1. Programming Languages and Paradigms
Imperative Programming

2. Imperative Programming
Variables, assignment, sequencing, iteration, procedures as units
State-based, assignment-oriented
Global variables, side effects
Program units: Data (Variables) and Computation (Statements and Routines)
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

3. Data and Computation Binding Data Computation Variables Data types
Assignments and expressions
Control structures
Subprograms / routines
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4. Binding Program units/entities have attributes Binding Binding time
e.g., a variable has a name, a statement has associated actions
Binding
setting the value of an attribute
Binding time
when binding occurs
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

5. Binding Time Language definition time Language implementation time
example: size of an int in Java
Language implementation time
example: size of an int in C
Compile-time
example: type of a particular variable in C/Java
Execution-time
example: memory address of a variable
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

6. Variable A named location in memory that can hold a value
Formally, a 5-tuple:
name
scope
type
l-value
r-value
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

7. Name and Scope Declaration Identifier rules and significant characters
Name bound during compile time
Scope
range of instructions over which variable name is known
namespaces
Blocks (as in Pascal or C)
Static vs dynamic scope binding
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8. Type Consists of Built-in/Primitive vs User-defined types
Set of values
Operations
Built-in/Primitive vs User-defined types
binding?
Implicit declarations
e.g., FORTRAN and first letter of a variable and first assignment in BASIC or Perl or PHP
Dynamic typing
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9. Why Use Data Types? Purpose: classification and protection Advantages:
note that all data are ultimately represented as bit-strings
Advantages:
abstraction
compile-time checking and resolution
explicit specification
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10. Complex Data Types User-defined enumeration types Composite types
Aggregations
cartesian product (records or structures)
mapping (arrays)
unions
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11. L-value and r-value l-value: address/location
lifetime
memory allocation
r-value: contents/encoded value
initialization
constants
* binding?
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12. Pointers Attributes Allocation and de-allocation Operators
referencing (address-of)
de-referencing
Synonyms and unnamed variables
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13. Control: Statements and Routines
Expressions and statements
Conditional execution
Iteration
Routines
Parameter Passing
Modules and Program Structure
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14. Expressions and Statements
Operands: variables, literals
Operators
Unary, binary, and others (functions?)
Value returned vs effect
Precedence
Statements
The semicolon: C (terminator) vs Pascal (separator)
Blocks: C { } vs Pascal (begin-end)
Control structures: decision, iteration, routines
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15. Conditional Execution
Conditions and boolean values
Relationship of conditions and int in C
Not adopted in Java
Short-circuiting
If-statements and dangling else
The switch statement
Restrictions
The break; statement
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16. Iteration For statement The while loop while vs do-while
Loop control variable
The while loop
while vs do-while
do-while versus repeat-until
Iteration using goto
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17. Routines Program unit; sequence of instructions Also a 5-tuple: name
scope
type
l-value
r-value
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18. Evolution: statements & goto, then structure, then routines
First there was machine language, then came Fortran:
Improved statement readability
The goto statement
Then structured programming:
Blocks
Control structures (e.g., while-do)
And routines/procedures:
Scope, activation, recursion
Parameter passing
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

19. Evolution: statements & goto, then structure, then routines
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

20. About Routines Functions versus procedures Parameter Passing
By value
By reference
Others? (call by value-result, call by name)
Procedures as parameters
Activation Records
Recursion
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

21. Functions versus Procedures
Difference: functions return a value, procedures don’t
Languages treat this distinction differently
C: procedures are just functions that don’t return anything (void)
Modula: functions are just procedures with a return type
Pascal: some restrictions on function parameters
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

22. Parameters Formal parameters: indicated in routine definition
Actual parameters: indicated in invocation
Parameter passing methods
Call by value
Call by reference
Call by value-result (copy-in, copy-out)
Others: Call by name/macro substitution
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23. Call by Value Formal parameter is a local variable
Actual parameter serves as an initial value for the formal parameter
Actual parameter could be an expression
Language examples:
In C, all parameters are called by value, use of pointers is just a workaround
In Pascal, call by value is the default, but call by reference is supported
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24. Call by Reference
Formal parameter serves as an alias/synonym for the actual parameter during the invocation
Formal and actual parameters share the same l-value
Actual parameter must be a variable (or have an l-value)
Language examples
Pascal (precede formal parameter declaration with var; also called pass by variable)
C++ (append an & to the variable type in formal parameter)
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

25. Call by Value Result Copy-in/copy-out
Actual parameters are initially copied into the formal parameters
Formal parameter values are copied back to actual parameters after routine completes execution
In and out parameters are sometimes distinguished
Language example: ADA (in, out, and in-out parameters)
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

26. Call by Reference versus Call by Value-Result
int a = 5;
change( a );
// value of a?
By Reference: a = 11
By Value-Result: a = 7
void change( int x ) {
x++;
a = 10; }
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

27. Macro Substitution In C, #define directive can contain parameters
Example: #define sqr(x) x*x
The statement ans = sqr(3*a); is expanded to become ans = 3*a*3*a;
Careful because sqr(3+1) expands to 3+1*3+1
Better version: #define sqr(x) (x)*(x)
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

28. Activation Records
Activation record (or frame): contains the data needed for the activation of a routine
Typical Contents
Local variables
Formal parameters
Function result, if applicable
Control link (to activation record of caller) and access link (to access other data within scope)
Activation records stored in a runtime stack
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.

29. Illustration Suppose the functions main and f are defined as follows:
void main() { f(); g(); }
void f() { g(); }
There are two different activations of g() when the program executes g f
main g
main
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30. Recursion Activation records using runtime stacks support recursion
Recursive calls create separate activations of the function
factorial …
main
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31. More About Activations
Control and access links serve to provide a method to get to calling frames and non-local data
More examples are available in a separate set of slides
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32. Modules and Program Structure
Programming in the Large
need to compose program through units
Modules
program units that interact with each another
modules in turn contain their own data and routines
Encapsulation
information hiding
independent modules with interfaces
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33. Modularity Considerations
Interface and Implementation
Compilation
Independent
Separate
Libraries
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34. Summary Imperative Programming is about On to the next paradigm . . .
Data (variables) and statements affecting that data
Control-flow constructs enrich statement specification
Routines and modules help impose program organization
On to the next paradigm . . .
Copyright 2006, by the authors of these slides, and Ateneo de Manila University. All rights reserved.