C 中不需要的双重重复

Question

假设我正在编写这段代码：

char c; // c from choise.
do{
    printf("Please choose one of the following operators:\n"); 
    printf("+ for Addition\n- for Subtraction\n* for Multiplication\n/ for Division\n");
    printf("= for Assignment and\nE to check if two variables have the same type and value.\n");
    scanf("%c", &c);
}while(c!='+' && c!='-' && c!='*' && c!='/' && c!='=' && c!='E');

主要问题是，当用户输入数字或字母时，除了上述运算符之外的所有其他内容，printf 中的每条消息都会在屏幕上出现两次。

例如，如果 c='A' 我们有：

Please choose one of the following operators:
+ for Addition
- for Subtraction
* for Multiplication
/ for Division
= for Assignment and
E to check if two variables have the same type and value.
A //wrong answer, the loop continues...
Please choose one of the following operators:
+ for Addition
- for Subtraction
* for Multiplication
/ for Division
= for Assignment and
E to check if two variables have the same type and value.
Please choose one of the following operators:
+ for Addition
- for Subtraction
* for Multiplication
/ for Division
= for Assignment and
E to check if two variables have the same type and value.
//here the program awaits the new value of c.

但是，如果 c='-' 或 c='+' 等，我们当然有

Please choose one of the following operators:
+ for Addition
- for Subtraction
* for Multiplication
/ for Division
= for Assignment and
E to check if two variables have the same type and value.
- // or +
//the program goes to the next line.

当我尝试在 while 或 for 循环中转换 do_while 循环时发生了同样的事情。

Answer 1

在用户键入错误字符后，stdin 缓冲区中仍然（至少）有一个换行符。

@haccks 的评论是解决问题的一种方法。另一种方法是通过调用 getchar 的短循环跟随 scanf，直到收到 EOF。

Answer 2

在%c前加一个space：scanf(" %c", &c);。这是因为从先前输入输入的换行符可能存储在 c 中。
您还可以在 scanf() 之后使用 getchar() 清除输入缓冲区。

C 中不需要的双重重复

Unwanted double repetition in C

c

loops

repeat

do-while