How to compare column from a file c++ - c++

Question

I have a file that has 3 columns with many rows. I want to compare the value either increase or decrease for 2 columns. But when I compare, the value always increases even it should be decreased. Any idea how to solve this? This is my coding to compare the value in the file. But I didn't get the desired output. anyone can help me?
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
ifstream fromFile;
fromFile.open("data.txt");
{
cout << "There was an error" << endl;
exit(1);
}
else
cout << "No error, file opened successfully" << endl;
fromFile.clear();
fromFile.seekg(0);
for(int i=1; i<=20; i++)
{
int a{}, b {}, c {};
fromFile >> a >> b >> c ;
cout<<" Year : " <<a<<" population : "<<b<<" car : "<<c<< endl;
if ( b < b && c < c )
cout << " population decrease and car production decrease " << endl;
if ( b > b && c > c );
cout << " population increase and car production increase " << endl;
if ( b > b && c < c )
cout << " population increase but car production decrease " << endl;
if ( b < b && c > c );
cout << " population decrease but car production increase " << endl;
}
//CLOSES FILES
fromFile.close();
return 0;
}

Answer 1

There are several problems with your code. I will first mention those and then show a corrected solution (one of many possible variants).
Some issues:
You should not use using namespace std. There are many expanations on stackoverflow with the explanation. Use full qualified names instead
The std::ifstream has a constructor that automatically opens the stream. No need for function open
The stream has an overloaded bool operator. You can simply test the state with if (fromFile)
You forgot the if statement, but have an else statement. So, your program will not compile. And is logically wrong
Because of the missing if the program will always exit immidiately
clear and seekg are not necessary. The filepointer is at the beginning after opening the stream.
You use a loop with fixed hardcoded magic number 20. What if the file has more or less rows? You need to use the condition end-of-file to end the loop
You compare always the variables with itself. Even the compiler will warn you. This cannot work. You need to read a variable, store it somewhere in a different variable, read again from the file and compare the current variable with the previous value. This is your main problem
No need to call the close-function of the stream. The destructor will call it implicitely for you, when the stream variable leaves its scope
You have not enough comparision if statements. Many cases, especially, if values are same than before, will not be handled
So, what do we need to do? I will explain with only one variable now. Let's take the year. We define a variable year and a second varibale previousValueYear. In a loop, we read the value from the file into year. Then, we compare the year variable with the previousValueYear, do the evaluation and show the output. And then, before reading the next value from the file, we copy the current value from year to previousValueYear. Then we can do the correct comparison in the next loop run.
The first comparision is a little bit tricky, because we do not have a previousValueYear. We will add an additional if statement with a boolean flag, to check for this condition.
Please see here the corrected version:
#include <iostream>
#include <fstream>
int main() {
// Open the source file
std::ifstream fromFile("data.txt");
// Check, if we could open the file
if (fromFile) { // bool operator of stream will be called
// OK, file could be opened. Defineour variables
int year{}, population{}, car{};
int previousValueYear{}, previousValuePopulation{}, previousValueCar{};
bool thisIsTheNotFirstLoopRun{};
// Read values in a loop and check, if the input worked
while (fromFile >> year >> population >> car) {
// Show read data
std::cout << "Year: " << year << " \tPopulation: " << population << " \tCar: " << car << '\n';
// In the first loop run, we will not do a comparision (There is only one value and nothing to compare
if (thisIsTheNotFirstLoopRun) {
// Do the comparisons
if ((population < previousValuePopulation) and (car < previousValueCar))
std::cout << "Population decrease and car production decrease\n";
if ((population > previousValuePopulation) and (car > previousValueCar))
std::cout << "Population increase and car production increase\n";
if ((population > previousValuePopulation) and (car < previousValueCar))
std::cout << "Population increase and car production decrease\n";
if ((population < previousValuePopulation) and (car > previousValueCar))
std::cout << "Population decrease and car production increase\n";
}
// Remember current values for next loop run
previousValueYear = year;
previousValuePopulation = population;
previousValueCar = car;
// Now, we will not have any longer the first loop run
thisIsTheNotFirstLoopRun = true;
}
}
else {
// Error, file could not be opened. Show message
std::cerr << "\nError: SourceFile could not be opened\n";
}
return 0;
}
Advanced C++ porgrammers would change a lot. But for now it is OK