How do you use string.erase and string.find? - c++

Question

Why cant i call string.find in string.erase like so: str.erase(str.find(a[1]),str.size())?
edit:code added
#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
// html tags
string tags[5]={"<!--...-->","<!DOCTYPE>","<a>","<abbr>","<acronym>"};
//
//check if string exists
int boolStringExists(string a, string b)
{
if(a.find(b)>0)
{
return 1;
}
if(a.find(b)<=0)
{
return 0;
}
}
//erase tag from string a
void eraseTags(string a,string b[])
{
for(int i=0; i<5;i++)
{
int x=(boolStringExists(a,b[i]));
while (x>0)
{
a.erase(a.find(b[i]),b[i].size());
x=(boolStringExists(a,b[i]));
}
}
}
int _tmain(int argc, _TCHAR* argv[])
{
fstream file;
file.open("h:\\a.htm");
string k,m;
while(getline(file, k))
m += k ;
eraseTags(m,tags);
return 0;
}
Gives this message: "this application has requested the runtime to terminate it in an unusual way.Please contact the application's support team for more information."

Answer 1

If the string is not found, find returns string::npos, and then your code wouldn't work and will give runtime error. See this gives error : https://ideone.com/NEhqn
So better write this:
size_t pos = str.find(a[1]);
if ( pos != std::string::npos)
str.erase(pos); //str.size() is not needed!
Now this doesn't give error : https://ideone.com/IF2Hy

Answer 2

There's nothing wrong with that call (assuming a[1] exists and is found in str at least once)
#include <iostream>
#include <string>
int main()
{
std::string str = "Hello, world!";
std::string a = "wwwwww";
str.erase(str.find(a[1]), str.size());
std::cout << str << '\n';
}
test run: https://ideone.com/8wibR
EDIT: Your full source code fails to check if b[1] is actually found in str. The function boolStringExists() returns 1 if a.find(b) is greater than zero, and the value of std::string::npos which it returns when b is not found in a IS greater than zero.
To fix this while keeping the rest of your logic intact, change that function to
//check if string exists
bool boolStringExists(string a, string b)
{
return a.find(b) != string::npos;
}

Answer 3

It seems you want to erase everything that comes after str.find(a[1]). In that case you can omit the second argument.
#include <iostream>
#include <string>
int main(int argc, char *argv[]) {
std::string str = "Hello, world!";
std::string needle = "o,";
str.erase(str.find(needle));
std::cout << str << "\n";
}
In this example I used needle instead of a[1], but the principle is the same.