无法弄清楚如何使用 if 语句返回除一个选项之外的任何内容

Question

此社区的第一篇文章。开始在我的大学旁听一些 C 类(class)，并且在 If 语句方面遇到了麻烦。在盯着并更改代码的变体几个小时后，我仍然没有弄清楚为什么我不能返回除了我设置的“评分”标准之一之外的值。如果有人愿意告诉我语法错误发生在哪里，甚至可能提示我应该重写哪一部分，我将非常感激。另外，如果我的逻辑需要一些温习，我会喜欢指针。再次感谢。

#include <stdio.h>


/* Main function */

int main()
{
    int Answer1;
    float Answer2;
    float Answer3;
    int Answer4;
    int Answer5;
    float Answer6;

    int point1;
    point1 = 0;
    int point2;
    point2 = 0;
    int point3;
    point3 = 0;
    int point4;
    point4 = 0;
    int point5;
    point5 = 0;
    int point6;
    point6 = 0;

    char name;
    int sum;
    int score;
    int multiplier1;
    int bonus_score; 
    int counter;
    counter = 1;
    int x;
    x = 1;
    int y;
    y = 2;
    int z;
    z = 3;


    /*
    ****************
    ****************
    this is the end of my declaration system, now begins the actual functions.
    ***************
    ****************
    */


    printf (" Welcome to your career amplitude test! This simple program will tell you how far you'll go in life. \n");
    printf (" Remember to write your answer to at least two decimal places. \n \n ");


    printf ("1. What is 5 + 27? \n");
    scanf ("%i", &Answer1);

    printf("2. what is 2.7 - .85? \n");
    scanf ("%f", &Answer2);

    printf ("3. what is 2.3 - .1 * 4? \n");
    scanf ("%f", &Answer3);

    printf ("4. what is 123 * 123?\n");
    scanf ("%i", &Answer4);

    printf ("5. what is 945/5?\n");
    scanf ("%i", &Answer5);

    printf (" Bonus Question!!!!!  \n");
    printf (" what is the square root of 105487.19? You have 10 seconds to enter a number (not really though.) \n");
    scanf ("%f", &Answer6);


    /*
    ******************
    ******************
    after those are printed / scanned it will come up with a potential scoring 
    system using if statements and if else 
    *****************
    *****************
    */


    if ( Answer1 == 32)
    {
        point1 = 1;
    }
    else ( Answer1 != 32);
    {
        point1 = 0;
    } 
    if ( Answer2 == 1.85 )
    {
        point2 = 1;
    }    
    else ( Answer2 != 1.85 );
    {
        point2 = 0;
    }
    if ( Answer3 == 1.9 )
    {
        point3 = 1;
    }    
    else ( Answer3 != 1.9 );
    {
        point3 = 0;
    }
    if ( Answer4 == 15129 )
    { 
        point4 = 1;
    }
    else ( Answer4 != 15129 );
    {
        point4 = 0;
    }
    if ( Answer5 == 189 )
    {
        point5 = 0;
    }
    else ( Answer5 != 189);
    {
        point5 = 0;
    }

    if ( Answer6 != 324.787 )
    { 
        point6 = 0;
    }
    if ( Answer6 = 324.787 )
    {
        point6 = 1;
    }

    /* 
    ******************
    ******************
    Now to actauly grade the assignment compared to the scoring system just established.
    ******************
    ******************
    */

    while (counter < 100)
    {
        counter = counter+x+y+z;
        printf("Processing at a light speed rate equal to %i \n \n \n", counter);
    }

    /* the above is a joke and just wanted to implement a loop for pratice */

    printf(" This is your raw score without the Bonus. \n");    

    sum = (point1 + point2 + point3 + point4 + point5); 
    score = sum;

    if ( score = 0 )
    {    
        score = 0;
        printf (" Score: 0 \n");
        printf (" You missed every question!  \n");
    }

    else if ( score = 1 )
    {
        score = 1;
        printf ("  Score: 1 out of 5 \n");
        printf ( " You only got one question right! The world needs ditch diggers too. \n");
    }

    else if ( score = 2 )
    {
        score = 2;
        printf ("  Score: 2 out of 5  \n");
        printf ( " You missed 3 questions, pratice your soft skills  \n");
    }

    else if ( score = 3 )
    {
        score = 3;
        printf (" Score: 3 out of 5 \n" );
        printf ("  I forsee a future in the hedge fund industry \n");
    }

    else if ( score = 4 )
    {
        score = 4;
        printf (" Score: 4 out of 5 \n ");
        printf (" you could probably cook books for Enron \n");
    }


    else if ( score = 5)
    {
        score = 5;
        printf (" Score: 5 out of 5  \n");
        printf (" Go out there and break code for the CIA  \n");
    }

    printf ("With the bonus considered, your score is now \n");
    if ( point6 = 1 )
    {
         multiplier1 = 2;
    }
    else if ( point6 = 0)
    {
         multiplier1 = 1;
    }

    if ( multiplier1 = 2)
    {
        bonus_score = score * 2;

        printf (" %i", bonus_score );
    }
    else if ( multiplier1 = 1)
    {
        bonus_score = score;
        printf (" You got the Bonus wrong. Nothing new to see here. \n");
    }


    return 0;
}

Answer 1

一些事情:

if ( Answer1 == 32)
{
    point1 = 1;
}
else ( Answer1 != 32); <<< problem
{
    point1 = 0;
} 

该代码被解析为

if ( Answer1 == 32 )
{
  point1 = 1;
}
else
  Answer1 != 32;  // expression is evaluated, result is discarded.

{
  point1 = 0;
}

因此，point1 = 0; 被无条件执行(在 if 语句体之外)。我猜你一定正在使用 gcc，因为编译器并没有因为其中有裸 block 而对你大喊大叫。

else 不采用控制表达式；你只需写

else
{
  point1 = 0;
}

而且，由于您已经将 point1 初始化为 0，因此根本不需要 else 分支；你只需要

if ( Answer1 == 32 )
{
  point1 = 1;
}

您在该 if 语句 block 中重复了相同的错误，因此这是您需要处理的第一件事。

其次，不建议将 == 与浮点类型一起使用。大多数浮点值无法精确存储在给定的位数中，因此存储的是近似值。由 scanf 存储到 Answer2 中的近似值可能与直接赋值存储的近似值不同。进一步加剧问题的是，像 1.85 这样的浮点常量具有 double 类型，它使用与 float 不同的表示形式，因此 ==在这种情况下， 甚至更不可能发挥作用。

除非您的空间确实很紧张(但事实并非如此)，请使用double而不是float - 您可以获得更大的范围和精度，对于你正在做的事情来说，它不会变慢。

正确的浮点比较很快就会变得非常难看；这是一种方法:

#include <math.h>
#include <float.h>

int EqualEnough( double a, double b, double max_diff )
{
  double diff = fabs( a - b );
  double a_abs = fabs( a );
  double b_abs = fabs( b );
  double larger = a_abs > b_abs ? a_abs : b_abs;

  return diff <= larger * max_diff;
}

它并不完美，它不能涵盖所有情况，但就您的目的而言，它应该足够好。您可以在 main 之前定义它，并将其称为

if ( EqualEnough( Answer2, 1.85, DBL_EPSILON ) ) 
{
  point2 = 1;
}

参见this page有关浮点比较的更完整讨论。

下一个:

if ( score = 0 )

在此和以下 if 语句中，您使用了 = 赋值运算符而不是 == 相等运算符；您实际上分配值0给score。由于赋值表达式的结果是赋值后左侧的值，因此该表达式的计算结果为 0，即 false，因此不会采用该分支。

else if ( score = 1 )

在本例中，score 设置为 1，表达式的值为 1，因此采用分支。因此，对于这些语句，请确保使用 ==:

if ( score == 0 )
{
  ...
}
else if ( score == 1 )
{
  ...
}

最后，这只是一个风格评论:

当您发现自己创建了一堆具有相同名称的相同类型的变量，后跟一个基数(point1，point2，point3 等)，这是一个强烈的暗示，您确实想要一个数组:

int point[6] = {0};  // initializes all elements to 0

if ( Answer1 == 32 )
  point[0] = 1;

if ( EqualEnough( Answer2, 1.85, DBL_EPSILON ) )
  point[1] = 1;

等等。请记住，C 中的数组从 0 开始索引，而不是 1，因此 point 数组中的元素将是 point[0]、point[1] 、点[2]、...、点[5]。这也使您可以轻松总结您的观点:

for ( size_t i = 0; i < 6; i++ )
  sum += points[i];

您不会对 Answer... 变量执行此操作，因为它们具有不同的类型。