n00b question.... dat file related

**hobbes67** · 06-23-2005

I am new to the world of C and C++, now this may sound like a stupid quesiton, but this is the same way i learned visual basic, but anyways i wanna practice reading and writing to a dat file.

now right now i have a dat file called people.dat and in that dat file i have 5 names

Tom Cruise
Lindsay Lohan
Brad Pitt
Britney Spears
Jack Johnson

now i want my practice application to read the dat file and display the list as is, then i want it to display reversed

Jack Johnson
Britney Spears
Brad Pitt
Lindsay Lohan
Top Cruise

i then want to write the reversed list to another dat file called reversed.dat...
I am able to read the dat file and display in the order it is in in the dat file, but to display it in reverse and write it reversed is where i get lost.

I know this is soo simple, but i wanna learn C/C++, there are too many vb programmers

thanks guys/gals

**mitakeet** · 06-23-2005

There are a lot of C/C++ programmers too!

Make a stab at the program and post back here with your problems. You will get MUCH better help that way!

**sand_man** · 06-23-2005

Well don't know what your code looks like but lets say you have an array of characters that you read from the file...

Code:

#include <stdio.h>
#include <string.h>

int main(void){
  char str[14] = "Thisisastring";
  int i,len = strlen(str);
  
  for(i=len-1;i>-1;i--){
    printf("%c",str[i]);
  }  
  
  return 0;
}

You just need to loop through the string backwards displayin each character one at a time instead of using printf("%s",str) to display the whole string

**Karthur** · 06-23-2005

In this case, he wants to read strings, store them, and then print them in reversed order, or read the whole file into a buffer, change all the newlines to nul chars, then scan from the back of the buffer, printing each string as a line, or ....

**Raptor007** · 06-23-2005

Originally Posted by sand_man

You just need to loop through the string backwards displayin each character one at a time instead of using printf("%s",str) to display the whole string

No, no, no... that would print the characters backwards, he wants to print the LINES backwards with each string intact.

Something like this should do it:

Code:

#include <stdio.h>
#include <stdlib.h>

int main () {

	// Declare all variables at the beginning.
	int i;
	FILE *filePointer;
	char stringPointers[5][100];

	// Open file in read text (rt) mode.
	filePointer = fopen ("myfile.dat", "rt");
	if (filePointer == NULL) {
		printf ("Error opening file: %s\n", "myfile.dat");
		exit (1);  // Exiting without 0 means there was a problem.
	}

	// Read 5 lines (up to 99 chars each) and then close the file.
	for (i = 0; i < 5; i++) {
		fgets (stringPointers[i], 100, filePointer);
	}
	fclose (filePointer);

	// Print in normal order.  Linefeeds should be in each string from fgets.
	for (i = 0; i < 5; i++) {
		printf ("%s", stringPointers[i]);
	}

	// Print a newline character to separate the outputs.
	printf ("\n");

	// Print in reverse order.
	for (i = 4; i >= 0; i--) {
		printf ("%s", stringPointers[i]);
	}

	return 0;
}

Or with dynamic allocation:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>

int main () {

	// Declare all variables at the beginning.
	int i;
	FILE *filePointer;
	char **stringPointers;

	// Allocate the array of 5 strings.
	// This is a pointer to pointers (works as char[][]).
	stringPointers = (char **) malloc (5 * sizeof (char *));

	// Allocate each string (up to 99 chars in length each).
	// This is a pointer to characters (works as char[]).
	for (i = 0; i < 5; i++) {
		stringPointers[i] = (char *) malloc (100);
	}

	// Open file in read text (rt) mode.
	filePointer = fopen ("myfile.dat", "rt");
	if (filePointer == NULL) {
		printf ("Error opening file: %s\n", "myfile.dat");
		exit (1);  // Exiting without 0 means there was a problem.
	}

	// Read 5 lines (up to 99 chars each) and then close the file.
	for (i = 0; i < 5; i++) {
		fgets (stringPointers[i], 100, filePointer);
	}
	fclose (filePointer);

	// Print in normal order.  Linefeeds should be in each string from fgets.
	for (i = 0; i < 5; i++) {
		printf ("%s", stringPointers[i]);
	}

	// Print a newline character to separate the outputs.
	printf ("\n");

	// Print in reverse order.
	for (i = 4; i >= 0; i--) {
		printf ("%s", stringPointers[i]);
	}

	// Clean up memory that was allocated with malloc (good habit).
	for (i = 0; i < 5; i++) {
		free (stringPointers[i]);
	}
	free (stringPointers);

	return 0;
}

**sand_man** · 06-24-2005

No, no, no...

Well thats what I get for not reading the original post properly

**hobbes67** · 06-24-2005

thanks for the help guys, i'm workin with all you gave me, and i'm trying to make it work but these arrays i have to do some research on.. i'll get it workin soon..

**Raptor007** · 06-24-2005

Something you might not be used to... all strings in C are arrays of characters, with a null terminator character telling it when the string ends.

Therefore this:

Code:

char string[100];

...defines a string of maximum length 99, because it needs space to put a '\0' after the used characters to tell it the string has ended.

Hope this helps!

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