Boolean Expressions in Visual Basic

Sometimes a programmer would like a statement, or group of statements to execute only if certain conditions are true. There may be a different statement, or group of statements that are to be executed when the condition is false. An expression that can be evaluated to true or false is called a Boolean expression. (Named after a mathematician named Boole.)

Examples of Boolean Expressions with relational operators:  (X and Y are variables)

X = 5 (True if X has a value equal to 5)
X > Y(True if X is greater than Y; example X is 7, Y is 3)
X < Y(True if X is less than Y; example X is 2, Y is 2.5)
X <> Y(True if X is not equal to Y; example X is 5, Y is 6)
X >= Y(X is greater than or equal to Y; example X is 4, Y is 3 or 4)
X <= Y(X is less than or equal to Y; example X is 4, Y is 4 or 5)

The relational operators can also be used to compare strings: s1<s2 means s1 comes before s2 alphabetically. "a" < "A". Equality is case sensitive.

s1= "NO" (True if s1 has a value equal to "NO")
s1 > s2 (True if the string s1 is greater than the string s2; example s1 is "B", s2 is "A")
s1 < s2

(True if s1 is less than txt; example s1 is "ready", s2 is "set")

s1 <> s2 (True if s1 is not equal to s2; example s1 is "YES", s2 is "yes")
s1 >= s2 (s1 is greater than or equal to s2; example s1 is "45", s2 is "400")
Strings are compared starting in left most position, 1st postion is same, "5">"0"
s1 <= s2 (s1 is less than or equal to s2; example s1 is "Ann", s2 is "Anna")
s1 is "ann", s2 is "ANN"

The IF Statement

If you want to execute one or more statements when a Boolean expression is true use the format shown below:

  If Boolean  exp. Then 
     statement 1      statement      ...    Else      false statement 1      false statement     ...   End If

The Else section is optional;. If there are no statements to execute when the Boolean expression is false, you may leave out the else section.

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