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Quizzes > Computers & Technology > Programming & Software

What Can't a C Variable Start With? Take the Quiz

Think you know what a C variable cannot start with? Challenge yourself now!

Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art on golden yellow background stylized letter C representing variables quiz on C programming naming rules

Use this C variable rules quiz to check what a C variable can't start with - like digits - so you write valid names in code. Get quick practice with clear answers to plug gaps before class or an exam, then go deeper with the variables and constants practice .

Which of these variable names is invalid because it begins with a digit?
_value
5value
value
value5
In C, identifiers must begin with an underscore (_) or a letter (A - Z or a - z) and cannot start with a digit. Starting with a digit makes the compiler interpret it as a numeric literal. Therefore, any variable beginning with a digit like "5value" is invalid. See .
Which of the following is not a valid C variable name because it is a reserved keyword?
total
myVar
count
int
Keywords in C have special meanings and cannot be used as identifiers. The word "int" is a reserved keyword used to declare integer types. Attempting to use it as a variable name will result in a compilation error. Read more at .
According to the C standard, identifiers beginning with two underscores are reserved for the implementation in all contexts. Which of the following names falls into this reserved category?
__temp
temp
temp__
_temp
The C standard reserves all identifiers beginning with two underscores for the implementation. Such names are used internally by compilers and standard libraries and should not be used in user code. "__temp" falls into this reserved category. Reference: .
Which of the following characters is valid as the first character of a C variable name?
@
 
1
A
A valid C identifier must start with a letter (A - Z, a - z) or an underscore. It cannot start with digits, punctuation, or whitespace. The character 'A' is a typical letter, making it valid. More details at .
Which of these names should be avoided because it is reserved by the C standard for the implementation (compiler or standard library)?
__helper
helper__
_helper
Helper
Identifiers beginning with two leading underscores are reserved for the implementation in all contexts. They should be avoided in user code to prevent clashes with compiler or library internals. "__helper" uses this pattern and is reserved. See .
Which of the following is not a valid C variable name because it contains an invalid character?
student_name
student1
student-name
_student
In C, identifiers can only contain letters, digits, and underscores after the first character. The hyphen '-' is not a valid character in identifiers and is interpreted as the subtraction operator. Therefore "student-name" is invalid. For more, see .
Which of the following is a valid C variable name?
2data
_data2
data 2
data-2
A valid C identifier must start with a letter or underscore and can be followed by letters, digits, or underscores. "_data2" meets these criteria: it begins with an underscore and only uses allowed characters. Names like "2data", "data-2", and "data 2" violate these rules. Learn more at .
Which of the following best describes the valid starting characters for a C variable name?
Letters and underscores
Digits only
Letters and digits
Digits and underscores
C variable names must begin with either a letter (A - Z, a - z) or an underscore (_). They cannot start with digits or other symbols. Therefore, the correct description of valid starting characters is letters and underscores. More information is available at .
Which of the following statements about identifiers starting with an underscore is correct according to the ISO C standard?
They are always invalid
They are reserved at file scope if followed by an uppercase letter
They are reserved at block scope if followed by a lowercase letter
They are never reserved
According to ISO C, identifiers that begin with an underscore followed by an uppercase letter are reserved for the implementation at file scope. Using this pattern in your own code risks name clashes with system or library identifiers. Therefore, such names should generally be avoided in global scope. See .
Which identifier starting pattern is reserved by the C standard in all contexts?
Two leading underscores
Uppercase letter
One underscore followed by lowercase letter
Lowercase letter
The C standard reserves any identifier that begins with two underscores in all contexts for the implementation. This ensures that compiler and library internals do not conflict with user-defined names. User code should never define identifiers starting with "__". Refer to for details.
Which of the following characters cannot start a C variable name but is used to denote preprocessor directives?
#
$
@
?
The '#' character is used in C to introduce preprocessor directives and is not allowed in identifiers. Any name starting with '#' will be interpreted as a directive, not a variable. Thus '#' cannot start a valid variable name. For more details, see .
What does the notation "\\uXXXX" in an identifier represent in C11, and under what condition can it serve as the first character?
A universal character name; it can start an identifier if it maps to a letter category
A hexadecimal numeric escape; only used in string literals
An octal escape sequence; cannot be used in identifiers
A Unicode combining mark; must follow a base character
In C11, a universal character name of the form \\uXXXX represents a Unicode code point in an identifier. Such a sequence can start an identifier only if the corresponding character belongs to the Unicode ID_Start category. Not all code points are valid starts - they must map to a letter or underscore equivalent. See for more information.
Identifiers starting with an underscore and a lowercase letter are reserved for which scope according to the C standard?
Any scope for user code
Block scope for the implementation
No scope; they are always allowed for user code
File scope for the implementation
Identifiers that start with an underscore followed by a lowercase letter are reserved for the implementation at file scope. This reservation applies to names visible across translation units, such as global variables. Blocks or functions can use these names only if not linked externally, but it is still best to avoid them at file scope. More at .
Which of the following identifier starting patterns is NOT reserved by the C standard in any context?
Starting with a lowercase letter
Starting with an underscore followed by an uppercase letter
Starting with an underscore followed by a lowercase letter
Starting with two underscores
The C standard reserves identifiers beginning with a single underscore and a capital letter, double underscores, or underscore & lowercase letter at file scope. Only names that start with a lowercase letter are not reserved in any context. This makes lowercase-starting identifiers the safest choice for user-defined globals. Learn more at .
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Study Outcomes

  1. Understand C variable naming rules -

    Describe the fundamental constraints of C identifiers, including why a C variable cannot start with a digit or special symbol.

  2. Identify invalid starting characters -

    Spot when a C variable begins with prohibited characters like numbers or symbols, preventing compilation errors.

  3. Analyze common naming pitfalls -

    Differentiate between valid and invalid variable names, recognizing issues such as reserved keywords or unconventional formats.

  4. Apply correct naming conventions -

    Construct clear, consistent identifiers that comply with C's rules and enhance code readability.

  5. Evaluate and correct variable names -

    Review sample code to detect naming violations and refactor identifiers for error-free compilation.

Cheat Sheet

  1. Valid Identifier Start Characters -

    According to ISO/IEC 9899:2018, a C variable must begin with a letter (A - Z or a - z) or an underscore (_), ensuring consistency across compilers (see C11 standard). For example, _count and data1 are valid starts, whereas 1data is not. Remember the rule: "start smart, choose letters or underscores."

  2. Digits and Special Symbols Are Off-Limits -

    In C, a C variable cannot start with a digit or symbol like $, %, or @, even if some compilers allow extensions (per university tutorials such as those at MIT OCW). Writing 9lives or $price will trigger a compile-time error. Keep in mind: "no numbers or special chars at the very beginning."

  3. Reserved Keywords and Namespaces -

    You can't use reserved words (e.g., int, return) as identifiers, as defined by K&R and the official ANSI C standard. Also, avoid leading underscores followed by uppercase letters or double underscores, since these are reserved for the implementation (GNU C manual). Choose names like myValue instead of __MyValue.

  4. Case Sensitivity and Naming Conventions -

    C is case-sensitive, so Value and value are distinct variables (per Stanford CS Education Library). While uppercase starts are allowed, many style guides (e.g., Linux kernel) recommend lowercase and snake_case for readability, like total_count. Consistency builds confidence and prevents subtle bugs.

  5. Mnemonic Tricks for Quick Recall -

    Use memory aids like "LUCK" - Letter or Underscore, Cannot start with a digit, Keywords forbidden - and remember "c variable cannot start with" anything else. This simple phrase helps cement naming rules from educational sites like GeeksforGeeks. A catchy mnemonic turns rules into reflexes!

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