为什么此循环无限重复？

Question

The purpose of this program is to count the digits in an alphanumeric input. However, I used a loop to not execute the program unless the input is alphanumeric .

This is the code:

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

int main(){
   int input,isanum,digitcount;
   printf("\nEnter a number: ");
   scanf("%d",&input);
   isanum=isalnum(input);

   while(isanum==0){
     printf("\nIncorrect input. Try again: ");
     scanf("%d",&input);
     isanum=isalnum(input);
   }
   digitcount=0;
   while(input!=0){
       input=input/10;
       digitcount++;
   }
   printf("\nNumber of digits = %d",digitcount);
   return 0;
}

The problem is with the loop. It keeps looping infinitely and ignores the scanf statement and I don't know why. Am I using isalnum() incorrectly here?

Answer 1

You need to check the return value of scanf() .

isalnum() takes a character, but you are passing an int. '1' is not the same as 1 in C.

You probably should consume an entire line each time, eg with fgets() , and then check if it fits your desired input format, eg with sscanf() .

As it stands, you never consume anything, you just keep trying to read a number but there isn't one there so it fails every time. Failing to check the return value of scanf() is a contributing factor to your not noticing this.

Answer 2

Look at how isalnum() is defined: it expects a char * . You, however, give it an int * . The data is stored completely different there.

Besides, if you are reading in an int, you know beforehand that it will be alphanumeric, right?