adding the string function example

lectures/01_introduction/README.md

@@ -287,6 +287,13 @@ object. There are several ways of constructing string objects:
 
 This [example program](getline.cpp) is the starting point to most of your homeworks. It shows how you can read information from a file, and write information into another file.
 
+## 1.16 Two Useful String Functions
+
+- find()
+- substr()
+
+This [example program](strings.cpp) which uses these two string functions, can also be very useful in your homeworks.
+
 <!--## 1.16	C++ vs. Java
 
 - Standard C++ library std::string objects behave like a combination of Java String and StringBuffer objects. If you aren’t sure of how a std::string member function (or operator) will behave, check its semantics or try it on small examples (or both, which is preferable).

lectures/01_introduction/strings.cpp

@@ -0,0 +1,33 @@
+#include <iostream>
+
+int main(int argc, char* argv[]){
+	std::string line("Why not change the world? Because I don't know how, do you know?");
+
+        int start = 0;
+	// starting from the position start, and search the string variable line,
+        // to find the first question mark.
+        int end = line.find("?", start);
+	int len = end - start;
+	// go from start to end, but exclude the character at end.
+	// when we use the substr(start, length) function on a std::string, 
+	// the substring includes the character at the start position, 
+	// and the length of the substring is length. 
+	// It does not include the character at the position start + length.
+        std::string myString = line.substr(start, len);
+
+	// print myString to console.
+	std::cout << myString << std::endl;
+	
+	start = end+1;
+	// with an updated start position,
+        // we now find the second question mark.
+        end = line.find("?", start);
+	len = end - start;
+	// go from start to end, but exclude the character at end.
+        myString = line.substr(start, len);
+
+	// print myString to console.
+	std::cout << myString << std::endl;
+
+	return 0;
+}