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.

Fig. 7.1 Characters in a 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 ].

Accessing a string character

#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.

Accessing a string character

#include <iostream>
#include <string>

int main() {
    string fruit = "orange";
    char letter = fruit[0];
    cout << letter << '\n';
}

Q1

What would replace the "?" in order to access the letter "b" in the string below?

#include <iostream>
using namespace std;

int main () {
  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".

Check answer

Q2

What is printed when the code below is run?

#include <iostream>
using namespace std;

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

lunch

When we <code>cout</code> a <code>string</code> 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.

Check answer

Q3

Click on each spot where a character in a string is accessed.

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

Show Hints

Q4

Construct a block of code that correctly prints the letter "a".

1. cout << x[7] << '\n';

   ↑ ↓ ← → ✓ Use

2. cout << x[8] << '\n'; #distractor

   ↑ ↓ ← → ✓ Use

3. string x;

   ↑ ↓ ← → ✓ Use

4. x = "It is warm outside!";

   ↑ ↓ ← → ✓ Use

5. x = "It is warm outside"

   ↑ ↓ ← → ✓ Use

Check answer