Which of the following are valid identifiers:

4th

new_file

C++Program3

first-Num

last name

file2018

Question

Which of the following are valid identifiers:

4th

new_file

C++Program3

first-Num

last name

file2018

BlackTom AI · Accepted Answer

Identifiers must start with a letter (A–Z or a–z) or an underscore (_) and may contain letters, digits (0–9), and underscores; they cannot contain spaces or most punctuation characters (such as + or -) and cannot begin with a digit. These are the standard rules used in C, C++, Java, Python, and many other languages.

1) Relevant knowledge points and concepts
- First character: letter or underscore only (not a digit).
- Subsequent characters: letters, digits, or underscores.
- No whitespace or punctuation (e.g., space, hyphen, plus signs).
- Keywords/reserved words would be disallowed, but none of the options are language keywords.

2) Step-by-step checking of each option
- 4th: begins with the digit “4”. That violates the rule that an identifier cannot start with a digit → invalid.
- new_file: begins with a letter, contains only letters and an underscore. Underscore is allowed → valid.
- C++Program3: contains “+” characters. Plus signs are punctuation/operators, not allowed in identifiers → invalid.
- first-Num: contains a hyphen “-”. Hyphen is the subtraction operator/punctuation, not allowed in identifiers → invalid.
- last name: contains a space. Spaces are not permitted in identifiers → invalid.
- file2018: begins with a letter and contains only letters and digits, which are allowed → valid.

3) Intermediate steps / checks
- For each candidate, verify first character rule, then scan remaining characters for any disallowed characters (spaces, punctuation). Only new_file and file2018 pass both checks.

4) Why the correct answers are correct
- new_file: starts with a letter and uses only letters and underscore — all legal.
- file2018: starts with a letter and uses only letters and digits — all legal.

5) Why other options are incorrect
- 4th: starts with digit → invalid.
- first-Num: contains hyphen → invalid.
- last name: contains space → invalid.
- C++Program3: contains plus signs → invalid.

6) Conclusion
The valid identifiers are new_file and file2018, which matches the provided answer.

Which of the following are valid identifiers: 4th new_file C++Program3 first-Num last name file2018Multiple choice

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