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coding question?

Posted on 2006-10-19
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Last Modified: 2010-04-15
hi, could someone explain to me what is the meaning of " ndigit[c-'0']++" in my following code?
thanks

-----------code--------------------
#include <stdio.h>

 main()  
   {
       int c, i, nwhite, nother, nline,ndigit[10];

       nwhite = nother = 0;
       nline=1;
       for (i = 0; i < 10; i++)
           ndigit[i] = 0;
       while ((c = getchar()) != EOF) {
           switch (c) {
           case '0': case '1': case '2': case '3': case '4':
           case '5': case '6': case '7': case '8': case '9':
           
               ndigit[c-'0']++;------------------------------------------->??
               break;
           case ' ':
           case '\t':
               nwhite++;
               break;
           case '\n':
               nline++;
               break;
           default:
               nother++;
               break;
           }
       }
       printf("digits =");
       for (i = 0; i < 10; i++)
           printf(" %d", ndigit[i]);
       printf(", white space = %d, line# = %d, other = %d\n", nwhite, nline, nother);
       return 0;
   }
0
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Question by:rmtogether
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9 Comments
 
LVL 84

Expert Comment

by:ozo
ID: 17770575
If the character set used is ASCII, in which the character values '0' ... '9' are contiguous, It counts the number of times c=='0' in ndigit[0], the number of times c=='1' in ndigit[1], ..., the number of times c=='9' in ndigit[9],
0
 

Author Comment

by:rmtogether
ID: 17770624
I still confused about the format [c-'0'], why use c minus zero ?
0
 
LVL 84

Expert Comment

by:ozo
ID: 17770646
Assuming you are using the ASCII character set
'0'-'0'==0
'1'-'0'==1
'2'-'0'==2
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Author Comment

by:rmtogether
ID: 17770801
thanks ozo

I understand it conts the number of times like c=='0' in ndigit[0], why can't I just use c like ndigit[c] without -'0'?
0
 
LVL 84

Expert Comment

by:ozo
ID: 17770831
in ASCII, '0'==48 and '9'==57, so you would have to declare int ndigit['9'+1] instead of int ndigit[10]
the for loops would also need to be changed to for (i = '0'; i <= '9'; i++)
0
 

Author Comment

by:rmtogether
ID: 17770890
thans,
So, at begining c is declare as integer, when goes to switch (c), c are converted to character. that is why we need use c-'0'-----> am I correct about this?

does case must use character? like case '0' ? can I use case 0 instead?
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LVL 84

Accepted Solution

by:
ozo earned 1000 total points
ID: 17770937
There is no conversion.  getchar() returns an int, and '0' is an integer constant.
If your character set is ASCII, '0'==48
you can use case 0: which is the same as case '\0':
or case 48: which is the same as case case '0': (assuming ASCII)

other ASCII character values
       0 nul    1 soh    2 stx    3 etx    4 eot    5 enq    6 ack    7 bel
       8 bs     9 ht    10 nl    11 vt    12 np    13 cr    14 so    15 si
      16 dle   17 dc1   18 dc2   19 dc3   20 dc4   21 nak   22 syn   23 etb
      24 can   25 em    26 sub   27 esc   28 fs    29 gs    30 rs    31 us
      32 sp    33  !    34  "    35  #    36  $    37  %    38  &    39  '
      40  (    41  )    42  *    43  +    44  ,    45  -    46  .    47  /
      48  0    49  1    50  2    51  3    52  4    53  5    54  6    55  7
      56  8    57  9    58  :    59  ;    60  <    61  =    62  >    63  ?
      64  @    65  A    66  B    67  C    68  D    69  E    70  F    71  G
      72  H    73  I    74  J    75  K    76  L    77  M    78  N    79  O
      80  P    81  Q    82  R    83  S    84  T    85  U    86  V    87  W
      88  X    89  Y    90  Z    91  [    92  \    93  ]    94  ^    95  _
      96  `    97  a    98  b    99  c   100  d   101  e   102  f   103  g
     104  h   105  i   106  j   107  k   108  l   109  m   110  n   111  o
     112  p   113  q   114  r   115  s   116  t   117  u   118  v   119  w
     120  x   121  y   122  z   123  {   124  |   125  }   126  ~   127 del
0
 

Author Comment

by:rmtogether
ID: 17775211
hi, ozo

thank you I got it.
as you said,  getchar() returns an int, and assign to c. So how about
case ' '
case '\t'
case '\n':
0
 
LVL 84

Expert Comment

by:ozo
ID: 17776677
that should be
case ' ':
case '\t':
case '\n':

' ' is sp  space
'\t' is ht  horizontal tab
'\n' is nl  newline
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