Difference between revisions of "Basic Pascal Tutorial/Chapter 3/IF"
(Added ; where missing and addition of compound statement example.) |
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Line 17: | Line 17: | ||
begin | begin | ||
Statement1; | Statement1; | ||
− | Statement2 | + | Statement2; |
end; | end; | ||
</syntaxhighlight> | </syntaxhighlight> | ||
Line 28: | Line 28: | ||
StatementIfFalse; | StatementIfFalse; | ||
</syntaxhighlight> | </syntaxhighlight> | ||
+ | |||
+ | Note there is no <tt>;</tt> following the statement before the <tt>else</tt>, even for the case with compound statements. | ||
+ | <syntaxhighlight> | ||
+ | if BooleanExpression then | ||
+ | begin | ||
+ | Statement1; | ||
+ | Statement2; | ||
+ | end | ||
+ | else | ||
+ | begin | ||
+ | Statement3; | ||
+ | Statement4; | ||
+ | end; | ||
+ | </syntaxhighlight> | ||
+ | |||
If the Boolean expression evaluates to <tt>FALSE</tt>, the statement following the <tt>else</tt> will be performed. Note that you may not use a semicolon after the statement preceding the <tt>else</tt>. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from. | If the Boolean expression evaluates to <tt>FALSE</tt>, the statement following the <tt>else</tt> will be performed. Note that you may not use a semicolon after the statement preceding the <tt>else</tt>. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from. |
Revision as of 18:34, 30 March 2019
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3Ca - IF (author: Tao Yue, state: unchanged)
The IF statement allows you to branch based on the result of a Boolean operation. The one-way branch format is:
if BooleanExpression then
StatementIfTrue;
If the Boolean expression evaluates to true, the statement executes. Otherwise, it is skipped.
The IF statement accepts only one statement. If you would like to branch to a compound statement, you must use a begin-end block to enclose the statements:
if BooleanExpression then
begin
Statement1;
Statement2;
end;
There is also a two-way selection:
if BooleanExpression then
StatementIfTrue
else
StatementIfFalse;
Note there is no ; following the statement before the else, even for the case with compound statements.
if BooleanExpression then
begin
Statement1;
Statement2;
end
else
begin
Statement3;
Statement4;
end;
If the Boolean expression evaluates to FALSE, the statement following the else will be performed. Note that you may not use a semicolon after the statement preceding the else. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from.
If you need multi-way selection, simply nest if statements:
if Condition1 then
Statement1
else
if Condition2 then
Statement2
else
Statement3;
Be careful with nesting. Sometimes the computer won't do what you want it to do:
if Condition1 then
if Condition2 then
Statement2
else
Statement1;
The else is always matched with the most recent if, so the computer interprets the preceding block of code as:
if Condition1 then
if Condition2 then
Statement2
else
Statement1;
You can get by with a null statement:
if Condition1 then
if Condition2 then
Statement2
else
else
Statement1;
Or you could use a begin-end block. But the best way to clean up the code would be to rewrite the condition.
if not Condition1 then
Statement1
else
if Condition2 then
Statement2;
This example illustrates where the not operator comes in very handy. If Condition1 had been a Boolean like: (not(a < b) or (c + 3 > 6)) and g, reversing the expression would be more difficult than NOTting it.
Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.