Find missing value in array using a for loop

Question

So I am trying to make a for loop that searches through an array and returns how many missing values it has found.

Here is my method:

 public void questionsMissed() {
 int missedQuestions = 0;
 for (int i = 0; i < 20; i++) {
 if (answers[i] == null){
 missedQuestions++;
 }
 }
 System.out.println("You have missed " + missedQuestions + " questions.");
 }

The error is where the if statement is. I am assuming it is because of the null. How can I go around finding missing values?

ERROR: incomparable types: int and

• Vicky Thakor: Show us the complete code and problem
• Eran: What error are you getting?
• Mureinik: ... and the error, please
• Compass: Possible errors just by looking at the code - answers out of scope, array out of bounds, etc.
• Lewis Menelaws: Sorry. Added all of the code and the error message.

Answer 1

You get the error because you are trying to compare a primitive data type, int, to null. Only reference types can be compared to null. Primitive types always have a value - they cannot be null, so the compiler tells you that the comparison does not make sense.

There are two ways you can go about fixing this:

• Switch the type of answers to Integer - This would let you check its values for null, at the expense of wrapping primitive integers into by-reference wrappers.
• You can designate an integer value, say, -1, as "missing" - This would let you keep int as the type of answer[]'s element, but there would be a special number that must be used everywhere in your program.

Since you are using char values for your answers, I would use the second approach, but first I would make answers and correctAnswers arrays of char. There is a convenient "special value" for the unused value - the null character 0円':

if (answers[i] == '0円') {
 ...
}

• Lewis Menelaws: Great answer. Thanks for the help and being specific with each answer.

Answer 2

Two things :-

There is nothing like missing values which you can compare using null. You can compare it to 0 as those defined array members are initialised to 0 (THANKS to dasblinkenlight for correcting me,I was confused with C),but that appears to be a poor decision/choice.

I'd recommend putting -1 for the missing values as is generally done in sentinel condition checking and then compare values like :-

public void questionsMissed() {
 int missedQuestions = 0;
 for (int i = 0; i < 20; i++) {
 if (answers[i] == -1){
 System.out.println("OOPS,the element here is missing!");
 }
 }
}

• Lewis Menelaws: Thank you for the answer. However, if I use == 0 doesn't that mean it wouldn't return anything or do most arrays have a default value of 0? Thank you.
• Am_I_Helpful: @Lewis-SORRY Lewis,edited my answer. In java,array members are always initialised by 0 whatever be the case so consider -1 for sentinel condition!
• Am_I_Helpful: @dasblinkenlight-THANKS,corrected. I was confused between C and Java as nowadays I am busy with some C coding!!! THANKS and +! for you. Also, I mentioned your point in my answer.

Answer 3

You compare a primitive type int to null. Primitive types are not objects and always have a value. Thus you can not compare them to null.

If you use an Integer[] you can mark missed questions by inserting null or you can compare them with '0円'.

But since you already use character literals I would use Character instead.

When you use a primitive wrapper type your methods will also get much easier. E.g.

Character[] answers = new Character[20];
....
public void questionsMissed() {
 int missedQuestions = Collections.frequency(Arrays.asList(answers), '0円');
 System.out.println("You have missed " + missedQuestions + " questions.");
}