If statement isn't being caught

Question

I can't for the life of me figure out why C is ignoring my if statement.

I'm trying to skip all the procedures in the while statement when the input is -1000 (so that it doesn't print before exiting the program). Here is my code:

int main()
{
  int count = 1;
  int grade1;
  int grade2;
  double sum;
  double average;

  printf("Please input a number of grades: \n");

  scanf("%d", &grade1);
  printf("Sum is: %d.000000 \n", grade1);
  printf("Average is: %d.000000 \n", grade1);
  count++;

  sum = grade1;

  while(grade2 != -1000) 
  {
    if(grade2 != -1000)
    {
      scanf("%d", &grade2);

      sum = sum + grade2;
      average = sum / count;

      printf("Sum is: %lf \n", sum);
      printf("Average is: %lf \n", average);

      grade1 = sum; //Converting the sum back into an int
      count++;
    }
  }
  return 0;
}

Here is a link to an image of my output. As you can see, even when grade2 is given -1000, the if statement is ignored, and another 2 lines are printed to the screen before the program exits. How can I fix this? Is this some sort of oddity of how C works?

Please don't post an image of the output, copy the output in your question. — Pablo, Jan 31 '18 at 22:16
And you should use `do{}while()` instead of `while(){}`, because you need to check the looping condition after the body runs. — Barmar, Jan 31 '18 at 22:17
There's no need for the `if` statement, since it's the same condition as `while`. — Barmar, Jan 31 '18 at 22:20
@Barmar I think OP actually wants/needs the `if` statement. The problem is that the `scanf` should be before the `if` instead of inside the `if` — Support Ukraine, Jan 31 '18 at 22:52
@4386427 Then he could just use `while(true)` and then `else { break; }` — Barmar, Jan 31 '18 at 22:53
@Barmar - True - there are many better ways to code this. But OP tried to fix the problem using the `if` - which is doable. So I think it has a value to explain how the problem is fixed while keeping the `if` — Support Ukraine, Jan 31 '18 at 22:58
BTW, little need to use `sum` as `double`. Just insure the average is calculated with `double` math, `int sum = 0; ...... double average = 1.0* sum / count;` — chux - Reinstate Monica, Jan 31 '18 at 22:59
Also, you don't initialize grade2, so the first test is random. — Lee Daniel Crocker, Jan 31 '18 at 23:57

Support Ukraine · Accepted Answer · 2018-01-31T22:53:25.190

When you do this the first time

  while(grade2 != -1000)

the variable grade2 is uninitialized.

Consequently your code has undefined behavior

Make sure to initialize it like:

int grade2 = 0; // To zero or whatever you want

Further - always check the value returned by scanf. So instead of

scanf("%d", &grade1);

do

if (scanf("%d", &grade1) != 1)
{
    // Add error handling here
}

Your next problem is that you don't scan grade2 before checking whether it is -1000. Move the scan before the if-statement.

Maybe what you want to do is:

  int grade2 = 0;
  while(grade2 != -1000) 
  {
      if (scanf("%d", &grade2) != 1)
      {
          // Add error handling here
      }
      if(grade2 != -1000)
      {
         ...

so that you scan for the first grade2 before you do the if(grade2 != -1000) and enters the calculation code

Written differently this could be:

  while(1) 
  {
      if (scanf("%d", &grade2) != 1)
      {
          // Add error handling here
      }
      if(grade2 == -1000) break;  // Terminate the while

      sum = sum + grade2;
      ....

Stephen Docy · Answer 2 · 2018-01-31T22:38:01.873

2

While it's true that grade2 should be initialized and the return for scanf() should be checked, that's not the main problem the poster is running into. The problem is that he checks

if(grade2 != -1000)

AFTER he has already processed grade2. He should move

scanf("%d", &grade1);

before

if(grade2 != -1000)

edited Jan 31 '18 at 22:38

answered Jan 31 '18 at 22:19

Stephen Docy

4,738
7
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Ah, yes, ofcourse. That makes total sense. Thank you Stephen! – King Arthur the Third Feb 01 '18 at 05:32

machine_1 · Answer 3 · 2018-01-31T22:41:32.933

The if statement in your while loop is redundant because, the loop will not iterate unless the condition that controls it is true, and the if statement comes directly after that, checking for the same condition, whilst grade2 is unchaged.

Instead, you need to move it to after the scanf() call because that will modify the variable grade2, and don't forget to initialize your variables before using them or else you'll have undefined behavior.

int main(void)
{

    //....
    int grade2 = 0; // initialized...
    //....

    while (grade2 != -1000)
    {
        scanf("%d", &grade2);
        if (grade2 != -1000)
        {
            sum = sum + grade2;
            average = sum / count;

            printf("Sum is: %lf \n", sum);
            printf("Average is: %lf \n", average);

            grade1 = sum; //Converting the sum back into an int
            count++;
        }
    }
}

If statement isn't being caught

3 Answers3