Answers of Concepts of Programming Languages Chapter 8

Review Questions

1. A control structure is a control statement and the collection of statements whose execution it controls
2. Bohm and Jacopini proves that all algorithms that can be expressed by flowcharts can be coded in a programming language with only two control statements : one for choosing between two control flow paths and one for logically controlled iterations
3. Block is a sequence of code, delimited by either braces or the do and end reserved words.
4. The design issue of all the selection and iteration control statement is : should the control structure have multiple entries?
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6. Python uses indention to specify compound statements and using a colon to introduce the then clause.
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9. Design issues of multiple selection statements are :

• What is the form and type of the expression that controls the selection?
• How are the selectable segments specified?
• Is execution flow through the structure restricted to include just a single selectable segment?
• How are the case values specified?
• How should unrepresented selector expression values be handled, if at all?

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11. The C switch statement has virtually no restrictions on the placement of the case expressions, whcih are treated as if they were normal statement labels.

12. C switch statement is based on ALGOL 68

13. C# has a static semantics rule that disallows the implicit execution of more than one segment. Beside, the control expression and the case statements can be strings in C#

14. Design issues of iterative statements are :

• What are the type and scope of the loop variable?
• Should it be legal for the loop variable or loop parameters to be changed in the loop, and if so, does the change affect loop control?
• Should the loop parameters be evaluated only once, or once for every iteration?

Problem Set

1. Design issues of two way selection are 

• What is the form and type of the expression that controls the selection?
• How are the then and else clauses specified?
• How should the meaning of nested selectors be specified?

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3. The issue was that when a selection statement is nested in the then clause of a selection statement, it is not clear to which if an else clause should be associated.

Example :

      if (a

           if(a==0)

               a = 9;

      else

          b = 0;