read loops

[Colonel Sanders]

My language of choice: C++

What is the proper sequence for a read loop?

I believe according to my teacher (who has been absent the last two days. . .) the proper way to make a read loop is:

// horrible pseudo code below. . . but it gets the point across

read >> data
while (!eof)
  {
  array[i] = data
  i++
  read >> data 
  }

The problem I am finding with this is that the loop is continuous- it stores data and then reads the next piece of data- the last run of the loop does not store the last read piece of data into the array so with the above loop I end up with a null value in array[i], the 2nd to last piece of data gets stored in array[i-1], and the last record is still in the variable data. I have found that if a add "array[i] = data" after the while loop the array gets all the values I need in the proper locations- but I don't really like that solution 'cause it seems cheap and I think/hope there is a better way to read data.

So if the problem is that the loop is storing, incrementing, and then reading, I thought I'd try this:

while (!eof)
  read >> data
    i++
  array[i] = data

But that give me a null value for array[0]

I have been having nightmares about this problem, I am out of ideas for how to do a good read loop. Any suggestions?

Also, I want to experiment with config files so I don't have to re-compile my program when I'm debugging and there is a line i don't want to run. I want my config files to be human readable, so I'd like to have something like:

sort: t
output: 3
blah. . .

I would then need a way to store t into the variable sort so that my sort will run. The only way that I can think of to implement config file reading would be like

read char
store into "sort" ( <<-- variable where char is stored would never change)
read num
store into "output"

The problem with that approach is if I edit the config file and remove the "sort: t" line, or I simply move that line than the read won't know where to store the value, I'd like to use the config file to determine what to store, and where to store it. And what if I have a blank line? like "out: " - how can I make the read move to the next line?

Thanks in advance for any input about solving these problems.

L J

[Force Flow]

ok, I'm not too familiar with C++ syntax, but I am familiar with the problem.

Here's bascially how it goes:

first, read the file into a variable outside of the loop.

In the while loop statement, check to see if that variable is null. If not null, continue into the loop.

store the value of the temporary variable where you want it (in this case, you want it in the array, I assume).

Increment the counter (i)

Read again into the temporary variable, then go back up to the while statement. If the temporary variable is not null, continue, else break out of the loop.

This is an example of java/c# code (again, I'm not familiar with c++, so I hope this helps)

int i = 0;
string[] myArray = new string[50]; //or however many array spaces you think you need...be generous
string newLine = myFile.ReadLine(); //or read char, or whatever you need here
while(newLine != null){
     myArray[i++] = newLine;
     newLine = myFile.ReadLine();
}

As for the reading the config file, throw in an if statment that says if the string equals "" (or char equals ''), continue to the next line. Null is only thrown when you reach the end of the file.

A note about this though...is this a "band-aid fix" on a problem, or part of an actual solution?

[alfie2]

use std::getline function, it will read one line at a time.

ie:

string str;
getline(in,str); // Get the frist line from the file, if any.
while ( in ) { // Continue if the line was sucessfully read.
processLine(str); // Process the line.
getline(in,str); // Try to get another line.
}

in "processLine" you can look for "sort" "t" so on...by parsing the string anyway you wanted.

of course, you have to check for eof with : " if(in=eof) then....".

alternatively, you can do it by creating your own "readline" procedure
and check for the end of each line by searching for "\n" character.

[Floppyman]

Originally Posted by Force Flow This is an example of java/c# code (again, I'm not familiar with c++, so I hope this helps) int i = 0; string[] myArray = new string[50]; //or however many array spaces you think you need...be generous string newLine = myFile.ReadLine(); //or read char, or whatever you need here while(newLine != null){ myArray[i++]; newLine = aFile.ReadLine(); } ?

Force, either I'm blind or your code is not actually storing any data in the array....

[Force Flow]

Originally Posted by Floppyman Force, either I'm blind or your code is not actually storing any data in the array....

Whoops...it isn't. That's what I get for using a textbox and not a complier. Thanks, I fixed it.

[Zick Boy]

I haven't coded for a while in C++ but I think I know your problem.

Use a "do...while" instead of a "while..." loop. You're not storing the last line because your code first check the end of the file and then performs. So it works like this:

1- check if end-of-file = false
2- store data in variable from previous read
3- jump to next line
4- read file (last one in file)

...begin loop again
1- check if end-of-file = true
Stops!

see, you read the last line but it didn't get the chance to store it because the file was already at the end.

on a do... while loop it checks end-of-file AFTER it performs all operations

do {
  array[i] = data
  i++
  read >> data
} while (!eof)

I hope this helps!

[Colonel Sanders]

According to the teacher the problem was simply that he forgot to add an blank line to the end of the file. Array[i] would still return a garbage value, but so long as I use loops that use less "while < i" then I am fine.

L J