c++ - 如何检查输入是否为字符，显示为 "invalid input"，否则照原样运行

Question

#include<stdio.h>
#include<string.h>
#include<stdlib.h.>
#include<locale>
using namespace std;

main()
{
    int b;
    printf("Enter a number:");
    scanf("%d",&b);
    char a = b;

    if (a >= 'a' || a <= 'z' || a >= 'A' || a <= 'Z' )
    {
        printf("Invalid input");
    }
    if(int b=a || b<=0)
    {
        printf("It's a Negative number");
    }
    else if(int b=a || b>0)
    {
        printf("It's a Positive number");
    }
    else if(int b=a && b==0)
    {
        printf("It's a Zero");
    }
}

Answer 1

bool 逻辑很接近，但有点误解。

您需要检查 AND z OR A AND Z 之间的值是否存在无效输入。

一旦您知道 ASCII 表示不对字母进行编码，您就可以将其转换为 int 并执行常规数学逻辑来检查它的正数、负数等...

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<locale>
using namespace std;

main()
{
    char b;
    printf("Enter a number:");
    scanf("%c",&b);
    // checking ASCII value of b isn't between a AND z OR A AND Z
    if ((b >=  'a' && b <= 'z') || (b >=  'A' && b <= 'Z'))
    {
        printf("Invalid input\n");
    } else {
        // convert ASCII representation to the actual int it encodes
        int a = b - '0';
        if(a<0)
        {
            printf("It's a Negative number\n");
        }
        else if(a>0)
        {
            printf("It's a Positive number\n");
        }
        else if(a==0)
        {
            printf("It's a Zero\n");
        }
    }

}