How to set end condition of string iterator in C++? - string

Question

i'm new in C++. I met a problem when doing exercise in C++ Primer 5th edition(Ex 9.43). The for loop can't stop in my function find_and_replace. Here is the code:
#include <iostream>
#include <string>
int find_and_replace(std::string& org_str, const std::string& str4find, const std::string& str4replace);
int main(int argc, char const *argv[])
{
std::string str("I am a very loooooong string to be process!");
int find_times;
find_times = find_and_replace(str, "a", "###");
std::cout << find_times << std::endl;
std::cout << str << std::endl;
return 0;
}
int find_and_replace(std::string& org_str, const std::string& str4find, const std::string& str4replace)
{
int find_times = 0;
if (org_str.size() < str4find.size())
{
std::cout << "Error: The original string is too short!" << std::endl;
return find_times;
}
else if (org_str == str4find)
{
org_str.assign(str4replace);
++find_times;
return find_times;
}
for (auto i = org_str.begin(), j = i + str4find.size(); j != org_str.end(); )
{
std::string temp(i, j);
// std::cout << temp << std::endl;
if (temp == str4find)
{
j = org_str.erase(i, j);
org_str.insert(i, str4replace.begin(), str4replace.end());
// std::cout << org_str << std::endl;
// std::cout << *i << std::endl;
j = i + str4find.size();
// std::cout << *j << std::endl;
++find_times;
}
else
{
++i;
++j;
}
}
if (org_str.substr(org_str.size() - str4find.size()) == str4find)
{
org_str.erase(org_str.size() - str4find.size(), str4find.size());
org_str.insert(org_str.end(), str4replace.begin(), str4replace.end());
++find_times;
}
return find_times;
}
I update iterator j and i after erase and insert operation, so i think, i and j are not invalid.
Anyone can tell me why the end condition: j != org_str.end() is not work?
SOLVED!
Although the cout output is correct, the i iterator is already invalid. Here is the new code. Just change few lines in for-loop.
#include <iostream>
#include <string>
int find_and_replace(std::string& org_str, const std::string& str4find, const std::string& str4replace);
int main(int argc, char const *argv[])
{
std::string str("I am a very loooooong string to be process!");
int find_times;
find_times = find_and_replace(str, "a", "###");
std::cout << find_times << std::endl;
std::cout << str << std::endl;
return 0;
}
int find_and_replace(std::string& org_str, const std::string& str4find, const std::string& str4replace)
{
int find_times = 0;
if (org_str.size() < str4find.size())
{
std::cout << "Error: The original string is too short!" << std::endl;
return find_times;
}
else if (org_str == str4find)
{
org_str.assign(str4replace);
++find_times;
return find_times;
}
for (auto i = org_str.begin(), j = i + str4find.size(); j != org_str.end(); )
{
std::string temp(i, j);
// std::cout << temp << std::endl;
if (temp == str4find)
{
i = org_str.erase(i, j);
//CHANGE BEGIN
//org_str.insert(i, str4replace.begin(), str4replace.end());
for (auto k = str4replace.begin(); k != str4replace.end(); ++k, ++i)
{
i = org_str.insert(i, *k);
}
//CHANGE END
// std::cout << org_str << std::endl;
// std::cout << *i << std::endl;
j = i + str4find.size();
// std::cout << *j << std::endl;
++find_times;
}
else
{
++i;
++j;
}
}
if (org_str.substr(org_str.size() - str4find.size()) == str4find)
{
org_str.erase(org_str.size() - str4find.size(), str4find.size());
org_str.insert(org_str.end(), str4replace.begin(), str4replace.end());
++find_times;

Answer 1

The i iterator become invalid after calling erase and insert, you should use the return values of that functions or recalculate the position starting from s.begin() again.
#include <iostream>
#include <string>
using std::cout;
using std::string;
int find_and_replace( string &s, const string &oldVal, const string &newVal ) {
int find_times = 0;
if ( s.size() < oldVal.size() ) {
std::cout << "Error: The original string is too short!" << std::endl;
return find_times;
}
if ( s == oldVal ) {
s.assign(newVal);
return ++find_times;
}
for ( auto i = s.begin(); i != s.end(); ++i ) {
// compare using iterators
bool is_equal = true;
auto k = i;
for ( auto j = oldVal.begin(); j != oldVal.end(); ++j, ++k ) {
if ( k == s.end() || *j != *k ) {
is_equal = false;
break;
}
}
if ( is_equal ) {
auto ie = s.erase(i, i + oldVal.size());
// This may not work with older version of g++
auto ii = s.insert(ie, std::begin(newVal), std::end(newVal));
i = ii + newVal.size() - 1;
++find_times;
}
}
return find_times;
}
int main() {
string str("I am a very loooooong string to be process!");
int find_times;
find_times = find_and_replace(str, "a", "###");
cout << find_times << '\n';
cout << str << '\n';
return 0;
}
The output (as you can see here) is:
2
I ###m ### very loooooong string to be process!