Why does replacing a string replace part of another string? - c++

Question

I wrote a small piece of code today about replacing a word from a text file.
Though it replaces the given word but it also removes some spaces and some part of other string.
I want it to replace given word only while keeping rest as it as.
I don't know what should I do. Any help would be appreciated!
Original Data of file:
Is anyone there?
Who survived?
Somebody new?
Anyone else but you
On a lonely night
Was a burning light
A hundred years, we'll be born again
Output when replaced "anyone" by "porter":
Is anyonportere?
Who survived?
Somebody new?
anporterlse but you
On a lonely night
Was a burning light
A hundred years, we'll be born again
Code:
#include<iostream>
#include<fstream>
#include<cstdlib>
#include<cstring>
using namespace std;
int main(int argc , char* argv[])
{
string old_word,new_word;
int no=0;
old_word=argv[1];
new_word=argv[2];
if(argc<4)
{
cout<<"\nSome Arguments are missing";
return 0;
}
if(strlen(argv[1])!=strlen(argv[2]))
{
cout<<"\nReplacement is not possible as size of New wor dis not equal to old word";
return 0;
}
fstream obj;
obj.open(argv[3],ios::in|ios::out);
if(!obj)
{
cout<<"\nError in file creating";
return 0;
}
string fetch_word;
while(1)
{
if(obj.eof())
{
break;
}
else
{
int pos=obj.tellg();
obj>>fetch_word;
if(fetch_word==old_word)
{
no++;
obj.seekp(pos);
obj<<new_word;
}
}
}
if(no==0)
{
cout<<"\nNo Replacement Done . Zero Replacement Found";
}
else
{
cout<<"\nReplacement Done . Replacement Found ="<<no<<endl;
}
obj.close();
return 0;
}

Answer 1

If we take the string "Is anyone there?"
After read the word "Is" the read head is on the space after the "Is" so tellg will return 2.
Now you're reading the next word, you skip white spaces and begin reading untill next white space character, you are reading the "anyone" word and put its replacement in the taken position (2).
so it should give you the string: "Isportere there?"
Not what you ment for, but not the result you've got.
to fix it you should ignore white spaces before reading the position:
like this:
//#include <cwctype> for iswspace
//eat white spaces
while(iswspace(obj.peek()))
obj.ignore();
//now read head is on the beginning of a word, you can take position.
int pos=obj.tellg();
Edit
You'll have to debug and see if the tellg returns 3 in the first line before you read the word "anyone". I sugggest to add some debug print for each replacement with the possition.
like:
if(fetch_word==old_word)
{
no++;
cout<<"Replacing in pos "<< pos <<endl;
obj.seekp(pos);
obj<<new_word;
}
Now you can check:
Does the pos was correct? (you can try to seekg and read the word again)
Does the seekp was succes? (you can use tellp to check!)
What happen when you just do obj.seekp(3); obj<<"porter"; does it replace the string in the correct position?

Answer 2

#include <cctype>
#include <cstdlib>
#include <string>
#include <fstream>
#include <iostream>
int main()
{
std::string old_word{ "anyone" };
std::string new_word{ "porter" };
if (old_word.length() != new_word.length()) {
std::cerr << "Sorry, I can only replace words of equal length :(\n\n";
return EXIT_FAILURE;
}
char const *filename{ "test.txt" };
std::fstream obj{ "test.txt", std::ios::in | std::ios::out };
if (!obj.is_open()) {
std::cerr << "Couldn't open \"" << filename << "\" for reading and writing.\n\n";
return EXIT_FAILURE;
}
std::string word;
for (std::streampos pos; pos = obj.tellg(), obj >> word;) {
if (word == old_word) {
obj.seekg(pos); // set the "get" position to where it were before extracting word
while (std::isspace(obj.peek())) // for every whitespace we peek at
obj.get(); // discard it
obj.seekp(obj.tellg()); // set the "put" position to the current "get" position
obj << new_word; // overwirte word with new_word
obj.seekg(obj.tellp()); // set the "get" position to the current "put" position
}
}
}