使用字符串查找和替换while循环内部时的std :: out_of_range

Question

So I have a task to convert all occurrences of some word in one string to another string. But there is problem with condition of while loop which makes this error

terminate called after throwing an instance of 'std::out_of_range'

what(): basic_string::replace

This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information. Process returned 3 (0x3) execution time : 2.751 s

My code is:

#include <iostream>
#include <string>
using namespace std;
int main ()
{
    string str2("three");
    string str("one three two four three three");
    while ( str.find(str2) != NULL ){
    str.replace(str.find(str2),str2.length(),"five");
    cout << str << endl; // i put it inside loop to see output
    }
    cout << str << endl;
    return 0;
}

Any suggestions?

Answer 1

You are checking if str.find(str2) had an occurrence comparing it to NULL , but this is wrong, because NULL is a macro that isn't meant for that and often expands into 0 , which can be a valid index. You should compare it to std::string::npos . After doing this change, your code will work.

Edit : std::string::npos corresponds to 18446744073709551615 when testing on coliru. So that clearly isn't a valid index in your string.

Answer 2

This condition

while ( str.find(str2) != NULL ){

does not make sense because a call of find can return std::string::npos that is not equal to zero. In this case the code has undefined behavior.

You can apply the following approach

std::string str2("three");
std::string str("one three two four three three");

const char *five = "five";
size_t n = std::strlen(five);

for (std::string::size_type pos = 0;
    ( pos = str.find(str2, pos) ) != std::string::npos; pos += n)
{
    str.replace(pos, str2.length(), five);
}

Answer 3

it's caused because str.find(str2) returns -1 if str2 is not existed in the str . You can use a variable pos to save the found position, so that you will not need to re-invoke find function. The solution is supposed as following:

#include <iostream>
#include <string>
using namespace std;
int main () {
  string str2("three");
  string str("one three two four three three");
  int pos = str.find(str2);
  while (pos > 0) {
    str.replace(pos, str2.length(), "five");
    pos = str.find(str2);
    cout << str << endl; // i put it inside loop to see output
  }
  cout << str << endl;
  return 0;
}