number input only (C language-gcc)

[mitchi]

hi, i am trying to create a program wherein it only accepts numbers. when other characters/symbols are entered the program tells me that i have an invalid input. is this possible with c language using gcc.

i just dont know what parameter to put in the condition.



if(var==???)
{
number input correct
}
else
{
invalid input
}

[slavik]

well, depends on how you want to do input.

you can do scanf, or you can read a line into a buffer and then check it for invalid characters ...

look at ctype.h :) (for starters)

[mitchi]

of course, use scanf for input, but how can i complete the condition? what am i supposed to do, to be able compare or differentiate numbers from characters/symbols?

[dwhitney67]

as slavik mentioned, look at ctype.h (in /usr/include/ctype.h)

or just run this command:

$ man isdigit
$ man isalpha

[samjh]

It depends on what var is.

If var is a string, then you can use ctype.h and its isdigit function to test whether each character in that string is a number. The reason why you'll need to test each character is because the isdigit function only checks for values between 0 to 9.

So first of all, do:
#include <ctype.h>

Then after retrieving the input value into var using scanf, run loop through your string to check whether each character is a numeric digit using the isdigit function.

Example (pardon the crappy code):

#include <stdio.h>
// Need ctype.h for isdigit and isalnum functions.
#include <ctype.h>

int main(void)
{
char var[10]; // This is the variable to store input.
int i = 0;
int varisnum = 0; // Is var all numbers? 1 for yes, 0 for no.

scanf("%s", var);

while (isalnum(var[i]) != 0) { // Loop until it a character is not alpha-numeric.
if (isdigit(var[i]) != 0) { // Is var[i] a numeric digit?
varisnum = 1;
} else {
varisnum = 0;
break; // If we encounter a non-numeric character, there is no need to keep looping, so just break out.
}
i++; // Move counter to the next element.
}

if (varisnum == 0)
printf("Input was not all numbers.\n");
else
printf("Input was all numbers.\n");

return 0;
}

[gnusci]

Here another code, it is similar to the code posted before by samjh, but I am aware that not whole the variable must be filled with data, so it scan for numbers while there are data in the string, here the code:



#include<stdio.h>
#include<ctype.h>


int is_number(char * pchar, int lnum){
int i, is_num=0;
for(i=0; i<lnum; i++){
if(!isalnum(*pchar)) break;
// printf("%c - ",*pchar);
if(!isdigit(*pchar)){
is_num++;
break;
}
pchar++;
}
// printf("\nis_num = %i\n",is_num);
return is_num;
}


int main(void){
char num[50];

printf("\n number: ");
scanf("%s",num);

if(!is_number(num,50)){
printf("number input correct\n");
}else{
printf("invalid input\n");
}
return 0;
}

[slavik]

int in_num;
scanf("%d", &in_num);


btw, never use scanf to read in a string ... you are just asking for a buffer overflow.