Repeated calls to sscanf

This is a discussion on Repeated calls to sscanf within the C Programming forums, part of the General Programming Boards category; Code: int convert_data(int id_limb){ char temp1[VECTOR_SIZE]; char temp2[VECTOR_SIZE]; char code; char code_temp; double velocities[8]; int i; for (i=0;i<6;i++) //initializing { ...

  1. #1
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    Repeated calls to sscanf

    Code:
    int convert_data(int id_limb){
    	char temp1[VECTOR_SIZE];
    	char temp2[VECTOR_SIZE];
    	char code;
    	char code_temp;
    	double velocities[8];
    	int i;
    
    	
    	for (i=0;i<6;i++)	//initializing
    	{
    		memcpy(temp1,buffer+i*VECTOR_SIZE,VECTOR_SIZE);
    		sscanf(temp1,"%s",temp2);
    		velocities[i]=atof(temp2);  //
    	}
    	memcpy(&codigo_temp,buffer+6*VECTOR_SIZE,1);
    	sscanf(&codigo_temp,"%c",&code);
    	printf("\nCode %c \n",code);
    	
    return(execute_comand(velocities, code, id_limb));
    
    }
    The overall project consist of an operator sending linear and angular velocities to a robotic arm as a string of chars through the lan. As of now we need to speed up the process (it's about 0.5 ms too slow). Is there any way to optimize this particular function? For example, a way to skip the repeated calls to sscanf?

    Thanks a lot

  2. #2
    C++ Witch laserlight's Avatar
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    buffer is global? You don't seem to initialise temp2.
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    Yeah. Eliminate the temp1 buffer and also the sscanf calls. Neither are necessary. Also eliminate the printf() call, except when debugging.

    When using sscanf, the %s format scans until it encounters either a whitespace or the end of the input string. As long as you guarantee you do not change buffer, you can do the same using a loop. Similarly, %c simply gets the next character. If all you're reading from the string is a single character, you are getting the first character.
    Right 98% of the time, and don't care about the other 3%.

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    Quote Originally Posted by grumpy View Post
    Yeah. Eliminate the temp1 buffer and also the sscanf calls. Neither are necessary. Also eliminate the printf() call, except when debugging.

    When using sscanf, the %s format scans until it encounters either a whitespace or the end of the input string. As long as you guarantee you do not change buffer, you can do the same using a loop. Similarly, %c simply gets the next character. If all you're reading from the string is a single character, you are getting the first character.

    I'm not sure I follow, could you please elaborate a bit more? Also, the string consist of 6 different characters at all times (3 linear components and 3 angular).

    @laserlight buffer is global. As for why I don't initialize temp2, I figured I didn't need to since scanf would write there anyway

  5. #5
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    I just described what sscanf() does with the %s and %c format specifiers. It is not hard to replicate that behaviour. And sscanf() involves an overhead (interpreting the format string, etc) that you can avoid with a bit of thought.
    Right 98% of the time, and don't care about the other 3%.

  6. #6
    and the hat of wrongness Salem's Avatar
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    If it's just a space separated list of numbers, then use this
    man page strtod section 3

    Note the last parameter, you use this to "walk" along the string.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    I was under the impression that memcpy simply copied the memory blocks, but but didn't format the data. Therefore I used sscanf to format them as strings, was my logic wrong?

  8. #8
    and the hat of wrongness Salem's Avatar
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    If you showed us what buffer contains, then we might be able to suggest a better way.

    Unless you have say "123456" and you expect to get 123 and 456 as separate numbers, you seem to be doing a lot of extra work.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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    char buffer[BUFSIZE]

    As of right now there are seven different elements in the string: x,y,z linear and x,y,z angular. There's an extra character E,C or H that tells the machine to exit the program, continue or go home. So yes, the remote machine should be able to tell them apart.

    Thanks everyone for the help

  10. #10
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    I'm goig to try
    Code:
    for (i=0;i<6;i++)    //inicializacion
    
        {
            strncpy(temp1,buffer+i*VECTOR_SIZE,VECTOR_SIZE);
            velocities[i]=atof(temp1);
         
    }
          strncopy(&codigo,buffer+6*VECTOR_SIZE,1);
    will report results and speed later

  11. #11
    and the hat of wrongness Salem's Avatar
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    But strncpy doesn't append a \0 to where you copy it from.
    Like I said, so long as you don't have contiguous numbers (there is some kind of space, tab, comma, alpha separator), then you can just do this instead.

    velocities[i]=atof(buffer+i*VECTOR_SIZE);

    atof will stop converting as soon as it comes across something which isn't a digit or a decimal point.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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