Chapter 5.2

Global Variables and Scope in Programming Languages

• In C, global variables are implicitly visible in all subsequent functions in the file, except those that include a declaration of a local variable with the same name.
• In C99, definitions of global variables usually have initial values, while declarations of global variables never have initial values.
• In C++, function prototypes declare names and interfaces of functions but do not provide their code, while function definitions are complete.
• In PHP, variables are implicitly declared when they appear in statements, and global variables can be made visible in functions through the $GLOBALS array or global declaration statement.
• Global variables in JavaScript are similar to those in PHP, but there is no way to access a global variable in a function that has declared a local variable with the same name.
• In Python, variables are implicitly declared when they appear as the targets of assignment statements, and a global variable can be referenced in a function but can only be assigned if declared global in the function.
• Nested functions in Python access variables through static scoping, and nonlocal variables must be declared nonlocal in the nested function.
• In F#, all names defined outside function definitions are globals, with their scope extending from their definitions to the end of the file.
• Static scoping in programming languages provides nonlocal access but can allow more access than necessary and present challenges related to program evolution and maintenance.
• Developers may discard structure when it gets in the way, leading to program designs that bear little resemblance to the original and encouraging the use of far more globals than necessary.
• This can result in all subprograms being nested at the same level in the main program, using globals instead of deeper levels of nesting.
• The final design may be awkward, contrived, and not reflect the underlying conceptual design, as it may lead to program restructuring and complex maintenance.

Scope and Lifetime of Variables in Programming Languages

• Dynamic scoping allows a subprogram to refer to different nonlocal variables during different executions, leading to less reliable and harder to understand programs.
• Local variables in dynamic scoping are visible to any other executing subprogram, making it impossible to protect them from accessibility.
• Dynamic scoping makes it challenging to statically type check references to nonlocal variables and makes programs more difficult to read.
• Accesses to nonlocal variables in dynamic-scoped languages take longer compared to static scoping.
• Despite its shortcomings, dynamic scoping eliminates the need to pass parameters from one subprogram to another, as they are implicitly visible.
• Dynamic scoping is not as widely used as static scoping due to the readability, reliability, and execution speed advantages of static-scoped languages.
• The scope and lifetime of a variable are related in some cases, but not in others. For instance, in C and C++, a variable declared with the static specifier has a static scope local to the function, but its lifetime extends over the entire program execution.
• The scope of a variable does not necessarily extend to the body of another function, but its lifetime may extend over the time during which the other function executes.
• The referencing environment of a statement in a static-scoped language includes variables declared in its local scope and those of its ancestor scopes that are visible.
• In Python, the referencing environment of a statement includes local variables and variables declared in the functions in which the statement is nested, creating new scopes and environments with each function definition.
• Techniques for implementing references to nonlocal variables in both static- and dynamic-scoped languages are discussed in Chapter 10 of the text.
• The provided Python program showcases the creation of scopes and referencing environments through function definitions and variable declarations.

Scope and Lifetime of Variables in Programming Languages

• Dynamic scoping allows a subprogram to refer to different nonlocal variables during different executions, leading to less reliable and harder to understand programs.
• Local variables in dynamic scoping are visible to any other executing subprogram, making it impossible to protect them from accessibility.
• Dynamic scoping makes it challenging to statically type check references to nonlocal variables and makes programs more difficult to read.
• Accesses to nonlocal variables in dynamic-scoped languages take longer compared to static scoping.
• Despite its shortcomings, dynamic scoping eliminates the need to pass parameters from one subprogram to another, as they are implicitly visible.
• Dynamic scoping is not as widely used as static scoping due to the readability, reliability, and execution speed advantages of static-scoped languages.
• The scope and lifetime of a variable are related in some cases, but not in others. For instance, in C and C++, a variable declared with the static specifier has a static scope local to the function, but its lifetime extends over the entire program execution.
• The scope of a variable does not necessarily extend to the body of another function, but its lifetime may extend over the time during which the other function executes.
• The referencing environment of a statement in a static-scoped language includes variables declared in its local scope and those of its ancestor scopes that are visible.
• In Python, the referencing environment of a statement includes local variables and variables declared in the functions in which the statement is nested, creating new scopes and environments with each function definition.
• Techniques for implementing references to nonlocal variables in both static- and dynamic-scoped languages are discussed in Chapter 10 of the text.
• The provided Python program showcases the creation of scopes and referencing environments through function definitions and variable declarations.

Scope and Lifetime of Variables in Programming Languages

• Dynamic scoping allows a subprogram to refer to different nonlocal variables during different executions, leading to less reliable and harder to understand programs.
• Local variables in dynamic scoping are visible to any other executing subprogram, making it impossible to protect them from accessibility.
• Dynamic scoping makes it challenging to statically type check references to nonlocal variables and makes programs more difficult to read.
• Accesses to nonlocal variables in dynamic-scoped languages take longer compared to static scoping.
• Despite its shortcomings, dynamic scoping eliminates the need to pass parameters from one subprogram to another, as they are implicitly visible.
• Dynamic scoping is not as widely used as static scoping due to the readability, reliability, and execution speed advantages of static-scoped languages.
• The scope and lifetime of a variable are related in some cases, but not in others. For instance, in C and C++, a variable declared with the static specifier has a static scope local to the function, but its lifetime extends over the entire program execution.
• The scope of a variable does not necessarily extend to the body of another function, but its lifetime may extend over the time during which the other function executes.
• The referencing environment of a statement in a static-scoped language includes variables declared in its local scope and those of its ancestor scopes that are visible.
• In Python, the referencing environment of a statement includes local variables and variables declared in the functions in which the statement is nested, creating new scopes and environments with each function definition.
• Techniques for implementing references to nonlocal variables in both static- and dynamic-scoped languages are discussed in Chapter 10 of the text.
• The provided Python program showcases the creation of scopes and referencing environments through function definitions and variable declarations.