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Date: Wed, 11 Dec 1996 22:34:20 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Fri, 27 Sep 1996 18:31:11 GMTContent-length: 14558<HTML><HEAD><TITLE>CS 302 Section 70 Lecture Notes - Week 4</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 4</H2><HR><DL> <DT>Topic: <DD>User-defined functions and subroutines. Input and output arguments.<P> <DT>Text: <DD>Chp. 6.1 - 6.6, 6.8<P> <DT>Notes: <DD> <HR> <CENTER> <H3>User-Defined Functions</H3> </CENTER> <UL> <LI>FORTRAN has <EM>pre-defined</EM> or <EM>built-in</EM> mathematical functions. <LISTING> X = SQRT(Y) * SIN(X*Y) + NINT(3.2) </LISTING> <LI>Functions are small independent program modules that compute commonly used values. Highly reusable. <LISTING> +----------+ In | | Out Y=9.0 ----> | SQRT(Y) | ----> 3.0 | | +----------+ </LISTING> <LI>Functions take one or more input values called <EM>arguments</EM>, to produce a single output value called the <EM>result</EM>. <LISTING> Y = COS(X) C = MOD(A, B) Function Arguments Result -------- --------- ------ COS X cosine of X MOD A, B remainder of A/B </LISTING> <LI>Functions simplify or break-down a larger problem into a series of smaller problems, called <EM>top-down design</EM>. </UL> <H4>User-Defined Functions</H4> <UL> <LI>Can define your own functions and use them in your program. <LI>Resemble <EM>mini-programs</EM>. Very similar structure. <LISTING> Function Header Argument Declarations Local Variable Declarations Main Executable Section RETURN END </LISTING> Example: <EM>compute the area of a circle</EM>. <LISTING> REAL FUNCTION AREAC (RADIUS) C Computes the area of a circle with C a radius of RADIUS. C Declare RADIUS argument REAL RADIUS C Declare local variables/constants REAL PI PARAMETER (PI = 3.14159) C Compute result AREAC = PI * (RADIUS ** 2) RETURN END </LISTING> </UL> <H4>Calling User-Defined Functions</H4> <UL> <LI>User-defined functions are written after the main program. <LISTING> PROGRAM TEST C ------------------------------------ C Main program C ------------------------------------ REAL TWRAD, TWAREA, AREAC PRINT *, 'Enter radius of tower' READ *, TWRAD PRINT *, 'Area is ', AREAC(TWRAD) STOP END C ------------------------------------ C User-defined Functions C ------------------------------------ REAL FUNCTION AREAC (RADIUS) ... RETURN END </LISTING> <LI>User-defined functions are <EM>called</EM> from the main program just like any other function; i.e. part of an arithmetic expression. <LI>User-defined functions can be called from inside other functions. <STRONG>Exception</STRONG>: functions cannot call themselves (called <EM>recursion</EM>). <LI><STRONG>Important</STRONG>: Function type must be declared like a variable in the main program (or wherever it is called from). <LISTING> REAL ..., AREAC </LISTING> </UL> <H4>Function Header</H4> <UL> <LI>The function header specifies the <EM>name</EM> of the function, the <EM>type</EM> of value it returns, and the <EM>name</EM> and <EM>number</EM> of input arguments. <LISTING> functype FUNCTION funcname (arguments) REAL FUNCTION AREAC (RADIUS) INTEGER FUNCTION MOD (DIVID, DIVIS) </LISTING> <LI>Can also have zero arguments, but unusual. <LISTING> INTEGER FUNCTION FOO ( ) </LISTING> <LI>Only the argument <EM>names</EM> are listed in the function header. </UL> <H4>Argument Declarations</H4> <UL> <LI>The type of the arguments is specified immediately <EM>after</EM> the function header but <EM>before</EM> any local variables. <LI>Declare arguments just like declaring variables. <LISTING> REAL RADIUS INTEGER DIVID, DIVIS </LISTING> <LI>Also called <EM>dummy arguments</EM>. </UL> <H4>Actual Arguments</H4> <UL> <LI>The value of the dummy arguments are initialized to the corresponding values in the function call, called the <EM>actual arguments</EM>. <LISTING> ... Y = 10 X = AREAC(Y) PRINT *, AREAC(3 * Y + 3) STOP END REAL FUNCTION AREAC (RADIUS) ... RETURN END Actual Argument Dummy Argument --------------- -------------- Y RADIUS = 10 3 * Y + 3 RADIUS = 33 </LISTING> <LI>Number and type of actual arguments must correspond to number and type of dummy arguments. Specified in same order. <LI>Actual arguments may be variables, literals or expressions. </UL> <H4>Local Variables</H4> <UL> <LI>As with the main program, user-defined functions may need to store intermediate results in variables. <LI>Variables declared within a function are called <EM>local variables</EM> because they can only be used locally within the function. <LI><STRONG>Important</STRONG>: Variables declared in another function or in the main program cannot be used within a function! <LI>Do not re-assign argument variables within a function (called <EM>side-effects</EM>). <LISTING> INTEGER FUNCTION FACT (N) INTEGER N INTEGER COUNT, TEMP </LISTING> Right: <LISTING> TEMP = 1 DO COUNT = 1, N TEMP = TEMP * COUNT END DO FACT = TEMP </LISTING> Wrong: <LISTING> DO COUNT = 1, N-1 N = N * COUNT END DO FACT = N </LISTING> </UL> <H4>Function Result</H4> <UL> <LI>The purpose of a function is to compute and return a <EM>result</EM>. <LI>The result of a function is the last value assigned to the function name using a normal assignment statement. <LISTING> funcname = value INTEGER FUNCTION FACT (N) ... FACT = TEMP RETURN END </LISTING> </UL> <H4><TT>RETURN</TT> Statement</H4> <UL> <LI>The <TT>RETURN</TT> statement exits the function and resumes execution in the main program (or wherever it was called from). <LI>Normally the last statement in a function before the <TT>END</TT>. </UL> <H4><TT>END</TT> Statement</H4> <UL> <LI>Always the last statement in a function. <LI>Specifies the end of the function definition. </UL> <H4>Order of Execution</H4> <UL> <LI>Program executes all the statements from program header to <TT>END</TT>. <LISTING> PROGRAM FOO | ... | END V </LISTING> <LI><TT>IF</TT> statement branches to execute different sections of code. <LISTING> IF (BAR .NE. 0) THEN | ... .---o---. ELSE | | ... --> <-- END IF | V </LISTING> <LI><TT>DO</TT> loop goes back and re-executes code. <LISTING> DO BAR = 1, 10 | ... + <-. END DO | | +--- | V </LISTING> <LI>When call a function, jumps down and execute all the function statements. Return back to the main program on <TT>RETURN</TT>. <LISTING> PRINT *, FACT(Y) Main Function STOP | ...> | END | . | * ...... | REAL FUNCTION FACT (N) | <..... | ... | . | RETURN V .... V </LISTING> </UL> <HR> <CENTER> <H3>Subroutines</H3> </CENTER> <UL> <LI><EM>Functions</EM> return a single value, usually a number, and implement some mathematical function. <LISTING> INTEGER FUNCTION FACT (N) C Computes the factorial of N </LISTING> <LI>Subroutines can return any number of values and can perform any sort of operation. <LISTING> SUBROUTINE GRAPH (MIN, MAX, POWER) C Print a graph of Y = X^POWER from X=MIN to X=MAX </LISTING> <LI>Subroutines also take one or more input values (i.e. arguments) but may or may not return any results. <LISTING> +----------+ MIN=0 In | | MAX=10 ----> | GRAPH | POWER=2 | | +----------+ </LISTING> </UL> <H4>User-Defined Subroutines</H4> <UL> <LI>Subroutines resemble functions. <LISTING> Subroutine Header Argument Declarations Local Variable Declarations Main Executable Section RETURN END </LISTING> <LI><STRONG>Note</STRONG>: Subroutine header does not return a value. <LI>Example: <EM>split a <TT>REAL</TT> number into to its whole and fractional parts</EM>. <LISTING> SUBROUTINE BREAK (X, WHOLE, FRAC) C Break a real number into its C whole and fractional parts C Declare arguments REAL X, FRAC INTEGER WHOLE WHOLE = INT(X) FRAC = X - REAL(INT(X)) RETURN END </LISTING> </UL> <H4>Calling Subroutines</H4> <UL> <LI>Subroutines are written with functions <EM>after</EM> the main program. <LI>Subroutines are explicitly <EM>called</EM> from the main program using the <TT>CALL</TT> statement. <LISTING> CALL GRAPH(1, 10, 2) CALL BREAK(10.3, IPART, FPART) </LISTING> <LI>Subroutines can be called from inside other subroutines or functions (but no recursion). </UL> <H4>Subroutine Header</H4> <UL> <LI>The subroutine header specifies the <EM>name</EM> of the subroutine and the <EM>name</EM> and <EM>number</EM> of arguments. <LISTING> SUBROUTINE subname (arguments) SUBROUTINE GRAPH (MIN, MAX, POWER) SUBROUTINE BREAK (X, WHOLE, FRAC) </LISTING> <LI>Can also have zero arguments, not unusual. <LISTING> SUBROUTINE MENU ( ) </LISTING> </UL> <H4>Argument Declarations</H4> <UL> <LI>As with functions, the <EM>type</EM> of the arguments is specified immediately after the subroutine header. </UL> <H4>Local Variables</H4> <UL> <LI>As with functions, subroutines may need to store intermediate results in variables. <LI>Local variables can only be used <EM>locally</EM> within the subroutine. </UL> <H4>Subroutine Results</H4> <UL> <LI>Some subroutines do not return any values; e.g. <TT>GRAPH</TT> just draws a graph on the screen. <LI>Other subroutines return one or more values; e.g. <TT>BREAK</TT> takes one input value and returns two output values. <LI>A result is returned to the main program when the arguments are re-assigned inside the subroutine. Arguments pass values both into the subroutine as well as <EM>out</EM> of it. <LISTING> +----------+ MIN=0 In | | MAX=10 ----> | GRAPH | POWER=2 | | +----------+ +----------+ In | | Out WHOLE=10 X=10.3 ----> | BREAK | ----> FRAC=0.3 | | +----------+ </LISTING> <LI><STRONG>Note</STRONG>: Unlike functions, the subroutine name is <STRONG>not</STRONG> assigned a value. </UL> <H4>Input Arguments</H4> <UL> <LI><EM>Input arguments</EM> pass values <EM>into</EM> the subroutine, just like functions. <LI>Input arguments should not be re-assigned. <LI>As with functions, the <EM>actual arguments</EM> in the subroutine call may be literals, variables or expressions. <LISTING> CALL GRAPH(X-10, X+10, 2) CALL BREAK(10.3, ...) </LISTING> </UL> <H4>Output Arguments</H4> <UL> <LI><EM>Output arguments</EM> pass values back <EM>out</EM> to the main program. Similar to function results except multiple values can be passed. <LI>Unlike input arguments, output arguments <STRONG>must</STRONG> be re-assigned to a new value to pass it back out. <LI><STRONG>VERY IMPORTANT</STRONG>: The actual arguments in the subroutine call must always be <STRONG>variables</STRONG>. <LI>These variables are re-assigned inside the subroutine, hence the new values get passed back out to the main program. <LISTING> CALL BREAK(10.3, IPART, FPART) PRINT *, IPART, FPART (10 0.3) STOP END SUBROUTINE BREAK (X, WHOLE, FRAC) ... WHOLE = INT(X) FRAC = X - REAL(WHOLE) RETURN END </LISTING> <LI>When the <EM>dummy arguments</EM> <TT>WHOLE</TT> and <TT>FRAC</TT> are re-assigned, the <EM>actual arguments</EM> <TT>IPART</TT> and <TT>FPART</TT> are re-assigned too. <LI>Actual and dummy argument names do <STRONG>not</STRONG> need to have the same name. <LI>Output arguments only pass values out. The original values of <TT>IPART</TT> and <TT>FPART</TT>, if any, are ignored. </UL> <H4>Input/Output Arguments</H4> <UL> <LI>Some arguments can pass values both into and out of the subroutine. Called <EM>input/output arguments</EM>. <LISTING> SUBROUTINE SORT (NUM1, NUM2) C Sorts two numbers so that NUM1<=NUM2 C Input/Output Arguments INTEGER NUM1, NUM2 C Local Variables INTEGER TEMP C Sort the numbers IF (NUM1 .GT. NUM2) THEN TEMP = NUM1 NUM1 = NUM2 NUM2 = TEMP END IF RETURN END Input arguments: NUM1, NUM2 Output arguments: NUM1, NUM2 </LISTING> </UL></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 27, 1996.</H5></ADDRESS></HTML>⌨️ 快捷键说明
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