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Question

This is a simple sample program:

#include <iostream>
#include <string>

using namespace std;

string replaceSubstring(string, string, string);

int main()
{
    string str1, str2, str3;

    cout << "These are the strings: " << endl;
    cout << "str1: \"the dog jumped over the fence\"" << endl;
    cout << "str2: \"the\"" << endl;
    cout << "str3: \"that\"" << endl << endl;
    cout << "This program will search str1 for str2 and replace it with str3\n\n";

    cout << "The new str1: " << replaceSubstring(str1, str2, str3);

    cout << endl << endl;
}

string replaceSubstring(string s1, string s2, string s3)
{
    int index = s1.find(s2, 0);

    s1.replace(index, s2.length(), s3);

    return s1;
}

It compiles however the function returns nothing. If I change return s1 to return "asdf" it will return asdf. How can I return a string with this function?

Accepted Answer

You never give any value to your strings in main so they are empty, and thus obviously the function returns an empty string.

Replace:

string str1, str2, str3;

with:

string str1 = "the dog jumped over the fence";
string str2 = "the";
string str3 = "that";

Also, you have several problems in your replaceSubstring function:

int index = s1.find(s2, 0);
s1.replace(index, s2.length(), s3);

std::string::find returns a std::string::size_type (aka. size_t) not an int. Two differences: size_t is unsigned, and it's not necessarily the same size as an int depending on your platform (eg. on 64 bits Linux or Windows size_t is unsigned 64 bits while int is signed 32 bits).
What happens if s2 is not part of s1? I'll leave it up to you to find how to fix that. Hint: std::string::npos ;)

Answer 2

string str1, str2, str3;

cout << "These are the strings: " << endl;
cout << "str1: \"the dog jumped over the fence\"" << endl;
cout << "str2: \"the\"" << endl;
cout << "str3: \"that\"" << endl << endl;

From this, I see that you have not initialized str1, str2, or str3 to contain the values that you are printing. I might suggest doing so first:

string str1 = "the dog jumped over the fence", 
       str2 = "the",
       str3 = "that";

cout << "These are the strings: " << endl;
cout << "str1: \"" << str1 << "\"" << endl;
cout << "str2: \"" << str2 << "\"" << endl;
cout << "str3: \"" << str3 << "\"" << endl << endl;

Answer 3

Assign something to your strings. This will definitely help.

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How to Return a String from a C++ Function? [Closed]

3 Answers

Sarah Jenkins