Reading/writing txt strings - c

Question

I'm trying to learn how to manipulate txt files in C at the moment. This program should read in population.txt (http://pastebin.com/Q5fNRuJG), find the highest population and display it, make a file with populations of greater than 1 million, make a file with all the irish cities, and print the total population. It only creates empty files, and prints the total population (now also prints most populous city). Can anyone help?
The program (compiled in Borland):
//Program that filters and processes data with fscanf()
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
main()
{
FILE *fp;
FILE *ireland;
FILE *mmplus;
int pop;
int highest_pop=0;
char country[50];
char city[50];
int total_pop=0;
fp = fopen("population.txt","r");
ireland = fopen("ireland_pop.txt","w");
mmplus = fopen("greater_than_1MM.txt","w");
//Checking if the file opened correctly
if (fp==NULL)
{
printf("Cannot open file.\n");
exit(1);
}//End if
//Scanning fp row by row
while (fscanf(fp, "%s" "%s" "%i", country, city , &pop) != EOF)
{
//Getting the total population
total_pop=total_pop+pop;
//Storing the highest pop
if (pop > highest_pop)
{
highest_pop=pop;
}//End if
//Finding 1 million+ cities
if(pop>=1000000)
{
fprintf(mmplus,"%s %s %d\n",country,city,pop);
}//End if
//If the city is in Ireland
if (strcmp("ireland",country) == 0)
{
fprintf(ireland,"%s %s %d\n",country,city,pop);
}//End if
}//End while
rewind(fp); //Fix 1
while (fscanf(fp, "%s" "%s" "%d", country, city , &pop) != EOF)
{
//Finding the city with the highest population
if (pop == highest_pop)
{
printf("The city with the highest population is %s, %s, with a population of %d",city, country, pop);
}//End if
}//end while
printf("The total population of all the cities is %d.",total_pop);
getchar();
fclose(fp);
fclose(ireland);
fclose(mmplus);
}

Answer 1

The first time I ran your program I got empty files. The second time I ran it I got correct output. The third time I got one correct file and one incorrect file.
I think I may have typed ctrl-C to end the program. So I made a couple of changes to the last few lines of the program - adding newline to the output, and closing the files before waiting for input. After that, it works every time, but I can't explain everything.
fclose(fp);
fclose(ireland);
fclose(mmplus);
printf("The total population of all the cities is %d.\n",total_pop);
getchar();
return 0;

Answer 2

here is one way to implement the algorithm
//note: the system function: perror()
// outputs the strerror() message,
// the first parameter text, etc
//Program that filters and processes data with fgets() and sscanf()
#include <stdio.h>
#include <stdlib.h>
#include <string.h> // strcpy(), strlen()
#include <unistd.h> // unlink()
int main()
{
FILE *fp = NULL; // input
FILE *ireland = NULL; // list irish cities
FILE *mmplus = NULL; // list cities > 1million population
int pop;
char country[50];
char city[50];
int total_pop=0;
int maxPopulation = 0;
char maxPopulationCity[50] = {'\0'};
char maxPopulationCountry[50] = {'\0'};
char buffer[200]; // input work area
if( NULL == (fp = fopen("population.txt","r") ) )
{ // then, fopen failed
perror( "fopen for population.txt failed" );
exit( EXIT_FAILURE );
}
// implied else, fopen successful
if( NULL == (ireland = fopen("ireland_pop.txt","w") ) )
{ // then, fopen failed
perror( "fopen for ireland_pop.txt failed" );
fclose(fp); // cleanup
exit( EXIT_FAILURE );
}
// implied else, fopen successful
if( NULL == (mmplus = fopen("greater_than_1MM.txt","w") ) )
{ // then, fopen failed
perror( "fopen for greater_than_1MM.txt failed" );
fclose(fp); // cleanup
fclose(ireland);
unlink("ireland_pop.txt");
exit( EXIT_FAILURE );
}
// implied else, fopen successful
//Scanning fp row by row
while ( fgets( buffer, sizeof buffer, fp ) )
{
// this code makes the following assumptions
// 1) city is all one word
// (I.E, no 'st paul'
// 2) country is all one word and 'ireland' is always lower case
// (I.E. no 'Ireland' and no 'United States'
// 3) pop contains no ',' and is always positive
// 4) data item delimiters are only ' ', \t
if( 3 != sscanf(buffer, " %s %s %i", country, city , &pop) )
{
//calculate the total population
total_pop += pop;
//Storing the highest pop found so far
if (pop >maxPopulation)
{
maxPopulation=pop; // save new largest population
strcpy( maxPopulationCity, city ); // save new city
strcpy( maxPopulationCountry, country ); // save new country
}//End if
//Finding 1 million+ cities
if(pop>=1000000)
{
fprintf(mmplus,"%s %s %d\n",country,city,pop);
}//End if
//If the city is in Ireland
if (strcmp("ireland",country) == 0)
{
fprintf(ireland,"%s %s %d\n",country,city,pop);
}//End if
if( buffer[strlen(buffer)-1] != '\n' )
{ // then, input line to long for buffer[]
// read chars until newline consumed
while( '\n' != getchar() ) ;
}// end if
} // end if
}//End while
printf("The city with the highest population is %s, %s, with a population of %d",
maxPopulationCity,
maxPopulationCountry,
maxPopulation);
printf("The total population of all the cities is %d.",total_pop);
getchar();
fclose(fp);
fclose(ireland);
fclose(mmplus);
return(0);
} // end function: main