COS 333: Chapter 8, Part 1

Summary

This lecture delves into statement-level control structures, focusing on selection statements like if-else and switch-case. It explores the evolution of control structures post-FORTRAN, emphasizing the sufficiency of two-way selection and pre-test loops over go-to statements. The lecture discusses design issues in selection statements, including control expression types, clause specifications, and nested selector disambiguation across various programming languages. It also covers multiple-way selection statements, comparing their implementation in languages like C, C#, Java, JavaScript, and Ruby, highlighting differences in syntax and semantics.

Takeaways

• 📚 The lecture introduces chapter eight on statement level control structures, focusing on selection statements and their design issues.
• 🔍 The discussion differentiates between control within expressions, among program units, and among program statements, with the latter being the focus of the chapter.
• 🔄 Historical context is provided on the development of control statements, highlighting the shift from hardware-specific to more generalized structures post-FORTRAN.
• 🛑 The redundancy of unconditional branching statements like 'go to' is established, with algorithms able to be represented using only two-way selection and pre-test loops.
• 🔑 Control structures are defined as comprising a control statement and the statements under its control, with a design issue being whether to allow multiple entry points.
• ❓ The use of 'go to' statements for multiple entry points is generally seen as reducing readability without significantly increasing flexibility.
• 🔼 Two-way selection statements are detailed, with 'if-else' constructs being the primary example, and their design issues such as control expression form and type, clause specification, and nested selector disambiguation.
• 🌐 Programming language variations in handling two-way selection statements are highlighted, including differences in how then and else clauses are specified and nested if-else structures are disambiguated.
• 🔠 The importance of the control expression's type is noted, with some languages allowing arithmetic expressions while others require boolean types.
• 🔀 Multiple way selection statements, akin to 'switch' or 'case' statements, are introduced with five key design issues discussed, including form and type of control expression, segment specification, and handling of unrepresented values.
• 📝 The chapter concludes with a look at iterative statements, unconditional branching, and guarded commands to be covered in the subsequent lecture.

Q & A

What is the main focus of the lecture on chapter eight?

The lecture on chapter eight focuses on statement level control structures, including an introduction to these structures and a detailed discussion on selection statements.

What are the different levels of control that can be discussed in programming languages?

The different levels of control in programming languages include control within expressions, control flow among program units (sub-programs or methods/functions), and control flow among program statements.

Why was there a heavy reliance on jumps or go-to statements in FORTRAN I?

FORTRAN I was designed to work with the IBM 704 hardware, which meant that the control statements were based directly on the hardware, making the control structures very specific and leading to a heavy reliance on jumps or go-to statements.

What was a significant outcome of the research and arguments in the 1960s regarding control statements?

A significant outcome was the understanding that all flowchart-represented algorithms can be coded with only two-way selection statements and pre-test logical loops, making unconditional branching statements like jumps or go-to statements unnecessary.

What is a control structure in the context of programming?

A control structure consists of a control statement, such as an if statement or a while statement, and the statements whose execution the control structure manages.

Why are go-to statements and multiple entry points generally considered to decrease readability in programming?

Go-to statements and multiple entry points can make the flow of a program less clear and more difficult to follow, which can lead to decreased readability and maintainability of the code.

What are the two general categories of selection statements?

The two general categories of selection statements are two-way selectors and multiple way selectors.

How does the use of parentheses affect the control expression in a two-way selection statement?

Parentheses are used to clearly delimit the control expression, ensuring that the condition is clearly defined and separated from the rest of the statement.

What is the purpose of an else clause in a selection statement used as an expression, as seen in some languages like ML or F#?

In languages where a selection statement is used as an expression and can return a value, the else clause is required to provide an alternative value when the condition is not met, ensuring that the expression always returns a value.

How does the design of switch statements in C and similar languages address the issue of control flowing through multiple segments?

The design requires an explicit unconditional branch, such as a break, at the end of each selectable segment to prevent control from flowing inadvertently to the next segment.

What is the difference between C and C# when it comes to multiple way selection statements?

C# differs from C by having a semantic rule that disallows implicit execution of more than one segment and by allowing both the control expression and the case constants to be strings.

How does Ruby handle multiple way selection statements with case expressions?

Ruby's case expressions can either use a sequence of when conditions without a control expression or use a case value as a control expression with when values, and they can return a result because they are treated as expressions.

What is the advantage of using boolean multiple-way selection statements over nested two-way selection statements?

Boolean multiple-way selection statements can improve readability by avoiding the complexity and potential confusion of nested if-then-else statements.