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I have to replace all the 'a' characters with "aaa" in a string.
So, what I'm trying to do. If the input is water, the output has to be like this: waaater.
I tried this:

#include <iostream> #include <string.h> using namespace std; int main () { char s[100], t[100]; int i; cout << "The string is: " << endl; cin.get (s, 100); strcpy (t, s); for (i = 0; i < strlen (s); i++) if (s[i] == 'a') strcat (strcpy (s + i, "aa"), t + i); cout << "The modified string is: " << endl; cout << s; }

but the output is Segmentation fault (core dumped).
I don't know what's wrong and I can't figure out what to change where.

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    Is there a reason that you cannot use std::string ?? Commented Mar 14, 2021 at 21:59
  • @mathematician1975 If I use std::string, will it work? Commented Mar 14, 2021 at 22:04
  • If you really want to, you can do this with native arrays by using a reader and a writer. The writer to the target buffer advances once char per read from the reader buffer unless the char is an a, in which case it writes three times, advancing after each. Be sure to terminate the target string when done, and of course, limit check. Commented Mar 14, 2021 at 22:07
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    What you're doing wrong is, each time through the loop you check the next letter of s and if you find an a you insert 2 more a into s but then the next letter in s is one that you just inserted so then you insert 2 more and then 2 more ... until you run out of space in s. To fix it, every time you insert into s you need to increase the index into s by 2 so you can get past what you just inserted - but you can't increase the index into t because you didn't insert anything into t. And that means you need two indexes: one for s and one for t: onlinegdb.com/r1Nuxzn7u Commented Mar 14, 2021 at 22:43
  • If you want to see how to do it with std::string see stackoverflow.com/questions/1494399/… or stackoverflow.com/questions/20406744/… Commented Mar 14, 2021 at 22:48

3 Answers 3

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A simple regex would do it.

#include <iostream> #include <regex> #include <string> int main() { std::string text = "water"; std::regex replace("a"); std::cout << std::regex_replace(text, replace, "aaa") << std::endl; return 0; }

It replaces "water" with "waaater" as you wanted.

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You are trying to modify s in-place, but after each replacement of 'a' with "aaa", you are not skipping the new as you just inserted, so you end up replacing them as well, ie water -> waaater -> waaaaater -> waaaaaaater, and so on in an endless loop until an error occurs.

Try this instead:

#include <iostream> #include <string.h> using namespace std; int main () { char s[100], t[100]; int i, j; cout << "The string is: " << endl; cin.get (s, 100); strcpy (t, s); for (i = 0, j = 0; i < strlen (s); i++, j++) { if (s[i] == 'a') { strcat (strcpy (s + i, "aa"), t + j); i += 2; } } cout << "The modified string is: " << endl; cout << s; }

Demo

Alternatively:

#include <iostream> #include <string.h> using namespace std; int main () { char s[100]; cout << "The string is: " << endl; cin.get(s, 100); int len = strlen(s); for (int i = 0; i < len; i++) { if (s[i] == 'a') { if ((len+3) > sizeof(s)) break; // no more space! memmove(s+(i+3), s+(i+1), sizeof(s)-(i+1)); s[i+1] = 'a'; s[i+2] = 'a'; i += 2; len += 2; } } cout << "The modified string is: " << endl; cout << s; }

Demo

However, if you use std::string instead of char[], you can then use the std::string::find() and std::string::replace() methods, eg:

#include <iostream> #include <string> using namespace std; int main () { string s; cout << "The string is: " << endl; cin >> s; string::size_type pos = 0; do { pos = s.find('a', pos); if (pos == string::npos) break; s.replace(pos, 1, "aaa"); pos += 3; } while (true); cout << "The modified string is: " << endl; cout << s; }

Demo

Alternatively, use std::string::insert() instead, eg:

#include <iostream> #include <string> using namespace std; int main () { string s; cout << "The string is: " << endl; cin >> s; string::size_type pos = 0; do { pos = s.find('a', pos); if (pos == string::npos) break; s.insert(pos, "aa"); // or: s.insert(pos, 2, 'a'); pos += 3; } while (true); cout << "The modified string is: " << endl; cout << s; }

Demo

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Assume old_string = "water". It's length is 5. It has 1 'a'. Therefore, new_string should be of length new_length = old_length + (2 * number_of_a). In our case, 7. Declare a char array with size new_length + 1 (+1 for null char).

char new_string[8];

Set a variable j to 0. Now run a while loop from 0 to 7 inclusive with i being the loop variable. Inside the loop, first check if i equals 7. If YES, set new_string[i] = '\0'; ELSE, set new_string[i] = old_string[j]; Inside the same ELSE block, now check if old_string[j] == 'a' If YES, then increment i and set new_string[i] = old_string[j]; Perform the previous line one more time. Now, outside the ELSE block, increment j, increment i.

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