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Date: Wed, 11 Dec 1996 22:34:30 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Fri, 20 Sep 1996 19:03:08 GMTContent-length: 9650<HTML><HEAD><TITLE>CS 302 Section 70 - Lecture Notes: Week 3,Part 1</TITLE></HEAD><BODY><H2><!WA0><!WA0><!WA0><A HREF="http://www.cs.wisc.edu/~tick/cs302.html#text" ><!WA1><!WA1><!WA1><IMG SRC="http://www.cs.wisc.edu/~tick/icons/arrowleft.gif" WIDTH=15 HEIGHT=15></A> Lecture Notes - Week 3,Part 1</H2><HR><DL> <DT>Topic: <DD>Conditional execution and logical expressions. <TT>IF/THEN/ELSE/END IF</TT> statement.<P> <DT>Text: <DD>Chp. 3.2, 3.4, 3.5, 3.8, 3.9<P> <DT>Notes: <DD> <HR> <CENTER> <H3>Conditional Execution</H3> </CENTER> <UL> <LI>So far, all statements are <EM>unconditionally</EM> executed, starting with the first and proceeding sequentially to the last. <LI>What if we don't want <EM>every</EM> statement to be executed <EM>every</EM> time we run the program? <LI>Want to <EM>conditionally</EM> execute some statements, depending on a condition which may change every time the program is run. </UL> <H4>IF/THEN/END IF Statement</H4> <UL> <LI>Use the <TT>IF/THEN/END IF</TT> statement to conditionally execute one or more statements (Note: <TT>END IF</TT> is two words). <LISTING> unconditional statements IF (condition) THEN conditional statements END IF </LISTING> <LI>When the <STRONG>condition</STRONG> is <EM>true</EM> the conditional statements are executed; when it is <EM>false</EM> they are skipped. </UL> <H4>IF/THEN/ELSE/END IF Statement</H4> <UL> <LI>Optional <EM>ELSE</EM> specifies another set of statements to be executed when the <STRONG>condition</STRONG> is <EM>false</EM>. <LISTING> IF (condition) THEN true conditional statements ELSE false conditional statements END IF IF (NUM . GE. 0) THEN PRINT *, 'Positive' ELSE PRINT *, 'Negative' END IF </LISTING> <LI><EM>Indent</EM> conditional statements three spaces (i.e. column 10). <LI><TT>IF</TT>, <TT>ELSE</TT> and <TT>END IF</TT> are on separate lines. </UL> <H4>Logical IF Statement</H4> <UL> <LI>If only have <EM>one</EM> conditional statement and no <TT>ELSE</TT> part then can put everything on a single line. <LISTING> IF (DAY .EQ. 1) PRINT *, 'Monday' </LISTING> <LI>Note: there is no <TT>THEN</TT> or <TT>END IF</TT>. </UL> <H4>Nested IF Statements</H4> <UL> <LI>Can <EM>nest</EM> <TT>IF</TT> statements within the conditional sections of other <TT>IF</TT> statements. <LISTING> IF (YEAR .LE. 4) THEN STATUS = 'Undergraduate' Fee = 4547.75 ELSE STATUS = 'Graduate' IF (YEAR . LE. 8) THEN FEE = 5852.25 ELSE FEE = 720.75 END IF END IF </LISTING> <LI>The second <TT>IF</TT> statement is itself conditionally executed depending on the condition of the first <TT>IF</TT> statement. <LI>Indent each level another 3 spaces (i.e. 3, 6, 9, ... spaces). Use indentation to line up statements so that they are easy to read and understand. </UL> <H4>General IF Statement</H4> <UL> <LI>Use the <EM>general IF statement</EM> when testing multiple conditions, each with their own set of statements to execute. <LISTING> IF (YEAR .EQ. 1) THEN PRINT *, 'Freshman' ELSE IF (YEAR .EQ. 2) THEN PRINT *, 'Sophomore' ELSE IF (YEAR .EQ. 3) THEN PRINT *, 'Junior' ELSE IF (YEAR .EQ. 4) THEN PRINT *, 'Senior' END IF </LISTING> <LI>The conditions are checked sequentially until one is found that is true. The rest are skipped even if subsequent conditions are also true! <LI>An optional <TT>ELSE</TT> clause is executed when none of the conditions listed are true. <LISTING> IF (LIGHT .EQ. 'R') THEN PRINT *, 'Stop' ELSE IF (LIGHT .EQ. 'O') THEN PRINT *, 'Prepare to stop' ELSE PRINT *, 'Proceed' END IF </LISTING> <LI>Use the general <TT>IF</TT> statement instead of multiple nested <TT>IF</TT> statements. </UL> <HR> <CENTER> <H3>Logical Expressions</H3> </CENTER> <UL> <LI>The <STRONG>condition</STRONG> tested in <TT>IF</TT> statements is a <EM>logical expression</EM> surrounded by parentheses. <LISTING> IF (logical-expression-1) THEN ... ELSE IF (logical-expression-2) THEN ... END IF </LISTING> <LI>Logical expressions are similar to arithmetic expressions except the result has only two possible values: <TT>.TRUE.</TT> and <TT>.FALSE.</TT> (Note the dots before and after). </UL> <H4>LOGICAL Variables</H4> <UL> <LI>A variable can store <TT>.TRUE.</TT> and <TT>.FALSE.</TT> using the <TT>LOGICAL</TT> variable type. <LISTING> LOGICAL RENEW </LISTING> <LI>To assign a <TT>LOGICAL</TT> value to a <TT>LOGICAL</TT> variable use the assignment statement, just like any other variable. <LISTING> logical-variable = logical-expression RENEW = .FALSE. </LISTING> </UL> <H4>Relational Operators</H4> <UL> <LI>Relational operators compare two <EM>numbers</EM> together to produce <TT>.TRUE.</TT> and <TT>.FALSE.</TT> These can be used to create complex logical expressions. <BLOCKQUOTE> <TT>.LT.</TT> - less than?<BR> <TT>.LE.</TT> - less than or equal to?<BR> <TT>.GT.</TT> - greater than?<BR> <TT>.GE.</TT> - greater than or equal to?<BR> <TT>.EQ.</TT> - equal?<BR> <TT>.NE.</TT> - not equal?<BR> </BLOCKQUOTE> <LISTING> DAY = 52 RENEW = DAY .GT. 14 (= .TRUE.) </LISTING> <LI>Relational operator names also start and end with a dot. <LI>Operands may be literals, variables or expressions of any type (except <TT>LOGICAL</TT>s). e.g. <TT>INTEGER</TT>s, <TT>REAL</TT>s or <TT>CHARACTER</TT> strings. <LISTING> RENEW = EXP(X) .LT. (Y * 43.7 + Z) </LISTING> <LI>Both operands must be of comparable types; i.e. both numbers or both <TT>CHARACTER</TT> strings. You can compare "apples to oranges"! </UL> <H4>Logical Operators</H4> <UL> <LI>Logical operators compare two <TT>LOGICAL</TT> values to produce <TT>.TRUE.</TT> or <TT>.FALSE.</TT>. <BLOCKQUOTE> <TT>.AND.</TT> - are both operands <TT>.TRUE.</TT>?<BR> <TT>.OR.</TT> - is either operand <TT>.TRUE.</TT>?<BR> <TT>.NOT.</TT> - inverts <TT>.TRUE.</TT> and <TT>.FALSE.</TT><BR> </BLOCKQUOTE> <LI>Logical operator names also start and end with a dot. <LI>The logical operators are defined using a <EM>truth table</EM> (see Pg. 98 for the definitions of <TT>.EQV.</TT> and <TT>.NEQV.</TT>). <PRE> Op-1 Op-2 | (Op-1 .AND. Op-2) | (Op-1 .OR. Op-2)------------------+---------------------+------------------.TRUE. .TRUE. | .TRUE. | .TRUE..TRUE. .FALSE. | .FALSE. | .TRUE..FALSE. .TRUE. | .FALSE. | .TRUE..FALSE. .FALSE. | .FALSE. | .FALSE. Op-1 | (.NOT. Op-1)--------+---------------.TRUE. | .FALSE..FALSE. | .TRUE. </PRE> <LI>Example: what is the definition of a bicycle? <LISTING> IF ((WHEEL .EQ. 2) .AND. (.NOT. POWRD)) THEN PRINT *, 'Bicycle' ELSE PRINT *, 'Not a bicycle' END IF </LISTING> </UL> <H4>Operator Precedence</H4> <UL> <LI><STRONG>VERY IMPORTANT</STRONG> - You must remember the precedence order of the logical and relational operators. This varies between different programming languages! <UL> <LI><EM>First</EM>: arithmetic operators <LI><EM>Second</EM>: relational operators (all have the same precedence) <LI><EM>Third</EM>: <TT>.NOT.</TT> <LI><EM>Fourth</EM>: <TT>.AND.</TT> <LI><EM>Fifth</EM>: <TT>.OR.</TT> <LI><EM>Last</EM>: <TT>.EQV.</TT> and <TT>.NEQV.</TT> </UL> <LI>Note the order of <TT>.AND.</TT> and <TT>.OR.</TT> - it is <EM>very</EM> easy to get it wrong, with disasterous results! <LI>Use parentheses to over-ride the default precedence if necessary. <LI>Example: <EM>if x and y are both greater than min then ...</EM> <LI><EM>Right:</EM> <LISTING> IF (X .GT. MIN .AND. Y .GT. MIN) THEN </LISTING> <LI><EM>Wrong:</EM> <LISTING> IF (X .AND. Y .GT. MIN) THEN </LISTING> <LI>When in doubt use parentheses. </UL> <H4>Comparing CHARACTER Strings</H4> <UL> <LI><TT>CHARACTER</TT> strings can also be compared using the relational operators. <LISTING> IF (STATUS .EQ. 'Graduate') THEN </LISTING> <LI>If the strings are the same length then they are compared character by character. Both strings must have <EM>exactly</EM> the same characters. <LI>If one string is shorter it is automatically <EM>padded</EM> with blanks before comparing. <LI>Normally only use <TT>.EQ.</TT> and <TT>.NE.</TT> when comparing <TT>CHARACTER</TT> strings; using other relational operators can lead to unpredictable results. <LI><EM>Right:</EM> <LISTING> ANSWER = 'No' ... IF (ANSWER .EQ. 'Yes') THEN </LISTING> <LI><EM>Wrong:</EM> <LISTING> ANSWER = 'No' ... IF (ANSWER .LT. 'Yes') THEN </LISTING> <LI>Comparisons are <EM>case sensitive</EM> - upper and lowercase characters are very different! <LISTING> CHARACTER *3 ANSWER ANSWER = 'Yes' ... IF (ANSWER .EQ. 'Yes') THEN (.TRUE.) ... IF (ANSWER .EQ. 'YES') THEN (.FALSE.) ... </LISTING></DL></BODY><HR><ADDRESS><H5>Copyright © 1996 Modified From<!WA2><!WA2><!WA2><A HREF="http://www.cs.wisc.edu/~bestor/bestor.html">Gareth S. Bestor</A> (<!WA3><!WA3><!WA3><A HREF="mailto:bestor@cs.wisc.edu">bestor@cs.wisc.edu</A>). Last modified September 19, 1996.</H5></ADDRESS></HTML>⌨️ 快捷键说明
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