last
if ( isalpha( ( unsigned char )t ) )
{
printf("It's char\n");
}
else if ( isdigit( ( unsigned char )t ) )
{
printf("It's int\n");
}
Firstly:
char t[] = "";
creates a buffer of exactly one character, then
scanf("%s", t);
will overrun that buffer for anything but an empty string input. Making scanf()
safe from overrun is not straightforward, but even then most naive implementation will have a practical buffer length eg ;
char t[128] = "" ;
If the expectation is to enter a string that can be converted to an int
, then 10 decimal digits is sufficient for all positive 32bit integers.
scanf("%10s", t);
char
and int
are data types, here the user only ever enters a string. Your question is really whether the user has entered somthing that may be interpreted as an integer or not.
isalpha()
and isdigit()
operate on single characters, but t
is a string.
The following will check the first character of the string t
:
if( isdigit(t[0]) )
{
printf("It's digit\n");
}
else
{
printf("It's not a digit\n");
}
Note that it makes little sense testing for isalpha()
because the union of all digits + all alpha, is still only a subset of all characters .
If in fact you simply wish to verify that the entire string is numeric then:
for( int i = 0; t[i] != 0 && isdigit(t[i]) i++ )
{
// nothing
}
if( t[i] == 0 )
{
printf("It's numeric\n");
}
else
{
printf("It's not entirely numeric\n");
}
Even then it is not a given that the numeric string can be represented by an int
; it also has to be in range. You might also want to consider the possibility of a -/+ sign at the start, and might consider ignoring trailing non-numeric digits.
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