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I am trying to compare an item from a List of type Strings to an integer. I tried doing this but I get an error saying that:

'value < is not a member of List[Int]'

The line of code that compares is something similar to this: 

if(csvList.map(x => x(0).toInt) < someInteger)

Besides the point of why this happens, I wondered why I didn't get an error when I used a different type of comparison, such as ' == '.

So if I run the line:

if( csvList.map(x => x(0).toInt) == someInteger)

I don't get an error. Why is that?

user_123945839432
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4 Answers4

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You can refer to Why == operator and equals() behave differently for values of AnyVal in Scala

Every class support operator ==, but may not support <,> these operators.

in your code

csvList.map(x => x(0).toInt)

it returns a List<int>, and application use it to compare with a int, so it may process a implicit type conversion. Even the compiler doesn't report it as a error. Generally, it's not good to compare value with different types.

Community
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kiro
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csvList.map(x => x(0).toInt) converts the entire csvList to a List[Int] and then tries to apply the operator < to List[Int] and someInteger which does not exist. This is essentially what the error message is saying.

There is no error for == since this operator exists for List though List[T] == Int will always return false.

Perhaps what you are trying to do is compare each item of the List to an Int. If that is the case, something like this would do:

scala> List("1","2","3").map(x => x.toInt < 2)
res18: List[Boolean] = List(true, false, false)
Brian
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The piece of code csvList.map(x => x(0).toInt) actually returns a List[Int], that is not comparable with a integer (not sure what it would mean to say that List(1,2) < 3). If you want to compare each element of the list to your number, making sure they are all inferior to it, you would actually write if(csvList.map(x => x.toInt).forall { _ < someInteger })

John K
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Let's start with some introductions before answering the questions Using the REPL you can understand a bit more what you are doing

scala> List("1", "2", "3", "33").map(x => x(0).toInt)
res1: List[Int] = List(49, 50, 51, 51)

The map function is used to transform every element, so x inside the map will be "1" the first time, "2" the second, and so on.

When you are using x(0) you are accessing the first character in the String.

scala> "Hello"(0)
res2: Char = H

As you see the type after you have mapped your strings is a List of Int. And you can compare that with an Int, but it will never be equals. 

scala> List(1, 2, 3) == 5
res0: Boolean = false

This is very much like in Java when you try 

"Hello".equals(new Integer(1));

If you want to know more about the reasons behind the equality problem you can check out Why has Scala no type-safe equals method?

Last but not least, you get an error when using less than because there is no less than in the List class.

Extra: If you want to know if the second element in the list is smaller than 2 you can do

scala> val data = List("1", "10", "20")
data: List[String] = List(1, 10, 20)
scala> 5 < data(1).toInt
res2: Boolean = true

Although it is a bit strange, maybe you should transform the list of string is something a bit more typed like a case class and then do your business logic with a more clear data model.

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Jonas Anso
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