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In this analogy, you would check outside to confirm that it is not raining. If the condition is true (meaning that it isn't raining), you wouldplay tennis, go for a walk, and then relax on the beach. If it is raining, (meaning that the test condition is false), you would make some popcorn and relax in front of theTV.
The else clause is optional
Note that when writing JavaScript code, the else clause shown in Figure 5 is optional. In other words, you might choose to direct the script to take some specific action if the condition is true, but simply transfer control to thenext sequential statement in the script if the condition is false.
A selection script example
Listing 3 shows a sample script containing two selection statements (commonly called if-else statements) in sequence.
| Listing 3 . A selection script example. |
|---|
<!-- File JavaScript03.html --><html><body><script language="JavaScript1.3">if(3>2){
document.write("3 is greater than 2.","<br/>")
}else{document.write("3 is not greater than 2.")
}//end ifif(3<2){
document.write("3 is less than 2.","<br/>")
}else{document.write("3 is not less than 2.")
}//end if</script></body></html> |
If 3 is greater than 2...
The conditional clause in the first if statement in Listing 3 uses the "greater-than" relational operator ">" to determine if the literal value 3 is greater than the literal value 2, producing the first line of output shown in Figure 6 .
| Figure 6 . Output from script in Listing 3. |
|---|
3 is greater than 2.
3 is not less than 2. |
The test returns true
Since 3 is always greater than 2, the statement in the line immediately following the first test in Listing 3 is executed and the code in the line following the word else is skipped producing the first line of output text shown in Figure 6 .
If 3 is less than 2...
The conditional clause in the second if statement in Listing 3 uses the "less-than" relational operator (see Figure 7 ) to determine if the literal value 3 is less than the literal value 2.
The test returns false
Since 3 isn't less than 2, the statement in the line immediately following the second test in Listing 3 is skipped and the statement in the line immediately following the second word else is executed producing the second line of output text shown in Figure 6 .
I doubt that I will a frequent need to use logical operators in the modules in this collection. If I do, I will explain them at the time. However, Iwill use relational operators.
The relational operators that are supported by JavaScript are shown in Figure 7 .
| Figure 7 . Relational operators. |
|---|
>Left operand is greater than right operand>= Left operand is greater than or equal to right operand<Left operand is less than right operand<= Left operand is less than or equal to right operand
== Left operand is equal to right operand!= Left operand is not equal to right operand |
As with the arithmetic operators discussed earlier, we will use these operators extensively as we work through the physics exercises in futuremodules.
The next item in the list under Structured programming is repetition . Before I can explain repetition, however, I need to explain the use of variables.
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