Extract first and last values from a file.

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4.00/5 (1 vote)

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Hi all,
I a function that reads a file and extract the values, how can I extract only the first and the. last values. To read I use this function:

C++

if (csf.Open(fname, CFile::modeRead)) // CStdioFile csf;
  { 
    while (csf.ReadString(buffer)) //CString buffer;

    {
       int out = 0;
       out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );
       // my calculations to process the double values here
    }
      csf.Close();
  }

Updated code: Based on KarstenK's code :-

C++

if (csf.Open(fname, CFile::modeRead)) // CStdioFile csf;
  {
    csf.ReadString(buffer);
   //here is first read in the buffer
    int out = 0;
    out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );
    pInitialTime = str1;

  while (csf.ReadString(buffer)) {
   //AFTER that is the last readin the buffer
    out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );
     pFinalTime = str1; 
 } 
  csf.Close();
  of.WriteString(FmtValues1(pInitialTime, pFinalTime)); // both pInitialTime and pFinalTime values take the last value
    }

Using this I can read each line and do processing, but how can I extract only the first and the last values?

Posted 15-Jun-12 1:59am

Sumal.V

Updated 15-Jun-12 4:36am

v4

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2 solutions

Solution 2

You just missed one semicolon in Karsten's code, the one at the end of the while statement. It is also no good practice to code a while statement like this. Also Karsten's code was not prepared for when the last line of the file is empty.

Here is a revised version that will help you better:

if (csf.Open(fname, CFile::modeRead)) // CStdioFile csf;
  {
    csf.ReadString(buffer);
        //here is first read in the buffer
    int out = 0;
    out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );
    pInitialTime = str1;

    // allocate a temp buffer
    CString tempBuffer;

    // now read all remaining lines of the file without decoding them 
    while (csf.ReadString (tempBuffer))
      if (tempBuffer.GetLength() > 0)
          buffer = tempBuffer;

    // now the last non-empty line is in the buffer and we decode it
    out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );
     pFinalTime = str1; 
 } 
  csf.Close();
  of.WriteString(FmtValues1(pInitialTime, pFinalTime)); // both pInitialTime and pFinalTime values take the last value
    }

Posted 15-Jun-12 5:28am

nv3

Comments

Sumal.V 15-Jun-12 11:39am

This results in both variables taking the same values as well..

Sumal.V 15-Jun-12 11:51am

And If I add the semicolon, I get run time error. when I checked the code in debug, I found that the program does not move after while();

That's the reason I took it off.

nv3 15-Jun-12 12:07pm

I suggest you set a breakpoint on line

if (tempBuffer.GetLength() > 0)

and look line by line what is in your tempBuffer. Compare that with the contents of your file as displayed in an editor. Then tell us that happens when you get to the last line of your data file.

Sumal.V 18-Jun-12 4:34am

Good morning!,I checked the code with a break point but realised that since the variable in both the if and while loops was str1, ie both pinitialTime and pfinalTime were assigned this str1 variable, at the end when the final time was taking the final value, initialtime was assigned the same value. I changed the variable names now as initstr1 and finalstr1 instead of str1 for both and that gives me the right values..

nv3 18-Jun-12 4:41am

That's what I told you in your previous question about copying strings. Setting a pointer to a string does not mean your make a copy of it.

Another option would have been to define:

CString initialTime, finalTime;
...
initialTime = str1;
...
finalTime = str1;

In this case the two CString object do make a copy of what is in str1 at that time. That is actually what I had assumed you would do, as you didn't show the definition of pInitialTime and pFinalTime in your code.

Sumal.V 18-Jun-12 4:56am

Yeah this works as well. I now wonder why I used w_char types instead of CString itself.I'm sure I had some problem earlier, I'm just blank now. Must maintain some documentation. But thanks for keeping up with me :)

nv3 18-Jun-12 5:05am

You'r welcome!

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KarstenK · Accepted Answer · 2012-06-15T02:24:00

Solution 1

C#

if (csf.Open(fname, CFile::modeRead)) // CStdioFile csf;
  {
    CString buffer;
    csf.ReadString(buffer);
   //here is first read in the buffer

    while (csf.ReadString(buffer));
   //AFTER that is the last readin the buffer

it looks so easy ;-)

Posted 15-Jun-12 2:24am

KarstenK

Comments

Sumal.V 15-Jun-12 8:48am

Ummm.. it doesn't look easy for me :(
if I try extract one value, that will apply for all the values, as it is in a loop. so everything the loop is run, the particular column value will b printed.
And I must go into the while loop to start reading the values .

KarstenK 15-Jun-12 8:54am

I hope you noticed the ";" at the end of the line
while (csf.ReadString(buffer));

Show the complete code your talking about....

Sumal.V 15-Jun-12 9:29am

Hi,I have updated my question.

KarstenK 15-Jun-12 9:33am

here is my update

if (csf.Open(fname, CFile::modeRead)) // CStdioFile csf;
{
CString buffer;
csf.ReadString(buffer);
//here is first read in the buffer
int out = 0;
out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );

while (csf.ReadString(buffer));
//AFTER that is the last readin the buffer
out = swscanf_s(buffer, _T("%[^,],%[^,],%[^,],%lf"),str1, 2046, str2, 2046, str3, 2046, &db1) );

Sumal.V 15-Jun-12 10:22am

I updated my code, but bothe the first and last values get the same values ie(last ). I have shown my updated code in the question.

Sumal.V 15-Jun-12 10:34am

Cheers, it works when I pull the initial value before the while loop starts. I will work out why both the variables take the same value, when they are printed after teh while loop.
Thank u very much :)

Sumal.V 15-Jun-12 9:33am

I cannot extract any values before this statement.
while (csf.ReadString(buffer));
There the variable gets no values/garbage values before that line of code .

nv3 15-Jun-12 11:30am

Karsten, I think he forgot to copy that semicolon at the end of the while statement. That's why I would not write it like this but better:

while (...)
;

That makes it clear that the while body is empty.

Also, if the last line is empty, that empty line sits in the buffer and the decoding fails. An empty line at the end is not all that unlikely, I think.