Check a string in another string in C++ - c++

Question

#include <iostream>
#include <string>
using namespace std;
int main() {
string str_1 = "Gandalf";
string str_2 = "dal";
for (int i = 0; i <= str_1.length() - 2; i++)
for (int j = 0; j <= str_2.length(); j++) {
if (str_2[j] == str_1[i]) {
if (str_2[j + 1] == str_1[i + 1]) {
if (str_2[j + 2] == str_1[i + 2])
cout << "true";
}
}
}
return 0;
}
I can do it but if length of str_2 is 4 characters, program doesn't work.
I want that program can work for every length of string
but how?

Answer 1

The function find below basically reproduces the behaviour of std::string::find (without the starting position parameter). You need to:
loop through the outer string, and at each step:
loop through the second string checking each character.
If any of these fail, drop back to the outer loop.
If we make it all the way through the inner loop, then the second string is there, and return the current position in the outer loop.
If we run out of space in the first string, just skip the rest.
Hopefully the comments make this clear. I also include a little utility function to turn the found position into true/false, and some tests.
#include <iomanip>
#include <iostream>
#include <string>
std::string::size_type find(const std::string& s1,
const std::string& s2)
// return the position of s2 within s1,
// else npos if it is not present.
{
using size_type = std::string::size_type;
size_type curPos = 0;
size_type lim = s1.size();
size_type innerLim = s2.size();
for (; curPos<lim; ++curPos) { // loop through s1
if (lim < curPos+innerLim) {
break; // not enough space left
}
size_type innerPos = 0;
for(; innerPos < innerLim // loop through s2, while matching
&& curPos + innerPos < lim
&& s1[innerPos+curPos] == s2[innerPos];
++innerPos) ; // do nothing in the loop
if (innerPos == innerLim) { // matched the whole loop
return curPos;
}
}
return std::string::npos; // never matched
}
bool contains(const std::string& s1,
const std::string& s2)
{
return find(s1, s2)!=std::string::npos;
}
int main()
{
std::cout
<< std::boolalpha
<< contains("abc", "") << '\n' // true
<< contains("abc", "abc") << '\n' // true
<< contains("abc", "bc") << '\n' // true
<< contains("abc", "abcd") << '\n' // false
<< contains("abc", "abd") << '\n' // false
<< contains("abc", "xyz") << '\n';// false
}
This does more than what you really need, but it most closely models the "real" answer (use the facilities the language provides). Plus it makes it not a great homework answer, but contains all the clues to write your homework answer.

Answer 2

You could try something like this:
for (int i = 0; i < str_1.length() - str_2.length(); i++) {
bool is_same = true;
for (int j = 0; j < str_2.length(); j++) {
if (str[i + j] != str_2[j]) {
is_same = false;
break;
}
}
if (is_same) {
std::cout << "true" << std:endl;
}
}
It iterates over every character in str_1 and checks whether the character sequence starting at that point is the same as str_2.