Conditional Expressions

 

The other two structures, selection and repetition, involve conditional expressions. Conditional expressions evaluate to being either true or false, and are important in programming because they can be used to signal the computer to take one action (or set of actions) if the condition is true and another set of actions if the condition is false. With selection, a set of actions is taken if a condition is true; an alternative set of actions is taken if the condition is false; with repetition, a set of actions is repeated as long as a certain condition prevails.

 

Conditional expressions in VB statements must be preceded by certain keywords, such as If or Do While. The conditional expression itself is formed as follows:

 

<expression> <relational operator> <expression>

 

The expression on either side of the relational operator can be a constant (string or numeric), variable (string or numeric), or an expression using symbols like +, -, etc.

 

The relational operator can be one of six symbols (or pair of symbols):

 

Relational Operator Meaning

< less than

> greater than

= equal to

<> not equal to (same as in algebra)

>= greater than or equal to (same as in algebra)

<= less than or equal to (same as in algebra)

 

The expressions on either side of the relational operator should be "compatible" - that is, you should only compare a numeric item to a numeric item or a string item to a string item. (As with assignment statements, VB will perform "implicit conversion" if you compare two data items of different types. If VB cannot perform a conversion, a "type mismatch" error will occur.)

 

Following are some examples of conditional expressions involving the numeric variables intI and intJ. Assume intI contains the value 5 and intJ contains the value 10:

 

What is being compared

Conditional Expression

True or False?

variable vs. constant

intI = 5

True

variable vs. constant

intI >= 5

True

variable vs. constant

intI <> 5

False

expression vs. variable

(intI + 5) >= intJ

True

variable vs. variable

intJ > intI

True

expression vs. expression

intJ * 2 < intI + 4

False

constant vs. variable

20 > intJ

True

 

When the conditional expression involves a character string compare, you can think of the relational operators as performing a comparison based on "alphabetical order". For example, the expression "A" < "D" is true, because "A" precedes "D" alphabetically. Likewise, the expression "Z" > "Q" is true, because "Z" follows "Q" alphabetically.

 

Following are some examples of conditional expressions involving the string variables strA and strB. Assume strA contains "MARY" and strB contains "JOE":

 

What is being compared

Conditional Expression

True or False?

variable vs. constant

strA > "BILL"

True

variable vs. constant

strA > "XYZ"

False

variable vs. variable

strB < strA

True

constant vs. variable

"ABC" > strB

False

 

Technically, when character string data is compared, each character is compared one by one from left to right until the computer can determine whether the first string is less than, equal to, or greater than the second string. The determination is based on the ASCII value of each character.

 

If you look at a chart of the ASCII characters, you should see that, generally speaking:

 

special characters < digits < uppercase letters < lowercase letters

 

Bear in mind that when digits are used in character string comparisons, the comparison is based on the digits' ASCII value. Consider these two examples:

 

Type of Comparison

Conditional Expression

True or False?

numeric

3 > 107

False

character string

"3" > "107"

True

 

 

Using And, Or, and Not

 

You can have two or more individual conditions in a conditional expression if you join those conditions with the keywords And or Or. The rules for multi-condition conditional expressions are as follows:

 

(1) ALL conditions joined by the keyword And must be TRUE in order for the entire conditional expression to be considered TRUE - otherwise, the expression will be evaluated as FALSE.

 

(2) If ANY of the conditions joined by the keyword Or are TRUE, the entire conditional expression will be considered TRUE - only if ALL the conditions joined by ORs are FALSE will the entire conditional expression be considered FALSE.

 

(3) If you mix Ands and Ors in the same conditional expression, And has precedence over Or.

 

(4) The And/Or precedence can be overridden with parentheses.

 

Examples follow. Assume the following variables contain the indicated values:

A = 4 B = 6 C = 10 D = 3 E = 2 F = 5 G = 4

 

Conditional Expression

True or False?

C > B And B > A And A >= G

True

C > B And D > C And A >= G

False

F = G Or B = G Or A = G

True

F = G Or B = G Or D = G

False

A <> B Or C = D And F < G

True

(A <> B Or C = D) And F < G

False

 

The truth-value of any condition can be negated by placing the keyword Not in front of the conditional expression. For example, if the condition A > B is true, then the condition Not A > B is false.

 

The keywords And, Or, and Not are called logical operators.

 

 

Advanced String Comparisons Using the Like Operator

 

When regular relational operators (like >, <, =) won't do, you can use the Like operator to compare a string against a specified pattern. The Like operator is similar to the LIKE operator found in the SQL of most DBMS products (although the syntax differs somewhat).

 

An expression using Like has this format:

 

string Like pattern

 

where string is any string expression and pattern is any string expression conforming to the pattern-matching conventions described below.

 

A Like expression can be used anywhere a conditional expression can be used, such as in an If statement or Do While statement:

 

If string Like pattern Then . . .

Do While string Like pattern

 

A Like expression (or any conditional expression) can also be assigned to a Boolean variable to yield a True or False value:

 

BooleanVariable = string Like pattern

 

If string matches pattern, the result is True; if there is no match, the result is False. If either string or pattern is Null, result is Null.

 

Built-in pattern matching provides a versatile tool for string comparisons. The pattern-matching features allow you to use wildcard characters, character lists, or character ranges, in any combination, to match strings. The following table shows the characters allowed in pattern and what they match:

 

Characters in pattern

Matches in string

?

Any single character.

*

Zero or more characters.

#

Any single digit (09).

[charlist]

Any single character in charlist.

[!charlist]

Any single character not in charlist.

 

A group of one or more characters (charlist) enclosed in brackets ([ ]) can be used to match any single character in string and can include almost any character code, including digits.

 

Note: To match the special characters left bracket ([), question mark (?), number sign (#), and asterisk (*), enclose them in brackets. The right bracket (]) can't be used within a group to match itself, but it can be used outside a group as an individual character.

 

By using a hyphen () to separate the upper and lower bounds of the range, charlist can specify a range of characters. For example, [A-Z] results in a match if the corresponding character position in string contains any uppercase letters in the range AZ. Multiple ranges are included within the brackets without delimiters.

 

Other important rules for pattern matching include the following:

 

         An exclamation point (!) at the beginning of charlist means that a match is made if any character except the characters in charlist is found in string. When used outside brackets, the exclamation point matches itself.

         A hyphen () can appear either at the beginning (after an exclamation point if one is used) or at the end of charlist to match itself. In any other location, the hyphen is used to identify a range of characters.

         When a range of characters is specified, they must appear in ascending sort order (from lowest to highest). [A-Z] is a valid pattern, but [Z-A] is not.

 

         The character sequence [] is considered a zero-length string ("").

 

This example uses the Like operator to compare a string to a pattern.

 

Dim blnMyCheck as Boolean

blnMyCheck = "aBBBa" Like "a*a" ' Returns True.

blnMyCheck = "F" Like "[A-Z]" ' Returns True.

blnMyCheck = "F" Like "[!A-Z]" ' Returns False.

blnMyCheck = "a2a" Like "a#a" ' Returns True.

blnMyCheck = "aM5b" Like "a[L-P]#[!c-e]" ' Returns True.

blnMyCheck = "BAT123khg" Like "B?T*" ' Returns True.

blnMyCheck = "CAT123khg" Like "B?T*" ' Returns False.

 

 

Now that you are an expert on conditional expressions, let us look at the selection and the repetition control structures.