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Questions and Answers
What is the main advantage of using dynamic variables over static variables?
What is the main advantage of using dynamic variables over static variables?
What is a disadvantage of using implicit heap-dynamic variables?
What is a disadvantage of using implicit heap-dynamic variables?
What is the main disadvantage of static variables?
What is the main disadvantage of static variables?
What is the main advantage of using stack-dynamic variables?
What is the main advantage of using stack-dynamic variables?
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What is a disadvantage of using explicit heap-dynamic variables?
What is a disadvantage of using explicit heap-dynamic variables?
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What is the definition of a binding in programming languages?
What is the definition of a binding in programming languages?
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What is the main advantage of using static variables?
What is the main advantage of using static variables?
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What is the binding time for associating operator symbols with operations?
What is the binding time for associating operator symbols with operations?
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What is a disadvantage of using stack-dynamic variables?
What is a disadvantage of using stack-dynamic variables?
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What type of binding occurs when a variable is declared with a data type?
What type of binding occurs when a variable is declared with a data type?
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What is the lifetime of a variable?
What is the lifetime of a variable?
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Which of the following languages use implicit declarations?
Which of the following languages use implicit declarations?
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What is the advantage of implicit declarations?
What is the advantage of implicit declarations?
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What type of binding occurs at runtime?
What type of binding occurs at runtime?
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Which of the following languages use dynamic binding?
Which of the following languages use dynamic binding?
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What is the disadvantage of implicit declarations?
What is the disadvantage of implicit declarations?
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What is a characteristic of a variable in programming languages?
What is a characteristic of a variable in programming languages?
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Which programming language has a limit of 30 characters for a name?
Which programming language has a limit of 30 characters for a name?
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What is the disadvantage of case sensitivity in names?
What is the disadvantage of case sensitivity in names?
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Which programming languages have predefined names that are mixed case?
Which programming languages have predefined names that are mixed case?
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What is a keyword in programming languages?
What is a keyword in programming languages?
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What is a potential problem with reserved words in programming languages?
What is a potential problem with reserved words in programming languages?
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In which programming language does a name starting with a dollar sign ($) specify its type?
In which programming language does a name starting with a dollar sign ($) specify its type?
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What is the characteristic of names in the C-based languages?
What is the characteristic of names in the C-based languages?
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What is the primary advantage of explicit heap-dynamic variables?
What is the primary advantage of explicit heap-dynamic variables?
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What is a common disadvantage of implicit heap-dynamic variables?
What is a common disadvantage of implicit heap-dynamic variables?
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What determines how references to names are associated with variables?
What determines how references to names are associated with variables?
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Which of the following is a characteristic of global variables?
Which of the following is a characteristic of global variables?
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What is the primary disadvantage of explicit heap-dynamic variables?
What is the primary disadvantage of explicit heap-dynamic variables?
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What is the scope of a variable?
What is the scope of a variable?
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What type of scoping is used in the given code snippet?
What type of scoping is used in the given code snippet?
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What is the reference of variable x in sub2?
What is the reference of variable x in sub2?
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What is big in relation to sub2?
What is big in relation to sub2?
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What is the scope of the variable count in the while loop?
What is the scope of the variable count in the while loop?
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Why do Java and C# not allow declaring variables in the inner block?
Why do Java and C# not allow declaring variables in the inner block?
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In which languages can variable declarations appear anywhere in a program unit?
In which languages can variable declarations appear anywhere in a program unit?
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In C#, what is the scope of a variable declared in a block?
In C#, what is the scope of a variable declared in a block?
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In which languages can variables be declared in the for statements' initialization expressions?
In which languages can variables be declared in the for statements' initialization expressions?
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Study Notes
Introduction to Variables
- Variables have several attributes: type, scope, lifetime, type checking, initialization, and type compatibility.
Names
- Length of names varies across languages:
- FORTRAN I: maximum 6 characters
- FORTRAN 95: maximum 31 characters
- COBOL: maximum 30 characters
- Ada: no limit, all characters are significant
- C++ and C#: no limit, but implementors often impose one
- PHP: names must begin with a $
- Perl: $, @, or % at the beginning specifies the type
- Case sensitivity:
- Disadvantage: readability (names that look alike are different)
- C-based languages are case sensitive
- Other languages are not case sensitive
- Special words:
- Keywords: special words in certain contexts, e.g., in Fortran
- Reserved words: special words that cannot be used as user-defined names
- Potential problem: many reserved words can cause collisions, e.g., COBOL has 300 reserved words
The Concept of Binding
- Binding: association between an attribute and an entity
- Binding time: when the association takes place
- Possible binding times:
- Language design time
- Language implementation time
- Compile time
- Load time
- Runtime
- Link time
Static and Dynamic Binding
- Static binding: binding occurs before runtime and remains unchanged
- Advantages: reliability, efficiency
- Disadvantages: less flexibility
- Dynamic binding: binding occurs during execution or can change during execution
- Languages that use dynamic binding: Python, Ruby, JavaScript, and PHP
- Advantages: flexibility
- Disadvantages:
- Less reliable
- Cost at execution time
Storage Bindings and Lifetime
- Allocation: getting a memory cell from a pool
- Deallocation: putting a memory cell back into the pool
- Lifetime: time during which a variable is bound to a memory cell
- Types of storage binding based on lifetimes:
- Static variables
- Stack-dynamic variables
- Explicit heap-dynamic variables
- Implicit heap-dynamic variables
Categories of Variables by Lifetime
- Static variables:
- Bound to memory cells before execution
- Advantages: efficiency, history-sensitive subprogram support
- Disadvantage: lack of flexibility
- Stack-dynamic variables:
- Storage bindings created during execution
- Advantages: allows recursion, conserves storage
- Disadvantages:
- Overhead of allocation and deallocation
- Subprograms cannot be history sensitive
- Inefficient references
- Explicit heap-dynamic variables:
- Allocated and deallocated by explicit directives
- Advantages: provides dynamic storage management
- Disadvantage: inefficient and unreliable
- Implicit heap-dynamic variables:
- Allocation and deallocation caused by assignment statements
- Advantages: flexibility
- Disadvantages:
- Inefficient
- Loss of error detection
Scope
- Scope: range of statements over which a variable is visible
- Local variables: declared in a program unit
- Nonlocal variables: visible within a block, but not declared there
- Global variables: special category of nonlocal variables
- Scope rules: determine how references to names are associated with variables
Static Scope
- Static scoping: binding names to nonlocal variables
- The scope of a variable is determined by the block structure
Blocks
- Blocks create static scopes
- The reference to a variable inside a block is hidden from the same variable outside the block
Declaration Order
- C99, C++, Java, and C# allow variable declarations to appear anywhere in a program unit
- Declarations may create scopes that are not associated with subprograms
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Description
This quiz covers the basics of variables, including properties such as type, scope, lifetime, type checking, initialization, and type compatibility. Learn about the concept of binding, named constants, and more.
