7.3. Extracting characters from a string

Strings are called “strings” because they are made up of a sequence, or string, of characters. Imagine a string of beads, where each character is a bead in the string.

The first operation we are going to perform on a string is to extract one of the characters. C++ uses square brackets ([ and ]) for this operation.

Take a look at the active code below. We extract the character at index 1 from string fruit using [ and ].

#include <iostream> #include <string> int main() { std::string fruit = "orange"; char letter = fruit[1]; std::cout << letter; }

The expression fruit[1] indicates that I want character number 1 from the string named fruit. The result is stored in a char named letter. When I output the value of letter, I get a surprise:

r

r is not the first letter of "orange". Unless you are a computer scientist. Programmers (and most programming languages) always start counting from zero. The 0th letter (“zeroeth”) of "orange" is o. The 1th letter (“oneth”) is r and the 2th (“twoeth”) letter is a.

Note

In C++, indexing begins at 0!

If you want the zereoth letter of a string, then you have to put zero in the square brackets.

The active code below accesses the first character in string fruit.

#include <iostream> #include <string> int main() { string fruit = "orange"; char letter = fruit[0]; cout << letter << endl; }

Q-3: What would replace the “?” in order to access the letter ‘b’ in the string below?

#include <string>

int main () {
  std::string bake = "bake a cake!";
  char letter = bake[?];
}

• 1
• Don't forget that computer scientists do not start counting at 1!
• 0
• Yes, this would access the letter 'b'.
• 2
• This would access the letter 'k'.

Q-4: What is printed when the code below is run?

#include <iostream>
#include <string>

int main () {
  std::string lunch = "hello";
  std::string person = "deejay";
  lunch[0] = lunch[3];
  std::cout << lunch;
}

• lunch
• When we cout a string we print its content not its name.
• jello
• Carefully check which string(s) we are indexing into.
• lello
• Correct! We copy the 'l' from position 3 of "hello" to position 0.
• heljo
• Consider which string(s) we are indexing into.

Q-5: Click on each spot where a character in a string is accessed.Remember, square brackets [] are used to access a character in a string.

int main() {
    string fruit = "apple";
    char letter = fruit[2];
    cout << fruit << endl;
    cout <<  fruit[4]  << endl;
}

Construct a block of code that correctly prints the letter “a”.

        string x;

x = "It is warm outside!";

x = "It is warm outside" #paired

cout << x[7] << endl;

cout << x[8] << endl; #distractor
        

You have attempted of activities on this page