scanf()返回已成功匹配和赋值的输入项的数目。如果不检查返回值（rv），则变量（monthpay）可能未初始化，使用它是未定义的行为。

void flush() {
   for(;;) {
     char ch;
     int rv = scanf("%c", &ch);
     if(rv == EOF || rv == 1 && ch == '\n') return;
   }
}

printf("Please enter your monthly wage:\t");
float monthpay;
for(;;) {
    int rv = scanf("%f", &monthpay);
    if(rv == EOF)
        return;
    if(rv == 1)
        break;
    printf("\nThat was invalid");
    printf("\n Please try again: ");
    flush();
}
printf("%.2f", monthpay);

在尝试扫描float之后，扫描并丢弃除换行符之外的空白，扫描丢弃并计数任何剩余的非换行符字符，扫描换行符。
%n说明符将提供扫描处理的字符计数，直到该说明符为止。
扫描集%*[ \t\f\v]中的星号指示scanf丢弃匹配字符。扫描集%*[^\n]中的插入符号指示scanf处理不匹配字符，因此将丢弃所有不是换行符的字符。%1[\n]指示scanf扫描一个字符，并且该字符必须是换行符。如果输入浮点数后仅跟空格，当scanned将为2且extra将为0时，循环将退出。

#include <stdio.h>

int main ( void) {
    char newline[2] = "";
    int extra = 0;
    int scanned = 0;
    float monthpay = 0.0;

    do {
        printf ( "enter monthly pay\n");
        scanned = scanf ( "%f", &monthpay);
        if ( scanned == EOF) {
            return 1;
        }
        extra = 0;
        scanf ( "%*[ \f\t\v]"); // scan and discard whitespace except newline
        scanf ( "%*[^\n]%n", &extra); // scan discard and count non-whitespace
        scanned += scanf ( "%1[\n]", newline);
    } while ( monthpay < 0 || scanned != 2 || extra != 0);

    return 0;
}

在尝试使用结果之前，应始终检查scanf的返回值，以确定输入是否已成功转换。
但是，对于基于行的用户输入，使用函数scanf通常是不可取的，因为它的行为不直观，它通常不消耗整行输入，这会引起各种各样的麻烦，例如scanf在输入流上留下换行符会引起this问题，如果用户输入6abc\n，那么情况就更糟了：在这种情况下，scanf将匹配6作为有效输入，但将abc\n保留在输入流中，这很可能导致下一个输入操作无法按预期运行，除非您事先从输入流中显式丢弃该行的剩余部分。
由于上述原因，通常最好总是一次将整行输入作为字符串读取，例如使用函数fgets，然后可以使用函数strtof尝试将字符串转换为数字。
下面是一个执行大量输入验证的示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>

float get_float_from_user( const char *prompt );

int main( void )
{
    float monthpay;

    //loop forever until user enters a valid number
    for (;;) //infinite loop, equivalent to while(1)
    {
        monthpay = get_float_from_user( "Please enter your monthly wage: ");
        if ( monthpay > 0 )
        {
            //input is ok, so we can break out of the loop
            break;
        }

        printf( "Please enter a positive number!\n" );
    }

    printf( "Input was valid. You entered: %.2f\n", monthpay );

    return 0;
}

float get_float_from_user( const char *prompt )
{
    //loop forever until user enters a valid number
    for (;;)
    {
        char buffer[1024], *p;
        float f;

        //prompt user for input
        fputs( prompt, stdout );

        //get one line of input from input stream
        if ( fgets( buffer, sizeof buffer, stdin ) == NULL )
        {
            fprintf( stderr, "Unrecoverable input error!\n" );
            exit( EXIT_FAILURE );
        }

        //make sure that entire line was read in (i.e. that
        //the buffer was not too small)
        if ( strchr( buffer, '\n' ) == NULL && !feof( stdin ) )
        {
            int c;

            printf( "Line input was too long!\n" );

            //discard remainder of line
            do
            {
                c = getchar();

                if ( c == EOF )
                {
                    fprintf( stderr, "Unrecoverable error reading from input!\n" );
                    exit( EXIT_FAILURE );
                }

            } while ( c != '\n' );

            continue;
        }

        //attempt to convert string to number
        errno = 0;
        f = strtof( buffer, &p );
        if ( p == buffer )
        {
            printf( "Error converting string to number!\n" );
            continue;
        }

        //make sure that number is representable as a "float"
        if ( errno == ERANGE )
        {
            printf( "Number out of range error!\n" );
            continue;
        }

        //make sure that remainder of line contains only whitespace,
        //so that input such as "6abc" gets rejected
        for ( ; *p != '\0'; p++ )
        {
            if ( !isspace( (unsigned char)*p ) )
            {
                printf( "Unexpected input encountered!\n" );

                //cannot use `continue` here, because that would go to
                //the next iteration of the innermost loop, but we
                //want to go to the next iteration of the outer loop
                goto continue_outer_loop;
            }
        }

        return f;

    continue_outer_loop:
        continue;
    }
}

此程序具有以下行为：

Please enter your monthly wage: abc
Error converting string to number!
Please enter your monthly wage: 6abc
Unexpected input encountered!
Please enter your monthly wage: 6e+2000
Number out of range error!
Please enter your monthly wage: -5
Please enter a positive number!
Please enter your monthly wage: 0
Please enter a positive number!
Please enter your monthly wage: 6.3
Input was valid. You entered: 6.30

我在上面的代码中使用的函数get_float_from_user是基于this answer of mine to another question中的函数get_int_from_user的，关于该函数如何工作的进一步解释，请参见上面的答案。