Chapter 3- Data Types and Operators Boolean Expressions Page 2 3 4

Introduction

The If Statement Page 2 3 4 5 6 7 8 9
Arithmetic Operations Page 2 3 4 5 6 Boolean Operators and Nested If Statements Page 2 3 4 5 6 7
Events and Sequential Processing Page 2 3 4 5 More Examples Page 2 3 4 5 6 7 8 9 10 11 12
Datatypes and Conversions Page 2 3 4 5 6 7 Using Check Box and Option Controls Page 2 3 4 5 6 7 8 9 10
Variable Declarations - Local and Global Page 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Exercises Page 2 3 4 5 6 7 8
Chapter 4- Selection Statements Review Questions
Introduction

4-6: More on Nested If Statements

Exercise 4-5: Zeller's Algorithm

Zeller's Congruence allows us to compute the day of the week that a given date corresponds to. For example, given a date such as 21 Jan 1937 the program will compute that the day of the week is Thursday. In fact Visual Basic already has a built-in function to do this task for us, the WeekDay function, so we are "re-inventing the wheel" so to speak - but lets do it anyway for practice.

Figure 4-9: The Zeller program interface

The Zeller algorithm has the following steps, assuming a date dd Month yyyy is entered:

  1. compute M from the Month as follows - if it's January M=11, February M=12, March M=1, April M=2 and so on to December M=10
  2. if it's January or February subtract 1 from the year, yyyy. This new value is to be used in the remaining steps.
  3. calculate C, the century, from the year (e.g. for 1937 C=19)
  4. calculate Y, the year in the century, from the year (e.g. for 1937 Y=37.)
  5. compute the value of:
    (26M - 2) div 10 + D + Y +
    Y div 4 + C div 4 - 2C + 7777
    where D is an integer value for the day of the month.
  6. find the remainder when this value is divided by 7. A remainder of 0 implies Sunday, 1 implies Monday and so on to 6 implies Saturday.

The figure shows the user interface. Notice that we calculate the day of the week by both Zeller's algorithm and the built-in Visual Basic function, as a means of double checking ourselves.