这个问题在这里已有答案：
检查发生在两个不同的位置。
也就是说，当你输入第一个和第二个数字时，输入a，s，m，d或q。
在任何检查中，如果检查结果为false，则应要求您重新输入输入。
我猜这可以通过在while循环检查中为数字部分放置scanf语句来完成，但是当我输入无效值（非数字）时，循环无限运行。
所以我一定做错了。我做了大部分的a，s，m，d和q部分工作。
但第二部分似乎永远不会起作用。为此，我将失败的尝试留在了while循环中，而是在//comments中。
任何帮助将不胜感激！到目前为止，这是我的代码：
#include <stdio.h>
#include <ctype.h>
int main(void)
{
char ch;
float num1,num2,answer;
printf("Enter the operation of your choice:\n");
printf("a. add s. subtract\n");
printf("m. multiply q. divide\n");
printf("q. quit\n");
while ((ch = getchar())!='q')
{
printf("Enter the operation of your choice:\n");
printf("a. add s. subtract\n");
printf("m. multiply q. divide\n");
printf("q. quit\n");
ch=tolower(ch);
if (ch=='\n')
continue;
else
{
switch(ch)
{
case 'a':
//The code below is what I have tried to make work.
//This code would also be copy pasted to the other cases,
//of course with the correct operations respectively being used.
//
//printf("Enter first number: ")
//while(scanf("%f",&num1)==0)
//{
// printf("Invalid input. Please enter a number.");
// scanf("%f",&num1);
//}
//printf("Enter second number: ")
//while(scanf("%f",&num2)==0)
//{
// printf("Invalid input. Please enter a number.");
// scanf("%f",&num2);
//}
//answer = num1 + num2;
//printf("%f + %f = %f\n",num1,num2,answer);
//break;
//
//I have also tried to make this work using do-while loops
printf("Enter first number: ");
scanf("%f",&num1);
printf("Enter second number: ");
scanf("%f",&num2);
answer = num1 + num2;
printf("%f + %f = %f\n",num1,num2,answer);
break;
case 's':
printf("Enter first number: ");
scanf("%f",&num1);
printf("Enter second number: ");
scanf("%f",&num2);
answer = num1 - num2;
printf("%f - %f = %f\n",num1,num2,answer);
break;
case 'm':
printf("Enter first number: ");
scanf("%f",&num1);
printf("Enter second number: ");
scanf("%f",&num2);
answer = num1 * num2;
printf("%f * %f = %f\n",num1,num2,answer);
break;
case 'd':
printf("Enter first number: ");
scanf("%f",&num1);
printf("Enter second number: ");
scanf("%f",&num2);
answer = num1 / num2;
printf("%f / %f = %f\n",num1,num2,answer);
break;
default:
printf("That is not a valid operation.\n");
break;
}
}
}
return 0;
}
再次感谢您的帮助！你会成为一个救生员！干杯! -Will S.
编辑：我的代码工作了！这是最终的代码......
#include <stdio.h>
#include <ctype.h>
int main(void)
{
char ch;
float num1,num2,answer;
printf("Enter the operation of your choice:\n");
printf("a. add s. subtract\n");
printf("m. multiply q. divide\n");
printf("q. quit\n");
while ((ch = getchar())!='q')
{
ch=tolower(ch);
//Ignore whitespace
if (ch=='\n')
continue;
else
{
switch(ch)
{
//Addition part
case 'a':
//First number
printf("Enter first number: ");
//Check to see if input is a number
while (scanf("%f",&num1)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
//Second number
printf("Enter second number: ");
while (scanf("%f",&num2)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
//Do math for respective operation
answer = num1 + num2;
//Print out result
printf("%.3f + %.3f = %.3f\n", num1,num2,answer);
break;
//Subtraction part
case 's':
printf("Enter first number: ");
while (scanf("%f",&num1)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
printf("Enter second number: ");
while (scanf("%f",&num2)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
answer = num1 - num2;
printf("%.3f - %.3f = %.3f\n", num1,num2,answer);
break;
//Multiplication part
case 'm':
printf("Enter first number: ");
while (scanf("%f",&num1)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
printf("Enter second number: ");
while (scanf("%f",&num2)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
answer = num1 * num2;
printf("%.3f * %.3f = %.3f\n", num1,num2,answer);
break;
//Division part
case 'd':
printf("Enter first number: ");
while (scanf("%f",&num1)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
printf("Enter second number: ");
while (scanf("%f",&num2)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
//Check for if number is a zero
while (num2==0)
{
printf("Please enter a non-zero number, such as 2.5, -1.78E8, or 3: ");
while (scanf("%f",&num2)==0)
{
printf("Invalid input. Please enter a number, such as 2.5, -1.78E8, or 3: ");
scanf("%*s");
}
}
answer = num1 / num2;
printf("%.3f / %.3f = %.3f\n", num1,num2,answer);
break;
//For if a non-valid operation is entered
default:
printf("That is not a valid operation.\n");
break;
}
}
printf("Enter the operation of your choice:\n");
printf("a. add s. subtract\n");
printf("m. multiply q. divide\n");
printf("q. quit\n");
}
printf("Bye.\n");
return 0;
}
回头看看，我可能没有if / else语句。
您的代码存在多个问题。首先是在这个循环中
while(scanf("%f",&num1)==0) //Taking Input Here Once
{
printf("Invalid input. Please enter a number.");
scanf("%f",&num1); //Again Taking input.
}
相反，你想要的是检查scanf()的返回值，如果它是0你将再次执行循环，所以这将是这样做的方式：
int l = 0;
while(l==0){ //Checking l, if it is zero or not, if zero running loop again.
printf("Invalid input. Please enter a number.");
l = scanf("%f",&num1); //Storing Return Value of scanf in l
}
scanf("%f" , &num1)或scanf("%f" , &num2)的行时，它将跳过所有空格并等待下一个输入。在输入与格式规范不匹配的情况下，输入不会被消耗并保留在输入缓冲区中。
int l = 0;
while(l==0){ //Checking l
printf("Invalid input. Please enter a number.");
l = scanf("%f",&num1); //scanf will look at the buffer if the input
//does not match, it will not be consumed
//and will remain in buffer.
}
换句话说，永远不会读取不匹配的字符。所以当你输入例如一个字符，当scanf继续在同一个字符上失败时，你的代码将无限循环。scanf("%f",&num2)调用时，由于输入，缓冲区中存在换行符\n字符，因此由于ch = getchar()，新行\n存储在ch中，并且随后的if条件满足并且循环再次执行。
if(ch =='\n')
continue;
while(scanf("%f",&num1)==0)
{
printf("Invalid input. Please enter a number.");
scanf("%f",&num1);
}
此循环每次迭代扫描两个数字。这不是你想要的。失去第二个scanf。
您还应该检查EOF和错误。
int result;
while((result = scanf("%f",&num1))==0)
{
printf("Invalid input. Please enter a number.");
}
if (result == EOF) .... report an error and exit ...