1
mapping (address => uint) itemsPerCreator;

if (itemsPerCreator[_creator] == null){}

Remix give me a error: ParseError: Expected primary expression.

_creator is a addres type variable. Inside the mapping each addres have a uint value.

  • What is null? – goodvibration Dec 26 '18 at 9:28
  • The if is inside a function. So I´m trying to evaluate if is a value inside the mapping. That mapping have one a address and a uint value. So im evaluating if the value inside the mapping is null – UnexpectedCharacter Dec 26 '18 at 9:29
  • It doesn't answer my question - where have you declared a uint variable named null? – goodvibration Dec 26 '18 at 9:30
  • null is not a variable. Is a value – UnexpectedCharacter Dec 26 '18 at 9:31
  • 1
    Any unmapped value is actually mapped to the zero value of the given type, which in your case is uint(0), or simply 0. – goodvibration Dec 26 '18 at 9:50
1

You are getting this error because there is nothing like null or undefined in solidity. An uint initializes with 0.

So, to check the zero-state of value(either default or intentionally updated) to the corresponding key in your mapping, you should write it as:

if (itemsPerCreator[_creator] == 0){}

As per the Solidity documentation:

A variable which is declared will have an initial default value whose byte-representation is all zeros. The “default values” of variables are the typical “zero-state” of whatever the type is. For example, the default value for a bool is false. The default value for the uint or int types is 0. For statically-sized arrays and bytes1 to bytes32, each individual element will be initialized to the default value corresponding to its type. Finally, for dynamically-sized arrays, bytes and string, the default value is an empty array or string.

  • The part to check the non-existence of value to the corresponding key in your mapping is wrong. A value exists, and it is 0. In addition to that, any key mapped to 0 can also be added in an explicit manner. – goodvibration Dec 26 '18 at 10:18
  • @goodvibration You're right, answer updated. – Aniket Dec 26 '18 at 10:28

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