It is important to validate data entered by users both from a programming point of view and a good user experience. If we do not validate the values then we have to handle invalid data in logic later on, which is often more work, to avoid bugs and issues. For the end user this will impact them when using the app as they will then have navigate back from what they are now doing in order to edit and correct the invalid data. Validating the value at the point of entry and giving meaningful error messages also helps the user by cutting down on simple typing mistakes.
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