输入，输出C程序

Question

So thats my programm: This program scans and prints numbers, but I want it to print "Mistake" if a character is entered. It produces an infinite loop if I put something like an "a" in. Why?

#include <stdio.h> 

int main(void){
    printf("Enter a number: \n");
    int a;
    while(scanf("%d",&a)!=EOF){
        if(46<'a'<58){
        }
        else{
            printf("Mistake.");
            return -1;
        }
        printf("%d\n",a);
    }
}

Answer 1

First of all, you cannot chain the relational operators in C. You need to change

 if(46<'a'<58){

to

if ((46 < 'a')  && ( 'a' < 58 )) {

to make it work.

That said, a char value is not a match for %d format specifier. You need to either

• use %c to scan the input as char and check the ASCII value
• use %d to scan the int input and check for the return value of scanf() for success. Also, in case scanf() fails for a non- int value, you need to clean up the input buffer before you loop again to avoid the infinite looping.

Answer 2

Read input as a string with fgets() , then test it as needed.

Using scanf("%d", ...) returns 0, 1, or EOF when it fails, reads an int or end-of-file is detected. Code could test that return value, but robust code simple does not use scanf() .

char buf[100];
while(fgets(buf, sizeof buf, stdin) != NULL) {
  int a;
  if (sscanf(buf, "%d",&a) == 1) {
    printf("%d\n",a);
  } else {
    puts("Mistake.");
  }
}

if(46<'a'<58) does not work. it first compares 46<'a' which is 1 :true and then compares 1 < 58 which is also true. @124...

Answer 3

46 < 'a' < 58 is always true, because 46 < 'a' evaluates to 1 , and then 1 < 58 evaluates to 1 . Also I suppose you want a , rather than 'a' , which is actually a literal. Additionally, I think your code has some logical problems.

Here is the refined code:

#include <stdio.h> 

int main(void){
    puts("Enter a number:");
    char a;
    while(scanf("%c", &a) != EOF) {
        if(('0' <= a && a <= '9') || a == '\n'){
            putchar(a);
        }
        else
        {
            puts("Mistake.");
            return -1;
        }
    }
}