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Understanding Decision Structures in Programming: If…Then Statements and More

This guide explores decision structures in programming, focusing on If…Then statements and their variations, including Else and ElseIf clauses. It explains how boolean expressions control program flow and highlights the use of relational operators for comparisons. Additionally, it covers nested structures for complex decisions, introduces the Select…Case statement as an alternative, and explains random number generation. The text also discusses algorithms, logical operators, message boxes, counter variables, and the purpose of checkbox controls in user interfaces.

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Understanding Decision Structures in Programming: If…Then Statements and More

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  1. Controlling Program FlowWith Decision Structures

  2. The If…Then Statement • Decision structures to control flow • True/False (boolean) decision structure • If a condition is true then certain statements will operate • If intGuess = 7 Then • Me.lblMessage.Text = “You guessed it” End If

  3. The If…Then Statement • Many If…Then statements will use relational operators • = • < • < = • > • > = • < >

  4. The If…Then…Else Statement • Else clause: evaluated when the If condition evaluates to false. • If intX = intSecret Then • Me.lblMessage.Text = “You Guessed it” Else Me.lblMessage.Text = “Try Again.” End If

  5. Nested If…Then…Else Statements • When you have an If…Then decision structure within an If…Then decision structure

  6. If…Then…ElseIf Statement • Used to decide between three or more actions

  7. Select…Case Statement • Another decision structure • Sometimes preferred over the if…then…elseif

  8. Select…Case Is Statement • Compares a range of values by using relational operators.

  9. Random Numbers • Rnd generates random numbers • Equal to or greater than 0, less than 1 • (High – Low +1) * Rnd () + Low • Eliminate everything after the decimal Int • Randomize

  10. Static Variables • Retains the value of a variable throughout program execution • Keep the scope as narrow as possible • Static intSecret As Integer

  11. Algorithms • An algorithm states the code in words • Determine a secret number • Get a number from the player • Compare the number and the secret # • If higher than… • If lower than… • If equal… • Pseudocode: an algorithm with some code

  12. Logical Operators • Joins two boolean expressions • Not, And, Or (order of operation) • If intGrade < 0 Or intGrade > 100 then • ‘out of range • If strItem = “book” And intquant > 10 then • ‘discount items over 10 • If Not strItem = “book” then • ‘done for every item except book me.radrock.checked

  13. Message Box • Dialog box • Created by using its own statement MessageBox.Show(“display”, “title bar”) • Place anywhere you want a messagebox to appear

  14. Counter Variables • A variable that is incremented by a constant number • Keeps track of number of guesses, button clicks, lives in a game • To increment a counter it is the counter = counter + constant intNumTries = intNumTries + 1 Test Grade

  15. Checkbox Control • Prefix - chk • Similar to radio buttons • More than one may be selected • .checked property To do Menu

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