how to store fscanf to print in reverse order?

Question

Hi i am trying to store fscanf into a data type in order to print it in reverse manner but cannot get it done.

here is my code

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

int main(int argc, char *argv[])
{
    FILE* input;
    FILE* output;

    input = fopen(argv[1],"r");
    output = fopen(argv[2],"w");

    int zipcode, population;
    while(fscanf(input,"%d %d\n",&zipcode, &population)!= EOF)
    {
        fwrite(&population, sizeof(int), 1, output);
    }
    return 0;
}

here is what i have been trying to do

for(i = fscanf(input,"%d %d\n",&zipcode, &population); i!=EOF; i++)
    {
        fwrite(&population, sizeof(int), 1, output);
    }

Answer 1

I think you've missed the point of fscanf (to provide portable and machine-independent data input) by unportably outputting the machine-dependent, internal representation of an int on your system.

How would you get the '2' out of 200? 200 / 100 results in 2. Is this any different to getting the '2' out of 256?

How would you get the '5' out of 256? Have you ever used the modulo (%) operator to obtain the remainder of a division? 256 % 100 results in 56. 56 / 10 results in 5.

What about the '6'? How would you swap the 6 and the 2? Presuming you've used the algorithm above to extract the 2 into a "multiples of one hundred" column, and the 6 into a "multiples of one" column, couldn't you swap them so that the 6 lies in the "multiples of one hundred" column and the two lies in the "multiples of one" column? You can use this approach to portably output in little endian, big endian or whatever mixed endianness you like... and there's a reverse approach to portably input in little endian, big endian or whatever mixed endianness you like. Can you work it out?

How would you get the 0x1f out of 0x1f23? How do you get the 0x23 out of that number? Interestingly, you can extract these "bytes" (octets, technically) using the same approach (division and modulo), but a different base: 0x100 instead of 10, 0x10000 instead of 100, 0x1000000 instead of 1000, etc. In fact, this is true for most number systems: binary, octal, decimal, hexidecimal... Perhaps a more interesting experiment might be implementing portable base negative two input/output, for both LSB (least significant bit first) and MSB (most significant bit first).

edit: In fact, there's a simpler way to achieve the result you've described in your comments: Obtain your integer modulo ten, and print that as a decimal digit. Then divide your integer by ten and continue with this number as your integer. Repeat these two steps until your integer is zero.

Answer 2

To print you data in revers for example population = 123456 you want to printf like 654321. simple way is: read population in a string instead a int. and define strrev() function to print string in reverse

As I understand from your comments your file is a sequence of zipcode and population something like:

 46804 3450103 37215 1337 47906 46849

and you want to write alternative numbers back to some output file, do like this(read comments to understand code):

#include<stdio.h>
#include<string.h>
#define SIZE 50
void strrev(char* st) {// string reverse function()
  char* f = st;      // points to first
  char* l = st + strlen(st) -1;  // points to last char
  char temp;  
  while(f < l){
    // swap two chars in string at f & l memory
    temp = *f;
    *f = *l;
    *l = temp;

    f++;
    l--;
  }
}
int main(int argc, char* argv[]){
    if(argc!=3) return 0;
    FILE* input  = fopen(argv[1],"r");
    FILE* output  = fopen(argv[2],"w");
    int zipcode;
    char population[SIZE] = {0};
    while(fscanf(input,"%d %s\n",&zipcode, population)!= EOF){
        // population is always alternative number 
          strrev(population);    // reverse the string 
        //printf("%s\n",population);
          fprintf(output,"%s ",population);  // write in output file
    }
    return 1;
}

And this works as follows:

:~$ cat inputfile 
 46804 3450103 37215 1337 47906 46849
:~$ ./a.out  inputfile outputfile
~$ cat outputfile 
3010543 7331 94864

This is one kind of simple solution.

EDIT Because you are commenting you need binary dump file. So I think you need outfile in binary formate: Just open output file in binary mode and use write function. For this I am writing a partial code(again read comments):

FILE* output  = fopen(argv[2],"wb");  
    //                             ^  open in write binary mode
int val;   // an extra variable int
while(fscanf(input,"%d %s\n",&zipcode, population)!= EOF){
      strrev(population);   // revers string        
      val = atoi(population);  // convert strint to int
      fwrite(&val,sizeof(int),1,output); // write in binary file
}

Answer 3

To convert endianess, you could refer to this link: convert big endian to little endian in C [without using provided func]

Else, you could refer to the function below as an example

int convert_endian(int n)
{
    int num = 0;
    while(n > 0) {
        num = (num * 10) + (n %10);
        n = n / 10;
    }
    return num;
}