Finding pattern in a text in C++ - c++

Question

I have written the following code to find the number of "ATA" in a text that is read to a string as "GCTATAATAGCCATA". The count returned should be 3 but it returns 0. When I check in debugger the string for text is initially created. However, when an empty string is passed to the function patternCount. Am I reading the contents of the file into the string text correctly?
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
void patternCount(string text, string pattern);
int main()
{
string text;
fstream file_("test.txt");
if(file_.is_open())
{
while(getline(file_,text))
{
cout << text << '\n';
}
file_.close();
}
cout << "Enter a string ";
string pattern;
getline(cin, pattern);
patternCount(text, pattern);
return 0;
}
void patternCount(string text, string pattern)
{
int count = 0;
size_t nPos = text.find(pattern, 0);
while (nPos != string::npos)
{
nPos = text.find(pattern, nPos + 1);
++count;
}
cout << "There are " << count <<" " << pattern << " in your text.\n";
}

Answer 1

This code just counts the number of occurrence of input string in the last line of text file. If that line is empty or no does not contain the string, The output result will be 0.
But I guess the OP wants to search a whole file, in which case the main function need be fixed accordingly.
std::ifstream file{"test.txt"};
std::ostringstream text;
std::copy(std::istream_iterator<char>{file}, std::istream_iterator<char>{},std::ostream_iterator<char>{text});
//...
patternCount(text.str(), pattern);

Answer 2

So if I understand correctly, you're not sure if you're reading correctly the contents from the file test.txt. If you want to read every content, then try this instead:
ifstream file_("test.txt");
string s,text;
file_>>s;
text=s;
while(file_>>s)
{
text=text+" "+s;
}
This should probably work. Note that reading from a file like filename>>string only reads till the first space. That's why I'm using the while. You can also use getline(), which reads the whole text with spaces. Also note that you should include fstream. Printing out the text should help more as well.

Answer 3

#include <iostream>
#include <fstream>
#include <string>
using std::cout;
using std::cerr;
using std::string;
int count = 0; // we will count the total pattern count here
void patternCount(string text, string pattern);
int main() {
cout << "Enter a string ";
string pattern;
std::getline(cin, pattern);
string text;
fstream file_("test.txt");
if(file_.is_open()){
while(std::getline(file_,text))
patternCount(text,pattern);
file_.close();
}else
cerr<<"Failed to open file";
cout << "There are " << count <<" " << pattern << " in your text.\n";
return 0;
}
void patternCount(string text, string pattern){
size_t nPos = text.find(pattern, 0);
while (nPos != string::npos) {
nPos = text.find(pattern, nPos + 1);
++count;
}
}
The Problem
Your code was good, there were no bugs in patternCount function.
But You were reading the file in an incorrect way. See, everytime you call std::getline(file_, text), the old result of the _text are overwritten by new line. So, in the end of the loop, when you pass text to patternCount function, your text only contains the last line of the file.
The Solution
You could have solved it in two ways:
As mentioned above, you could run patternCount() to each line in while loop and update a global count variable.
You could append all the lines to text in while loop and at last call the patternCount function.
Whichever you prefer, I have implemented the first, while second one is in other answers.