How to replace Hi with Bye in a file - c++

Question

I want to replace hi with a bye by reading a file and outputting another file with the replaced letters.
#include <iostream>
#include <fstream>
using namespace std;
int main() {
ifstream myfile;
ofstream output;
output.open("outputfile.txt");
myfile.open("infile.txt");
char letter;
myfile.get(letter);
while (!myfile.eof()) {
if (letter == 'H') {
char z = letter++;
if (z == 'i')
output << "BYE";
}
else output << letter;
}
output.close();
myfile.close();
return 0;
}
My outputs are repeated capital I's that is repeated infinity times.
Here is my input file
Hi
a Hi Hi a
Hi a a Hi

Answer 1

Don't check eof
The eof method is returning the location of the input stream read pointer, and not the status of the get. It is more like telling you whether or not get will succeed, so you could write something like:
while (!myfile.eof()) {
char letter;
myfile.get(letter);
//...
}
In this way, you would at least be getting a new letter at each iteration, and the loop ends when the read pointer reaches the end of the input.
But, there are other cases that might cause the get to not succeed. Fortunately, these are captured by the stream itself, which is returned by get. Testing the status of the stream is as easy as treating the stream as a boolean. So, a more idiomatic way to write the loop is:
char letter;
while (myfile.get(letter)) {
//...
}
Peek at the next letter
When you want to look at the next letter in the input following the detected 'H', you perform an increment.
char z = letter++;
But, this does not achieve the desired result. Instead, it just sets both letter and z variables to the numerical successor of 'H' ('H' + 1), and does not observe the next letter in the input stream.
There is another method you can use that is like get, but leaves the input in the input stream. It is called peek.
char z;
auto peek = [&]() -> decltype(myfile) {
if (myfile) z = myfile.peek();
return myfile;
};
if (peek()) {
//...
}
And now, you can check the value of z, but it is still considered input for the next get on letter.
Close to what you implemented
So, the complete loop could look like:
char letter;
while (myfile.get(letter)) {
if (letter == 'H') {
char z;
auto peek = [&]() -> decltype(myfile) {
if (myfile) z = myfile.peek();
return myfile;
};
if (peek() && z == 'i') {
myfile.get(z);
output << "BYE";
continue;
}
}
output << letter;
}
With this approach, you will be able to correctly handle troublesome cases like HHi as input, or the last letter in the input being an H.

Answer 2

Your two lines:
myfile.get(letter);
while (!myfile.eof()) {
are wrong.
First off you only read letter once, hence your infinite loop.
Secondly you don't use eof in a while loop.
You want something more like:
while (myfile.get(letter)) {
Also:
char z = letter++;
is wrong, you want to read another letter:
myfile.get(z);
but you have to be careful that you get something, so
if(!myfile.get(z)) {
output << letter;
break;
}
So finally:
char letter;
while (myfile.get(letter)) {
if (letter == 'H') {
char z;
if(!myfile.get(z)) {
output << letter;
break;
}
if (z == 'i') {
output << "BYE";
}
else output << letter << z;
}
else output << letter;
}
But now we are consuming the character after any H which may not be desirable.
See #jxh's answer for a way to do this with look ahead.

Answer 3

There is a dedicated function to replace patterns in strings. For example, you could use std::regex_replace. That is very simple. We define, what should be searched for and with what that would be replaced.
Some comments. On StackOverflow, I cannot use files. So in my example program, I use a std::istringstream instead. But this is also an std::istream. You can use any other std::istream as well. So if you define an std::ifstream to read from a file, then it will work in the same way as the std::istringstream. You can simply replace it. For the output I use the same mechanism to show the result on the console.
Please see the simple solution:
#include <iostream>
#include <sstream>
#include <regex>
// The source file
std::istringstream myfile{ R"(Hi
a Hi Hi a
Hi a a Hi)" };
// The destination file
std::ostream& output{ std::cout };
int main() {
// Temporary string, to hold one line that was read from a file
std::string line{};
// Read all lines from the file
while (std::getline(myfile, line)) {
// Replace the sub-string and write to output file
output << std::regex_replace(line, std::regex("Hi"), "Bye") << "\n";
}
return 0;
}