Reading errors

One small slip. The whole file fails.

One small slip in a data file, a missing comma or a wrong end tag, and software will not read it at all. The good news is that checkers say where they gave up. This lesson is about reading that message.

An end tag that does not match

Here is a small X M L file, and what a checker says about it. The checker reports a mismatched tag, at line four. Look at the last line. The element was opened as person, but the end tag says persons. A start tag and its end tag must match. Fix the end tag, so it reads person, and the checker is happy.

• Ada Example
• 1974-12-25
• Ada Example
• 1974-12-25

A comma that is missing

Now a small JSON file. The checker reports that it expected a comma, at line three. The red line is where the checker noticed. But the real mistake is the line before, shown in amber. The first pair has no comma after it. In JSON, a single comma separates one pair from the next. Add the comma at the end of line two, and the file reads correctly. Remember this: the line a checker reports can be just after the mistake.

• {
• "name": "Ada Example"
• "born": "1974-12-25"
• }
• {
• "name": "Ada Example",
• "born": "1974-12-25"
• }

A slip, and the rule behind it

Match the slip to the rule. A wrong end tag breaks X M L's rule that elements nest properly and tags match. A missing comma breaks JSON's rule that a single comma separates one pair from the next. A repeated name in a JSON object breaks the rule that names should be unique, and the standard warns that software may then behave unpredictably. And when X M L finds a fatal error, the standard says the processor must report it, and must not carry on as normal.

• A wrong XML end tag · Tags must match and nest properly · tag · navy
• A missing JSON comma · One comma between pairs · box · navy
• A repeated JSON name · Names should be unique · key · amber
• An XML fatal error · Must be reported; no normal processing · warning · slate

Check yourself

Pause and think. A checker reports a JSON error at line three. Where might the real mistake be? The answer: on line three, or on the line just before it.

• On line 3, or on the line just before it
• Only on line 1
• Nowhere; the checker is wrong
• Only at the end of the file

Keep these in mind

One small slip can make the whole file unreadable. A checker reports where it noticed the problem. The real mistake may be the line before. And each format's rules say what a slip breaks.

• One small slip can make the whole file unreadable
• A checker reports where it noticed the problem
• The real mistake may be the line before
• Each format's rules say what a slip breaks