filtering input to only numbers. C

Question

Here's a basic question:

How can I take the input from a user and only accept numbers? I know I need to use this start:

    do{
    ch=getchar();
     }while (ch != '\n');

But I know it's not enough. This will block every input, including numbers, so I need to break when input is number. also how do I break not after the first digit of the number?

I tried looking this up with no luck.

thanks!

Answer 1

When you need to perform error checking, do different things based on the input, etc., it's best to read user input line by line and process each line as you see fit.

// Make it large enough for your needs.
#define LINE_LENGTH 200

char line[LINE_LENGTH];

// Keep reding lines of text until there is nothing to read.
while ( fgets(line, sizeof(line), stdin) != NULL )
{
   // Process contents of line.
}

If you expect to see only one number per line, you can use sscanf to extract numbers from each line.

while ( fgets(line, sizeof(line), stdin) != NULL )
{
   int num;
   if ( sscanf(line, "%d", &num) == 1 )
   {
      // Got a number. Use it.
   }
}

Answer 2

The probably easiest way to read in a number is using scanf:

int number;
if (scanf("%d",&number) == 1) {
  printf("successfully read number %d\n",number);
} else {
  printf("not a number.\n");
}

It skips leading white spaces and then takes characters from stdin as long as these match an integral number format. Note that you still might have to press "enter" before your program will proceed, because the operating system might buffer the input (beyond your control).

Note that a ch=getchar() will take also the first non-digit value from stdin, which can then not be consumed by any further access to stdin anymore. scanf, in contrast, keeps this character in the buffer for later use.

Answer 3

Just check if the input is some value between '0' and '9'

#include <stdio.h>

int main(void)
{
    size_t len = 0;
    char str[32];
    int ch;

    while (len < sizeof(str) - 1) {
        ch = getchar();
        if ((ch >= '0') && (ch <= '9')) {
            str[len++] = (char)ch;
        } else {
            break;
        }
    }
    str[len] = '\0';
    puts(str);
    return 0;
}

Answer 4

simple, nice and neat solution:), I used to solve similar questions when I started programming, this will let you only input numbers

#include <stdio.h>

int main()
{
char ch;
  do{
    ch=getchar();
   }while (!(ch>=48 && ch<=57));
printf("yay,we got a number : %c !",ch);
return 0;
}