Simple Line Parsing

This is a discussion on Simple Line Parsing within the C Programming forums, part of the General Programming Boards category; Hi, I'm one month old in C and am here with a simple query. I want to parse a space ...

  1. #1
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    Simple Line Parsing

    Hi,

    I'm one month old in C and am here with a simple query. I want to parse a space delimited line in C and read each word and execute some functions based upon those words. e.g.

    Say I've :

    I am 27 years old\n

    I need to get I, am, 27, years and old in some variables. Also there is a little twist, if I've any numeral, I need it as a numeral and not a string.

    Could anyone please write a few lines of code and help me in doing this?

    Appreciate your guidance.

    Thanks,
    Angkar

  2. #2
    cwr
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    The best way to learn C is by writing your own code. Some hints:

    You can use strchr to find a character in a string (space for example).

    You can use strtol to convert part of a string to a number, and to determine whether it successfully converted.

  3. #3
    Just Lurking Dave_Sinkula's Avatar
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    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Thanks

    Thanks Dave. And could you please explain what the following line is doing from the "=" till the end?

    printf("token[%2d] = \"%*.*s\"\n", i, (int)len, (int)len, token);

    Thanks,
    Angkar

  5. #5
    Just Lurking Dave_Sinkula's Avatar
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    Code:
    printf("token[%2d] = \"%*.*s\"\n", i, (int)len, (int)len, token);
    A double-quote (escaped within a quoted string). A string to print. Minimum field width. Precision (the maximum number of characters to print for a string).
    Last edited by Dave_Sinkula; 12-26-2005 at 01:10 PM. Reason: Bah, colors.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Also what's the significance of *token in the for loop condition?
    Lots of new things to learn.

    Thanks,
    angKar

  7. #7
    Just Lurking Dave_Sinkula's Avatar
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    It is checking for the null termination character.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Frequently Quite Prolix dwks's Avatar
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    Why not just use strtok() to separate the tokens?
    dwk

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  9. #9
    Just Lurking Dave_Sinkula's Avatar
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    Perhaps one of these reasons.

    http://www.die.net/doc/linux/man/man3/strtok.3.html
    Never use these functions. If you do, note that:
    These functions modify their first argument.

    These functions cannot be used on constant strings.

    The identity of the delimiting character is lost.

    The strtok() function uses a static buffer while parsing, so it's not thread safe.
    Maybe just to avoid getting overly friendly with a function to perhaps avoid later on.

    [edit]Or perhaps to be able to detect empty fields.
    Last edited by Dave_Sinkula; 12-27-2005 at 01:20 PM.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    But suppose I don't want to print the separated tokens and just want to store them in an array. How to do that? (I mean, what function is an alternative to the %s manipulation you are doing in the printf?)

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    Frequently Quite Prolix dwks's Avatar
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    Well, just store the variable somewhere instead of printing it.

    Like with sprintf().
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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    My printf is showing null ! May be I'm not being clear:

    Say I've an i/p as :

    1 2 3
    1 2 4
    2 3 4

    separated by spaces and \n s.

    Now I use Dave's code as:
    Code:
    /*s is a single line i/p say 1 2 3 */
    while(fgets(s,1000,f)!=NULL) 
      {
          char *token=s;
          char *buffer;
          int i,n;
          
          printf("line %d:\n",++k);
          for(i=0;*token;i++)
          {
            size_t len= strcspn(token,"  \n");
    	n = sprintf(buffer,"token[%2d] = %*.*s\n", i, (int)len, (int)len,   token");
    	printf ("[%s] is a %d chars string\n",buffer,n);
    	token+=len+1 ;
          }
       }
    I'm getting buffer as null.Is my syntax correct? Also what I want is after first iteration of while loop buffer should be as buffer[0]=1, buffer[1]=2 and buffer[2]=3.

    Thanks,
    Angshu

  13. #13
    ATH0 quzah's Avatar
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    buffer doesn't have any space allocated for it. It's just a pointer, which doesn't point anywhere. Actually, I should say, "It's just a pointer, which could be pointing anywhere." Since in fact you never initialize it, it is in fact pointing some place ... wherever.


    Quzah.
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    Thanks. But how to solve that?

  15. #15
    cwr
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    By allocating space for buffer to point to, using malloc, or by changing buffer to a char array instead of a pointer.

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