📄 http:^^www.cs.wisc.edu^~bestor^cs110^week4.html
字号:
Date: Wed, 13 Nov 1996 23:16:31 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Thu, 18 Apr 1996 03:22:58 GMTContent-length: 9687<HTML><HEAD><TITLE>CS 110 Section 2 Lecture Notes - Week 4</TITLE></HEAD><BODY><H2><!WA0><!WA0><!WA0><A HREF="http://www.cs.wisc.edu/~bestor/cs302/cs302.html#text" ><!WA1><!WA1><!WA1><IMG SRC="http://www.cs.wisc.edu/~bestor/icons/arrowleft.gif" WIDTH=15 HEIGHT=15></A> Lecture Notes - Week 4</H2><HR><DL> <DT>Topic: <DD>Repetition and loops. <TT>DO/END DO</TT>, <TT>DO WHILE/END DO</TT> and <TT>DO/CONTINUE</TT> statement.<P> <DT>Text: <DD>Chp. 4.1 - 4.4, 4.6, 4.7, 4.9, 4.10<P> <DT>Notes: <DD> <HR> <CENTER> <H3>Repetition and Loops</H3> </CENTER> <UL> <LI><EM>Conditional execution</EM> allows you to optionally execute <STRONG>different</STRONG> sections of code. <LI><EM>Repetition</EM> or <EM>looping</EM> allows you to <STRONG>re-execute</STRONG> the same section of code several times. <LI>Example: Add up 50 numbers entered one per line. <LISTING> SUM = 0 READ *, NUM SUM = SUM + NUM READ *, NUM SUM = SUM + NUM . . . READ *, NUM SUM = SUM + NUM PRINT *, 'The total sum is', SUM </LISTING> or more simply <LISTING> SUM = 0 [re-execute the following 50 times:] READ *, NUM SUM = SUM + NUM PRINT *, 'The total sum is', SUM </LISTING> <LI>Repetition is more <EM>efficient</EM> and <EM>flexible</EM>. </UL> <H4>DO/CONTINUE Statement (FORTRAN-77)</H4> <UL> <LI>To re-execute one or more statements use the <TT>DO/CONTINUE</TT> statement. <LISTING> DO label count = start, end, inc statements to re-execute label CONTINUE<BR> 12345678901234567890123456780 ----------------------------- DO 10 CTR = 1, 50, 1 READ *, NUM SUM = SUM + NUM 10 CONTINUE </LISTING> <LI><STRONG>inc</STRONG> defaults to 1 if not specified. <LISTING> DO 10 CTR = 1, 50 </LISTING> <LI>Use a negative value for <STRONG>inc</STRONG> to count down. <LISTING> DO 10 CTR = 50, 1, -1 </LISTING> <LI><STRONG>count</STRONG> must be an <TT>INTEGER</TT> or <TT>REAL</TT> variable. </UL> <H4>How a DO Loops Work</H4> <UL> <LI><STRONG>count</STRONG> is initialized to start before the first iteration. After each iteration <STRONG>count</STRONG> is incremented by <STRONG>inc</STRONG>. The loop terminates when <STRONG>count</STRONG> > <STRONG>end</STRONG> (Note: not >=). <LI>The <STRONG>start</STRONG>, <STRONG>end</STRONG> and <STRONG>inc</STRONG> may be literals, variables or expressions. <LISTING> MAX = 50 DO 10 CTR = 1, MAX, SQRT(9)-2 </LISTING> <LI>Normally use <TT>INTEGER</TT> values for <STRONG>start</STRONG>, <STRONG>end</STRONG> and <STRONG>inc</STRONG>. <LISTING> end - start + inc Number of iterations = ----------------- inc </LISTING> <LI>If <STRONG>start</STRONG> > <STRONG>end</STRONG> the loop isn't executed at all! <LISTING> MIN = 55 DO 10 CTR = MIN, 50, 1 </LISTING> <LI>What if <STRONG>start</STRONG>, <STRONG>end</STRONG> or <STRONG>inc</STRONG> are changed inside the loop? <LISTING> MIN = 1 MAX = 50 DO 10 CTR = MIN, MAX MAX = CTR + 5 PRINT *, 'spam' 10 CONTINUE </LISTING> <LI>The <STRONG>start</STRONG>, <STRONG>end</STRONG> or <STRONG>inc</STRONG> are evaluated only once at the beginning. Only their <EM>initial</EM> values are important. </UL> <H4>Labels</H4> <UL> <LI>The <STRONG>label</STRONG> is the line number of the <TT>CONTINUE</TT> statement to indicate the end of the loop. Every statement up to the <TT>CONTINUE</TT> is re-executed. <LISTING> DO 10 CTR = 1, 50 ... 10 CONTINUE </LISTING> <LI>The <STRONG>label</STRONG> is written in columns 2 though 5. The <TT>DO</TT> and <TT>CONTINUE</TT> statements start in column 7. <LI>If you have multiple <TT>DO</TT> loops then keep the labels in ascending order and use multiples of 10 for the different numbers. <LISTING> DO 10 FOO = 1, 50 ... 10 CONTINUE DO 20 BAR = 1, 10 ... 20 CONTINUE </LISTING> </UL> <HR> <CENTER> <H3>Conditional Loops</H3> </CENTER> <UL> <LI><TT>DO</TT> statement re-executes the same statements a pre-determined number of times. <LI>What if don't know the number of iterations ahead of time? <LISTING> SUM = 0 READ *, NUM [do the following until NUM equals 99] SUM = SUM + NUM READ *, NUM PRINT *, 'The total sum is', SUM </LISTING> </UL> <H4>DO WHILE Statement (FORTRAN-90)</H4> <UL> <LI>Tests a condition on each iteration. If the condition is <TT>.TRUE.</TT> then re-execute the loop. <LISTING> DO WHILE (condition) statements to re-execute END DO<BR><BR> DO WHILE (NUM .NE. 99) SUM = SUM + NUM READ *, NUM END DO </LISTING> <LI>The <STRONG>condition</STRONG> is a <EM>logical expression</EM>, just like an <TT>IF</TT> statement. <LI>Can re-write a <TT>DO/CONTINUE</TT> loop as a <TT>DO WHILE</TT> loop. <LISTING> DO 10 COUNT = MIN, MAX, 2 PRINT *, COUNT 10 CONTINUE </LISTING> is the functionally the same as <LISTING> COUNT = MIN DO WHILE (COUNT .LE. MAX) PRINT *, COUNT COUNT = COUNT + 2 END DO </LISTING> <LI>The <STRONG>condition</STRONG> must contain a variable who's value is changed inside the loop. Otherwise the <STRONG>condition</STRONG> always remains <TT>.TRUE.</TT> (i.e. an <EM>infinite loop</EM>) <LI>Right: <LISTING> DO WHILE (NUM .NE. 0) PRINT *, NUM READ *, NUM END DO </LISTING> <LI>Wrong: <LISTING> DO WHILE (NUM .NE. 0) PRINT *, NUM SUM = SUM + NUM END DO </LISTING> </UL> <H4>FORTRAN-77 "DO WHILE" Loop</H4> <UL> <LI>The <TT>DO WHILE</TT> statement is not standard FORTRAN-77 and may not be available on all compilers. But sometimes we cannot use the FORTRAN-77 <TT>DO/CONTINUE</TT> loop. <LI>To implement the equivalent of a <TT>DO WHILE</TT> loop in FORTRAN-77 we can use an <TT>IF</TT> statement and a <TT>GOTO</TT> statement. <LISTING> DO WHILE (NUM .NE. 0) ... END DO </LISTING> can be written as <LISTING> 10 IF (NUM .NE. 0) THEN ... GOTO 100 END IF </LISTING> <LI>The <TT>GOTO</TT> statement "jumps" to resume executing the statement at the specified label. <LI><STRONG>WARNING</STRONG>: <STRONG>Never</STRONG> use <TT>GOTO</TT> statements anywhere in your program <STRONG>except</STRONG> if you have a strict FORTRAN-77 compiler and then <STRONG>only</STRONG> to implement <TT>DO WHILE</TT> loops. <LI><STRONG>SECOND WARNING</STRONG>: Anyone caught using <TT>GOTO</TT> statements in this class will automatically receive a mark of <STRONG>zero</STRONG> for that assignment. Just say <STRONG>NO</STRONG> to <TT>GOTO</TT>s! </UL> <H4>Nested Loops</H4> <UL> <LI>Just as <TT>IF</TT> statements can be nested inside each other, so can <TT>DO/CONTINUE</TT> and <TT>DO WHILE</TT> loops. <LISTING> DO 20 X = 1, 10 FACT = 1 DO 10 COUNT = 1, X FACT = FACT * COUNT 10 CONTINUE PRINT *, X, '! =', FACT 20 CONTINUE </LISTING> <LI>The outer loop is executed 10 times (X=1..10). Inner loop is executed X number of times, depending on the current value of X. <LI>Nested loops must use different counter variables. <LI>Inner loop's <TT>CONTINUE</TT> statement comes before the outer loop's <TT>CONTINUE</TT> statement. </UL> <H4>Indenting</H4> <UL> <LI>As with <TT>IF</TT> statement, indent all re-executed statements 3 additional spaces. </UL> <HR> <CENTER> <H3>FORTRAN-90 Loops</H3> </CENTER> <UL> <LI>Repitition and loops are vital for any programming languages to be useful. <LI>Standard FORTRAN-77 only has the <TT>DO/CONTINUE</TT> loop. <LI>FORTRAN-90 provides more powerful looping mechanisms (aside: the <TT>DO WHILE</TT> loop is also supported by many "non-standard" FORTRAN-77 compilers, such as Microsoft FORTRAN). </UL> <H4>DO/END DO Loop (FORTRAN-90)</H4> <UL> <LI>Same as the FORTRAN-77 <TT>DO/CONTINUE</TT> loop but without the <TT>CONTINUE</TT> statement and no label. <LI>The end of the loop is indicated by the <TT>END DO</TT> statement. <LISTING> DO 10 ADD = 1, 10 READ *, NUM SUM = SUM + NUM 10 CONTINUE </LISTING> is the same as <LISTING> DO ADD = 1, 10 READ *, NUM SUM = SUM + NUM END DO </LISTING> <LI>The <TT>DO/END DO</TT> can be nested like other loops. <LI>The <STRONG>counter</STRONG> must be an <TT>INTEGER</TT> variable. <LI><STRONG>Important</STRONG>: Always use the <TT>DO/END DO</TT> loop instead of the <TT>DO/CONTINUE</TT> loop in this class. It is also supported by Microsoft FORTRAN. </UL> <H4>Which Loop Do I Use?</H4> <UL> <LI>Use the <TT>DO/END DO</TT> (FORTRAN-90) instead of the <TT>DO/CONTINUE</TT> (FORTRAN-77) if possible. <LI>Use the <TT>DO WHILE/END DO</TT> (FORTRAN-90) instead of the <TT>IF/THEN/GOTO</TT> (FORTRAN-77) unless you have a strict FORTRAN-77 compiler. </UL></DL></BODY><HR><ADDRESS><H5>Copyright © 1996 <!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 April 17, 1996.</H5></ADDRESS></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -