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Text File | 1986-02-07 | 16.3 KB | 342 lines

.NT A NOTE ABOUT THE LESSONS in C .b4-24R5C4 These were written while the author was ~Ilearning~N the language and since .R6C4 they are ~Ifree~N ( to copy and/or distribute ) there is a money-back .R7C4 guarantee on the accuracy of each and every statement in the lessons (!) .R9C4 The ~Idisplay~N program was written ( in C ) in order to provide a vehicle .R10C4 for displaying the lessons. .R12C5 .B P.J.Ponzo .B Dept. of Applied Math .B Univ. of Waterloo .B Ontario N2L 3G1 .K16,32 PonzoTUTOR .WNT More FUNCTIONS, including main() You may recall, in a previous lesson, that we passed ~b~Ifloat~N arguments ~b~Ix~N and ~b~Iy~N to a function ( which we called ~b~Iaverage(x,y)~N ) which returned the average: (x+y)/2. It was something like: .WN 1 ~b~Imain() { ~N 2 ~b~I float x,y,a; ~N 3 ~b~I printf("\n Enter two numbers (separated by a space) : "); ~N 4 ~b~I scanf("%f%f",&x,&y); ~N 5 ~b~I a=average(x,y); ~N 6 ~b~I printf("\n The average of %f and %f is %f",x,y,a); ~N 7 ~b~I} ~N 8 ~b~Ifloat average (a,b); ~N 9 ~b~Ifloat a, b; ~N 10 ~b~I{ ~N 11 ~b~I float z; ~N 12 ~b~I z=(a+b)/2; ~N 13 ~b~I return(z); ~N 14 ~b~I} ~N The function ~b~Iaverage(a,b)~N was invoked in line 5, where we passed it ~Icopies~N of the variables ~b~Ix~N and ~b~Iy~N. The function itself starts in line 8 and it makes no difference what name the function gives to its arguments ..they are, after all, only private copies of the two variables which ~b~Imain()~N uses. Whatever ~b~Iaverage~N does with its copies, it does NOT effect the ~Ioriginal~N ~b~Ix~N and ~b~Iy~N. .WR15C1 .R15C1 We could have simplified the function by replacing lines 11, 12, 13 by: ~b~I return( (a+b)/2 ); ~N Anyway, we mentioned that ~b~Iprintf()~N, ~b~Iscanf()~N, ~b~Igetchar()~N and ~b~Imain()~N were ALL functions. Whereas WE write the functions ~b~Imain()~N and (in this example) ~b~Iaverage()~N, the others are a ~Istandard~N part of the ~IC-library~N. .WN Some C-compilers automatically give your ~b~Imain()~N program access to the ~Ist~Nan~Id~Nard ~Ii~Nnput ~Io~Nutput~N ( ~Istdio~N ) routines such as ~b~Iprintf()~N, etc. BUT, SOME DO NOT. To be sure that you include these ~Istdio~N functions in your ~b~Imain()~N program (assuming you will be using them) you should begin your program... ~b~I#include <stdio.h>~N ~IDON'T FORGET TO BEGIN WITH THIS !!~N ~b~Imain() { ~N ~b~I ........ ~N ~b~I ........ ~N ~b~I} ~N When the C-compiler sees the ~b~I#include <stdio.h>~N it will append to your ~b~Imain()~N program all the ~Iio~N functions in the C library. .WN .T don't forget STDIO.H .WN Now, if ~b~Iprintf()~N and ~b~Iscanf()~N and ~b~Iaverage()~N etc. can be passed arguments, why not ~b~Imain()~N ??? As a matter of fact, we CAN pass arguments to ~b~Imain()~N. Suppose your ~b~Imain()~N program was expected to add a series of numbers and that you compiled/linked it with the name ~Isum~N. Now you could run the ~Isum~N program by typing its name, ~Isum~N, after which a ~b~Iscanf()~N function call (in the program) would input the numbers which were to be added. Then your program would ~b~Iprintf()~N the sum. BUT, wouldn't it be nice to run the ~Isum~N program by typing: ~Isum 10.5 -15.23 6.7~N and have the ~Isum~N program run, and also have the numbers ~I10.5 -15.23~N and ~I6.7~N passed to the program as arguments ? .K19,32 main(?,?) We'll talk about how to do this in C.... .WK19,60 wunderbar! .WNT The main() arguments .R4C1 ~V0 #include <stdio.h> ~N 1 ~b~Imain(number,term) ~N 2 ~b~Iint number; /* first main() argument */ ~N 3 ~b~Ichar *term[]; /* second main() argument */ ~N 4 ~b~I{ /* NOW start main() */ ~N 5 ~b~I ........... ~N 6 ~b~I ........... /* program goes here */ ~N 7 ~b~I} /* end of main() */ ~N .R15C1 Here we ~V#include~N the stdio.h library (we'll need this stuff). .WR4C1 0 ~b~I#include <stdio.h> ~N ~V1 main(number,term) ~N .R15C1 .R15C1 NOW our main program expects two arguments called ~b~Inumber~N and ~b~Iterm~N. .WR5C1 1 ~b~Imain(number,term) ~N ~V2 int number; /* first main() argument */ ~N ~V3 char *term[]; /* second main() argument */ ~N .R15C1 .R15C1 The first argument (here called ~b~Inumber~N) is ALWAYS an ~b~Iint~Neger! The second argument (here called ~b~Iterm~N) is ALWAYS declared as: ~b~Ichar *name[]~N; name of second argument goes in here (in our example, it's ~b~Iterm~N). .WR6C1 2 ~b~Iint number; /* first main() argument */ ~N .R15C1 .R15C1 BECAUSE it is declared ~b~Ichar *term~N we see that ~b~Iterm~N is a ~r~Ipointer~N! BECAUSE we refer to it as ~b~Iterm[]~N we see that ~b~Iterm~N is an ~IARRAY~N! In fact, the second argument is ALWAYS an ARRAY of POINTERS which point to ~b~Ichar~Nacter strings (that's why we said ~b~Ichar~N). .WR15C1 .R15C1 If we called upon our (compiled/linked) program with the command string: ~Isum 10.5 -15.23 6.7~N then this contains ~I4~N elements, namely: ~Vsum~N and ~V10.5~N and ~V-15.23~N and ~V6.7~N (separated by a space). It is the number ~I4~N which gets passed to ~b~Imain()~N as its first argument! ( ..in our example, ~b~Inumber~N is the ~b~Iint~Neger ~I4~N). .WR15C1 .R15C1 And the second argument? The second argument which gets passed to ~b~Imain()~N is (are?) the strings: ~Vsum~N and ~V10.5~N and ~V-15.23~N and ~V6.7~N BUT, ~b~Imain()~N receives this list of strings as an ARRAY of POINTERS. term[0] points to the string ~Vsum~N term[1] points to the string ~V10.5~N term[2] points to the string ~V-15.23~N term[3] points to the string ~V6.7~N .WR15C1 .R15C1 NOTE: when we refer to the ~Istring -15.23~N we mean a collection of 7 characters: '-' and '1' and '5' and '.' and '2' and '3' and '\0'. (remember the '\0' which terminates strings?) We may pick out the numbers ~I10.5~N and ~I-15.23~N etc. by referring to ~b~Iterm[1]~N and ~b~Iterm[2]~N etc. Of course, they are NOT (really) numbers (?!@#$) but strings of ASCII characters. To add them up, we must convert them to ~b~Ifloat~Ning point numbers.... .WK6,32 GO!GO!GO! .WN .T Some SUM programming ... .WN 0 ~b~I#include <stdio.h> ~N 1 ~b~Imain(number,term) ~N 2 ~b~Iint number; /* first main() argument */ ~N 3 ~b~Ichar *term[]; /* second main() argument */ ~N 4 ~b~I{ /* NOW start main() */ ~N 5 ~b~I float next, sum=0; /* couple of floats */ ~N 6 ~b~I int n; . /* an int to count terms */ ~N 7 ~b~I for (n=1; n<number; n++); { /* loop thru' terms */ ~N 8 ~b~I next=atof(term[n]); /* CONVERT next term */ ~N 9 ~b~I sum=sum+next; /* add it to sum */ ~N 10 ~b~I printf("\n %8.3f",next); /* print each term */ ~N 11 ~b~I } /* end of for-loop */ ~N 12 ~b~I printf("\nSUM=%8.3f,sum); /* print the sum */ ~N 13 ~b~I} /* end of main() */ ~N .R8C1 ~V7 for (n=1; n<number; n++); { /* loop thru' terms */ ~N .R15C1 Here we begin our loop, which sums the terms. Beginning with ~b~Iterm[1]~N (the first number) we let ~b~In~N cycle thru' the terms (incrementing each time, with ~b~In++~N). .WR8C1 7 ~b~I for (n=1; n<number; n++); { /* loop thru' terms */ ~N ~V8 next=atof(term[n]); /* CONVERT next term */ ~N .R15C1 .R15C1 Here's our conversion of the ASCII string (one for each number) to a ~b~Ifloat~N. (This ~Ia~Nscii ~Ito~N ~If~Nloat function is part of any respectable C-library ...which is one reason why we #included stdio.h). Note that we give to ~b~Iatof()~N the ~r~Ipointer~N to the ASCII string, namely ~b~Iterm[n]~N (for each value of ~b~In~N in our for-loop). .WR9C1 8 ~b~I next=atof(term[n]); /* CONVERT next term */ ~N ~V9 sum=sum+next; /* add it to sum */ ~N .R15C1 .R15C1 Here we add the ~b~Ifloat~N ( generated in Line 8 by ~b~Iatof()~N ) to our ~b~Isum~N ( which was initialized to ~b~I0~N in Line 5 ). .WR10C1 9 ~b~I sum=sum+next; /* add it to sum */ ~N ~V10 printf("\n %8.3f",next); /* print each term */ ~N .R15C1 .R15C1 Just to prove that ~b~Iatof()~N is doing its job, we'll print each ~b~Ifloat~N which it generates ...each on a ~b~I\n~Newline. .WR11C1 10 ~b~I printf("\n %8.3f",next); /* print each term */ ~N ~V11 } /* end of for-loop */ ~N ~V12 printf("\nSUM=%8.3f,sum); /* print the sum */ ~N .R15C1 .R15C1 We end the for-loop, after which we ~b~Iprintf()~N our ~b~Isum~N (using ~I8~N positions on the screen, and printing to ~I3~N decimal places). .WR12C1 11 ~b~I } /* end of for-loop */ ~N 12 ~b~I printf("\nSUM=%8.3f,sum); /* print the sum */ ~N ~V13 } /* end of main() */ ~N .R15C1 .R15C1 Finally, we come to the end of ~b~Imain()~N. .K19,32 magnifique .WN 0 ~b~I#include <stdio.h> ~N 1 ~b~Imain(number,term) ~N 2 ~b~Iint number; /* first main() argument */ ~N 3 ~b~Ichar *term[]; /* second main() argument */ ~N 4 ~b~I{ /* NOW start main() */ ~N 5 ~b~I float next, sum=0; /* couple of floats */ ~N 6 ~b~I int n; . /* an int to count terms */ ~N 7 ~b~I for (n=1; n<number; n++); { /* loop thru' terms */ ~N 8 ~b~I next=atof(term[n]); /* CONVERT next term */ ~N 9 ~b~I sum=sum+next; /* add it to sum */ ~N 10 ~b~I printf("\n %8.3f",next); /* print each term */ ~N 11 ~b~I } /* end of for-loop */ ~N 12 ~b~I printf("\nSUM=%8.3f,sum); /* print the sum */ ~N 13 ~b~I} /* end of main() */ ~N In response to ~Isum 10.5 -15.23 6.7~N, our program will print: ~r~I 10.500~N Note: Each term is printed after 4 spaces, then ~r~I -15.230~N occupies a field width of ~I8~N and is ~r~I 6.700~N right-justified! ~r~ISUM= 1.970~N Note: 4 spaces was just enough for:~r~ISUM=~N. .K19,62 looks nice .WN .R10C1 Although ( in our program ) ~b~Iterm[0]~N is supposed to point to the ~Iname~N of the program ( in our example it was ~Isum~N ) some C-compilers do NOT provide this! In particular, MS-DOS does NOT pass the ~Iname~N to the compiled program, so C-compilers running under MS-DOS do not (CANNOT) implement this feature of the C language. Instead, reference to this ~Izero~Nth string may yield garbage or a blank (NULL) string or some home-made string! .b8-17 .K19,32 noname? .WN .T That's all folks! .K16,32 au revoir! .q