Difference between revisions of "Basic Pascal Tutorial/Chapter 3/IF"
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The <tt>IF</tt> statement allows you to branch based on the result of a Boolean operation. The one-way branch format is: | The <tt>IF</tt> statement allows you to branch based on the result of a Boolean operation. The one-way branch format is: | ||
− | < | + | <syntaxhighlight> |
if BooleanExpression then | if BooleanExpression then | ||
StatementIfTrue; | StatementIfTrue; | ||
− | </ | + | </syntaxhighlight> |
If the Boolean expression evaluates to <tt>true</tt>, the statement executes. Otherwise, it is skipped. | If the Boolean expression evaluates to <tt>true</tt>, the statement executes. Otherwise, it is skipped. | ||
The <tt>IF</tt> statement accepts only one statement. If you would like to branch to a compound statement, you must use a <tt>begin-end</tt> to enclose the statements: | The <tt>IF</tt> statement accepts only one statement. If you would like to branch to a compound statement, you must use a <tt>begin-end</tt> to enclose the statements: | ||
− | < | + | <syntaxhighlight> |
if BooleanExpression then | if BooleanExpression then | ||
begin | begin | ||
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Statement2 | Statement2 | ||
end; | end; | ||
− | </ | + | </syntaxhighlight> |
There is also a two-way selection: | There is also a two-way selection: | ||
− | < | + | <syntaxhighlight> |
if BooleanExpression then | if BooleanExpression then | ||
StatementIfTrue | StatementIfTrue | ||
else | else | ||
StatementIfFalse; | StatementIfFalse; | ||
− | </ | + | </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. | ||
If you need multi-way selection, simply nest <tt>if</tt> statements: | If you need multi-way selection, simply nest <tt>if</tt> statements: | ||
− | < | + | <syntaxhighlight> |
if Condition1 then | if Condition1 then | ||
Statement1 | Statement1 | ||
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else | else | ||
Statement3; | Statement3; | ||
− | </ | + | </syntaxhighlight> |
Be careful with nesting. Sometimes the computer won't do what you want it to do: | Be careful with nesting. Sometimes the computer won't do what you want it to do: | ||
− | < | + | <syntaxhighlight> |
if Condition1 then | if Condition1 then | ||
if Condition2 then | if Condition2 then | ||
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else | else | ||
Statement1; | Statement1; | ||
− | </ | + | </syntaxhighlight> |
The <tt>else</tt> is always matched with the most recent <tt>if</tt>, so the computer interprets the preceding block of code as: | The <tt>else</tt> is always matched with the most recent <tt>if</tt>, so the computer interprets the preceding block of code as: | ||
− | < | + | <syntaxhighlight> |
if Condition1 then | if Condition1 then | ||
if Condition2 then | if Condition2 then | ||
Line 55: | Line 55: | ||
else | else | ||
Statement1; | Statement1; | ||
− | </ | + | </syntaxhighlight> |
You can get by with a null statement: | You can get by with a null statement: | ||
− | < | + | <syntaxhighlight> |
if Condition1 then | if Condition1 then | ||
if Condition2 then | if Condition2 then | ||
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else | else | ||
Statement1; | Statement1; | ||
− | </ | + | </syntaxhighlight> |
Or you could use a <tt>begin-end</tt> block. But the best way to clean up the code would be to rewrite the condition. | Or you could use a <tt>begin-end</tt> block. But the best way to clean up the code would be to rewrite the condition. | ||
− | < | + | <syntaxhighlight> |
if not Condition1 then | if not Condition1 then | ||
Statement1 | Statement1 | ||
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if Condition2 then | if Condition2 then | ||
Statement2; | Statement2; | ||
− | </ | + | </syntaxhighlight> |
This example illustrates where the not operator comes in very handy. If Condition1 had been a Boolean like: <tt>(not(a < b) or (c + 3 > 6)) and g</tt>, reversing the expression would be more difficult than NOTting it. | This example illustrates where the not operator comes in very handy. If Condition1 had been a Boolean like: <tt>(not(a < b) or (c + 3 > 6)) and g</tt>, reversing the expression would be more difficult than NOTting it. |
Revision as of 14:36, 24 March 2012
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 to enclose the statements:
if BooleanExpression then
begin
Statement1;
Statement2
end;
There is also a two-way selection:
if BooleanExpression then
StatementIfTrue
else
StatementIfFalse;
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.
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