Shareware Grab Bag/Shareware Grab Bag.iso/007/ctut.arc/LESSON8

Text File | 1986-02-07 | 25KB | 478 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
    FILES FILES FILES
.R4C1
    Whereas ~b~Igetchar()~N and ~b~Iprintf()~N will get characters from the
    keyboard and print to the screen (the 'standard' input/output), we can
    ask other functions available in the C-library (and ones we write
    ourselves) to communicate with a ~Ifile on disk~N. Consider the excerpt:

1  ~b~Imain(number,name)                                    ~N
2  ~b~Iint number;                                          ~N
3  ~b~Ichar *name[];                                        ~N
4  ~b~I{                                                    ~N
5  ~b~I    FILE *fp, *fopen();                              ~N
6  ~b~I    fp=fopen(name[1],"r");                           ~N
7  ~b~I    if (fp == NULL) {                                ~N
8  ~b~I        printf("\nSorry, can't open %s",name[1]);    ~N
9  ~b~I        exit(0);                                     ~N
10 ~b~I    }                                                ~N
11 ~b~I    /*  rest of program goes here */                 ~N
.w
    The above program is expected to read a file on disk. We compile/link
    the program, giving it the name ~Iread~N. Then we type: ~Iread letter~N.
    The arguments to ~b~Imain()~N are the ~b~Inumber~N ~I2~N and the ARRAY
    of POINTERS ( ~b~Iname[]~N ) pointing to the strings: ~Iread~N and ~Iletter~N.
.WR10C1
~V2  int number;                                          ~N
~V3  char *name[];                                        ~N
.R20C1




.R20C1
    We declare the two arguments of ~b~Imain()~N to be ~b~Iint~N and ~b~Ichar *~N.
    In our example, ~b~Inumber~N will have the value ~I2~N and ~b~Iname[0]~N will
    be a pointer to the string ~Iread~N and ~b~Iname[1]~N will point to ~Iletter~N.
    Now we are ready to begin ~b~I{~N our ~b~Imain()~N program ....
.WR10C1
2  ~b~Iint number;                                          ~N
3  ~b~Ichar *name[];                                        ~N
4  ~b~I{                                                    ~N
~V5      FILE *fp, *fopen();                              ~N
.R20C1




.R20C1
    To ~Iopen~N a disk file, we use ~b~IFILE~N which calls upon the C-library
    to look after the mechanics of communicating with the operating system in
    order to access the disk. After ~b~IFILE~N come two ~r~Ipointers~N (we
    know they're pointers because of the ~b~I*~N ..right?).
.WR20C1




.R20C1
  Of course, if we intend to use the C-library, we had better ~V#include~N it!
.WK10,60
 stdio.h!
.R8C1
~V0  #include <stdio.h>                                   ~N
.WR20C1
    ~b~Ifp~N is a ~r~Ipointer~N variable which points to a FILE.
    ~b~Ifopen()~N is a function which ~Ireturns~N a ~r~Ipointer~N to a FILE.
.WR13C1
5  ~b~I    FILE *fp, *fopen();                              ~N
~V6      fp=fopen(name[1],"r");                           ~N
.R20C1


.R20C1
    This calls upon the ~b~Ifopen()~N function to ~Iopen~N a file. The pointer
    to the file ( which is ~Ireturn~Ned by ~b~Ifopen()~N ) is assigned to ~b~Ifp~N.
    The ~b~I"r"~N means that access to the file is for ~I"r"~Neading ( as
    opposed to "w"riting or "a"ppending ).
.WR20C1




.R20C1
    How do we tell ~b~Ifopen()~N the name of the file which we want to open...?
    ~b~Iname[]~N is an ARRAY of pointers, each pointing to a string.
.WR20C1


.R20C1
    When we typed ~Iread letter~N, we passed to our ~b~Imain()~N program the
    two strings ~Iread~N and ~Iletter~N. Now the pointer ~b~Iname[0]~N points to
    ~Iread~N and ~b~Iname[1]~N will point to the string ~Iletter~N ...
.WR20C1



.R20C1
    ...so we give the name of the file to ~b~Ifopen()~N, namely ~b~Iname[1]~N.
.K10,60
of course
.WR14C1
6  ~b~I    fp=fopen(name[1],"r");                           ~N
~V7      if (fp == NULL) {                                ~N
~V8          printf("\nSorry, can't open %s",name[1]);    ~N
~V9          exit(0);                                     ~N
.R20C1




.R20C1
    The function ~b~Ifopen()~N is clever. If it can't open the file for some
    reason (beer-on-the-disk) it ~Ireturn~Ns a NULL pointer.
    We check, in Line 7, IF ~b~Ifp~N is EQUAL ( ~b~I==~N ) to ~b~INULL~N.
    If so, we apologize ( Line 8 ) and ~b~Iexit~N the program ( Line 9 ).
.WR15C1
7  ~b~I    if (fp == NULL) {                                ~N
8  ~b~I        printf("\nSorry, can't open %s",~Vname[1]~N~b~I);    ~N
9  ~b~I        ~Vexit(0)~N~b~I;                                     ~N
.R20C1




.R20C1
    In Line 8 we ~b~Iprintf~N ~b~Iname[1]~N as a ~b~I%s~Ntring (of course).
    The ~b~Iexit(0)~N (in Line 9 ) is new. It will exit our ~b~Imain()~N
    and return you to the operating system (e.g. DOS ). ( It also ~Icloses~N
    any open files ).
.WR20C1




.R20C1
    NOTE: We gave to ~b~Ifopen()~N the string which ~b~Iname[1]~N points to,
          and we also give it to ~b~Iprintf()~N (namely ~Iletter~N, in this
          example).
.WNK10,32
  phew!!
.WN
1  ~b~Imain(number,name)                                    ~N
2  ~b~Iint number;                                          ~N
3  ~b~Ichar *name[]                                         ~N
4  ~b~I{                                                    ~N
5  ~b~I    FILE *fp, *fopen();                              ~N
6  ~b~I    fp=fopen(*name[1],"r");                          ~N
7  ~b~I    if (fp == NULL) {                                ~N
8  ~b~I        printf("\nSorry, can't open %f",name[1]);    ~N
9  ~b~I        exit(0);                                     ~N
10 ~b~I    }                                                ~N
11 ~b~I    /*  now we read from the file */                 ~N
~V12     char c;                                          ~N
~V13     while ( (c=getc(fp)) != EOF  )                   ~N
~V14         printf("%c",c);                              ~N
~V15 }                                                    ~N
.w

    Now we declare a ~b~Ichar c~N ( Line 12 ) and continually ~b~Igetc()~N
    ~Ifrom the file which we opened~N ( in Line 6 ). Reference to this open
    file is via its ~Ifile pointer~N ~b~Ifp~N ... hence we used ~b~Igetc(~Ffp~N~b~I)~N
    to tell the ~b~Igetc()~N function which file to ~b~Iget~N the next
    ~b~Ic~Nhar from. ( ~b~Igetc()~N is in ~b~Istdio.h~N ).
.WR17C1





.R17C1
    We (in this example) ~b~Iprintf()~N the ~%c~Nhar to the screen, ~b~Iwhile~N
    ~b~Ic~N is NOT EQUAL ( ~b~I!=~N ) to the ~b~IEOF~N  character (which ~Ishould~N
    indicate ~IE~Nnd ~IO~Nf ~IF~Nile ..if all goes well ..).
.WR17C1





.R17C1
.Q
How many errors in the above program ?
4
.R21C1
    Line 3 needs a ~Isemi-colon~N!
.R3C1
3  ~b~Ichar *name[] ~Ftch! tch!~N~b~I~N
.WR21C1
    Line 6 should give ~b~Iname[1]~N to ~b~Ifopen()~N, NOT ~b~I*name[1]~N
.R6C1
6  ~b~I    fp=fopen(~F*~N~b~Iname[1],"r");                          ~N
.WR21C1
    Line 8 needs a ~b~I%s~Ntring format, NOT a ~b~I%f~Nloat !
.R8C1
8  ~b~I        printf("\nSorry, can't open %~Ff~N~b~I",name[1]);    ~N
.WR21C1
    ....and we need to ~b~I#include <stdio.h>~N
.K10,60
as Line 0
.WNK10,32
funtastic!
.WN
0  ~b~i#include <stdio.h>                                   ~N
1  ~b~Imain(number,name)                                    ~N
2  ~b~Iint number;                                          ~N
3  ~b~Ichar *name[];                                        ~N
4  ~b~I{                                                    ~N
5  ~b~I    FILE *fp, *fopen();                              ~N
6  ~b~I    fp=fopen(name[1],"r");                           ~N
7  ~b~I    if (fp == NULL) {                                ~N
8  ~b~I        printf("\nSorry, can't open %s",name[1]);    ~N
9  ~b~I        exit(0);                                     ~N
10 ~b~I    }                                                ~N

    Let's begin with this introduction ( ...same as before, but with ~Cbugs~N
    removed!) and continue with a program which will read a file on disk
    and count the number of times the letters 'a' through 'z' occur.

    We'll compile/link the program under the name ~Icount~N and we will
    type ~Icount sam~N to count the letters 'a'-'z' in a file called ~Isam~N.
.K19,32
how nice!
.WN




.T
    counting from 'a to 'z'
.WN
~Vchar c;                                         /* characters from file */~N
~Vint n[26], i;                                   /* count for each letter*/~N
~b~Ifor (i=0;i<26;i++) n[i]=0;                      /* counts set to zero   */~N
~b~Ifor (i=0;i<1000;i++)  {                         /* go thru 1000 chars   */~N
~b~I    c=getc(fp);                                 /* get a char from file */~N
~b~I    if (c>='a'&& c<='z')  {                     /* is it between a & z? */~N
~b~I         n[c-'a']++;                            /* yes? increment count */~N
~b~I    }                                           /* end of if            */~N
~b~I}                                               /* end of for           */~N
~b~Iprintf("\nIn the file %s : ",name[1]);          /* print file name      */~N
~b~Ifor (i=0;i<=24;i+=2)                            /* go thru alphabet     */~N
~b~I     printf("\n%c%s%3d%s : %c%s%3d%s",          /* a format string      */~N
~b~I            'a'+i,  " occurs ",n[i],  " times", /* print letter and     */~N
~b~I            'a'+i+1," occurs ",n[i+1]," times");/* count for each letter*/~N
~b~I}                                               /* end of main()        */~N
.R17C1
    Here we declare some stuff, notably the ARRAY ~b~In[26]~N which will hold
    the number of times each of the 26 letters from 'a' to 'z' occur ...hence
    it's an ~b~Iint~Neger ARRAY.
.WR1C1
~b~Ichar c;                                         /* characters from file */~N
~b~Iint n[26], i;                                   /* count for each letter*/~N
~Vfor (i=0;i<26;i++) n[i]=0;                      /* counts set to zero   */~N
.R17C1



.R17C1
    Here we set all the integers in the ARRAY to 0  ..you can never tell what
    garbage might be in those memory locations!
.WR3C1
~b~Ifor (i=0;i<26;i++) n[i]=0;                      /* counts set to zero   */~N
~Vfor (i=0;i<1000;i++)  {                         /* go thru 1000 chars   */~N
~V    c=getc(fp);                                 /* get a char from file */~N
~V    if (c>='a'&& c<='z')  {                     /* is it between a & z? */~N
.R17C1


.R17C1
    Now we go through 1000 characters in the file and ~b~Igetc(fp)~N each one,
    giving the file ~r~Ipointer~N ~b~Ifp~N to the library function ~b~Igetc()~N
    and getting in ~Ireturn~N a ~b~Ichar~Nacter ~b~Ic~N which we check to see
    if it's GREATER or EQUAL to the character ~b~I'a'~N  AND  ( ~b~I&&~N )
    if it's LESS or EQUAL to the character ~b~I'z'~N.
.WR4C1
~b~Ifor (i=0;i<1000;i++)  {                         /* go thru 1000 chars   */~N
~b~I    c=getc(fp);                                 /* get a char from file */~N
~b~I    if (c>='a'&& c<='z')  {                     /* is it between a & z? */~N
~V         n[c-'a']++;                            /* yes? increment count */~N
.R17C1





.R17C1
    ~Iif~N we get a character between 'a' and 'z' we increment the appropriate
    member of our ARRAY ~b~In[]~N ....but how ??
.WR17C1


.R17C1
    Now ~b~Ic~N is a (single) byte in memory, containing the 'value' of the
    ~b~Ic~Nharacter. This 'value' would be ~I65~N for an 'a' and ~I66~N
    for a 'b', etc. (in ASCII). If the 'value' of the ~b~Ic~Nharacter returned
    by ~b~Igetc()~N were ~I69~N (for example) then we must have an 'e' and we
    would increment ~b~In[4]~N (which holds the number of times an 'e' occurs).
    BUT, the 'value' of ~b~Ic~N minus the 'value' of 'a' is ~I69-65=4~N ...
    ...that is, ~b~Ic-'a'~N has the 'value' ~I4~N ...
    SO we increment ~b~In[c-'a']~N ( which WILL increment n[4] ).
.WR7C1
~b~I         n[c-'a']++;                            /* yes? increment count */~N
~V    }                                           /* end of if            */~N
~V}                                               /* end of for           */~N
.R17C1








.R17C1
    Now we end the ~Iif~N and the ~Ifor~N.
    The numbers in the ARRAY ~b~In[]~N will give us what we want.
.WR8C1
~b~I    }                                           /* end of if            */~N
~b~I}                                               /* end of for           */~N
~Vprintf("\nIn the file %s : ",name[1]);          /* print file name      */~N
.R17C1


.R17C1
    Now (just for kicks) we print the ~b~Iname[1]~N of the file we opened...
.WR10C1
~b~Iprintf("\nIn the file %s : ",name[1]);          /* print file name      */~N
~Vfor (i=0;i<=24;i+=2)                            /* go thru alphabet     */~N
.R17C1

.R17C1
    Now we go through all 26 letters ( and all 26 'counts' stored in the
    ARRAY ~b~In[]~N ) but two-at-a-time ( which is why we used ~b~Ii+=2~N
    which adds ~I2~N to ~b~Ii~N each time through the ~Ifor~N loop.
.WR11C1
~b~Ifor (i=0;i<=24;i+=2)                            /* go thru alphabet     */~N
~V     printf("\n%c%s%3d%s : %c%s%3d%s",          /* a format string      */~N
~V            'a'+i,  " occurs ",n[i],  " times", /* print letter and     */~N
~V            'a'+i+1," occurs ",n[i+1]," times");/* count for each letter*/~N
.R17C1



.R17C1
    Now we print each letter as a ~n~I%c~Nharacter, giving to ~b~Iprintf()~N
    the numbers 'a'+0, then 'a'+1, then 'a'+2, etc. (meaning, the ASCII
    'values' ~I65~N then ~I66~N then ~I67~N etc.).
    We also print ~b~In[0]~N, ~b~In[1]~N, ~b~In[2]~N etc. which are the
    various 'counts' (for an 'a', 'b', 'c', etc.)
.WR12C1
~V     printf("\n~N~b~I%c%s%3d%s~N~V:%c%s%3d%s",            /* a format string      */~N
.R17C1





.R17C1
    Note that we choose (for variety) to specify a format which says to print:
    First a ~b~I%c~Nharacter then a ~b~I%s~Ntring~N then a ~b~I%d~Necimal (field width ~I3~N)
    then another ~b~I%s~Ntring~N.
.WR12C1
~V     printf("\n~F%c%s%3d%s~N~V:~F%c%s%3d%s~N~V",            /* a format string      */~N
.R17C1



.R17C1
    ...and we do this TWICE (before we go to a ~b~I\n~Newline) which is why
    we increased ~b~Ii~N by ~I2~N each time through the loop (as in ~b~Ii+=2~N).
.WR12C1
~b~I     printf("\n%c%s%3d%s : %c%s%3d%s",          /* a format string      */~N
~V            'a'+i,  " occurs ",n[i],  " times", /* print letter and     */~N
~b~I            'a'+i+1," occurs ",n[i+1]," times");/* count for each letter*/~N
.R17C1


.R17C1
    This is one of the TWO printouts on each line of the screen:
    First the ~b~I%c~Nharacter (as ~b~I'a'+i~N) then the ~b~I%s~Ntring (" occurs ")
    then the ~b~I%d~Necimal ( ~b~In[i]~N ) with field width ~I3~N, then the ~b~I%s~Ntring
    " times".
.WR13C1
~b~I            'a'+i,  " occurs ",n[i],  " times", /* print letter and     */~N
~b~I            'a'+i+1," occurs ",n[i+1]," times");/* count for each letter*/~N
~V}                                               /* end of main()        */~N
.R17C1




.R17C1
    ..and this (would you believe) is the end of ~b~Imain()~N !

    (We really ~Ishould~N provide that 4-space indent!)
.WK2,32
printout??
.WN
    For the program we just wrote (called ~Icount.c~N, before it was compiled
    and linked) we get:

~IA>count count.c~N

~r~IIn the file count.c : ~N
~r~Ia occurs  18 times : b occurs   3 times~N
~r~Ic occurs  19 times : d occurs   4 times~N
~r~Ie occurs  32 times : f occurs  22 times~N
~r~Ig occurs   3 times : h occurs   9 times~N
~r~Ii occurs  25 times : j occurs   0 times~N
~r~Ik occurs   0 times : l occurs   5 times~N
~r~Im occurs  11 times : n occurs  31 times~N
~r~Io occurs  19 times : p occurs   9 times~N
~r~Iq occurs   0 times : r occurs  23 times~N
~r~Is occurs   9 times : t occurs  22 times~N
~r~Iu occurs   7 times : v occurs   0 times~N
~r~Iw occurs   1 times : x occurs   1 times~N
~r~Iy occurs   2 times : z occurs   3 times~N
.K10,60
 pretty!
.WN
~b~Ifor (i=0;i<=24;i+=2)                            /* go thru alphabet     */~N
~b~I     printf("\n%c%s%3d%s : %c%s%3d%s",          /* a format string      */~N
~b~I            'a'+i,  " occurs ",n[i],  " times", /* print letter and     */~N
~b~I            'a'+i+1," occurs ",n[i+1]," times");/* count for each letter*/~N

    The mechanism for printing two-to-a-line is awkward (!)
    We really want to let ~b~Ii~N go from ~b~I0~N to ~b~I25~N and, each time ~b~Ii~N
    is exactly divisible by ~I2~N, printf a ~b~I\n~Newline.

    This 'exact divisibility' can be tested by dividing ~b~I~N by ~I2~N and checking
    if the remainder is zero.

    To get the remainder, when ~b~Ii~N is divided by ~I2~N, we use: ~b~Ii%2~N
    (which gives this remainder!).

.W
~b~Ifor (i=0;~Vi<26;i++~N~b~I)                              /* go thru alphabet     */~N
~V     if (i%~F2~N~V==0)   printf("\n");                /* newline if i%2==0    */~N
~b~I     printf("%c%s%3d%s",                        /* a format string      */~N
~b~I            'a'+i, " occurs ",n[i], " times");  /* print letter count   */~N
.R22C1
    ...much nicer code, since we can easily print 3 or 4 to a line by changing
    the number ~V~F2~N.
.WN


.T
   That's all folks!
.K16,32
au revoir!


.q