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Verify the correctness of the following program segment to exchange the values of x and y: temp = x y = temp At the beginning of this program segment, x and y have certain values. Thus we may express the actual precondition as x = a and y = b. The desired postcondi- tion is then x = b and y = a. Using the assignment rule, we can work backward from the postcondition to find the earlier assertions (read the following from the bottom to the top). {y = b, x = a} temp = x {y = b, temp = a} {x = b, temp = a} y = temp {x = b, y = a} The first assertion agrees with the precondition; the assignment rule, applied repeatedly, assures us that the program segment is correct.

Verify the correctness of the following program segment to exchange the values of x and y: temp = x y = temp At the beginning of this program segment, x and y have certain values. Thus we may express the actual precondition as x = a and y = b. The desired postcondi- tion is then x = b and y = a. Using the assignment rule, we can work backward from the postcondition to find the earlier assertions (read the following from the bottom to the top). {y = b, x = a} temp = x {y = b, temp = a} {x = b, temp = a} y = temp {x = b, y = a} The first assertion agrees with the precondition; the assignment rule, applied repeatedly, assures us that the program segment is correct.

Database System Concepts
Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
Problem 1PE
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Verify the correctness of the following program segment to exchange the values of x and y: temp = x y = temp At the beginning of this program segment, x and y have certain values. Thus we may express the actual precondition as x = a and y = b. The desired postcondi- tion is then x = b and y = a. Using the assignment rule, we can work backward from the postcondition to find the earlier assertions (read the following from the bottom to the top). {y = b, x = a} temp = x {y = b, temp = a} {x = b, temp = a} y = temp {x = b, y = a} The first assertion agrees with the precondition; the assignment rule, applied repeatedly, assures us that the program segment is correct.
Transcribed Image Text:Verify the correctness of the following program segment to exchange the values of x and y: temp = x y = temp At the beginning of this program segment, x and y have certain values. Thus we may express the actual precondition as x = a and y = b. The desired postcondi- tion is then x = b and y = a. Using the assignment rule, we can work backward from the postcondition to find the earlier assertions (read the following from the bottom to the top). {y = b, x = a} temp = x {y = b, temp = a} {x = b, temp = a} y = temp {x = b, y = a} The first assertion agrees with the precondition; the assignment rule, applied repeatedly, assures us that the program segment is correct.
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