Week 6

Recall that everything inside the computer is represented in 0s and 1s (binary). That means that everything, even chars and strings are represented as integers under the hood.

ASCII is an encoding system which represents every character with a given number. For example, an uppercase A is represented by the number 65 and an uppercase Z is represented by the number 90.

Here’s a glimpse of the ASCII table that illustrates the correspondence between characters and numbers that represent them:

You should try to memorize all the ASCII values. However, you should know the following:

Let’s now write a program that prints all the English letters in the alphabet (first, uppercase A–Z, and then lowercase a–z), along with their ASCII values. The program should print something like this:

A: 65
B: 66
C: 67
...

a: 97
b: 98
c: 99
...

Here’s one way to do it:

int main()
{
    // mapping for uppercase letters
    for (int i = 65; i < 65 + 26; i++) (1)
    {
        cout << (char) i << ": " << i << endl; (2)
    }

    cout << endl;

    // mapping for lowercase letters
    for (int i = 97; i < 97 + 26; i++) (3)
    {
        cout << (char) i << ": " << i << endl;
    }
}

It turns out that we can also cast from chars to ints, so let’s write a second version of that program, where we avoid magic numbers:

#include <iostream>
using namespace std;

int main()
{
    // mapping for uppercase letters
    for (char c = 'A'; c <= 'Z'; c++) (1)
    {
        cout << c << ": " << (int) c << endl; (2)
    }

    cout << endl;

    // mapping for lowercase letters
    for (char c = 'a'; c <= 'z'; c++) (3)
    {
        cout << c << ": " << (int) c << endl;
    }
}

Being able to represent characters with numbers gives us the ability to manipulate them, which opens lots of possibilities (e.g. capitalizing strings). More on this in a week or so!

Write a program that prints this Battleship board using loops. Take care to ensure that all spaces match this example:

   1  2  3  4  5  6  7  8  9  10
A  o  o  o  o  o  o  o  o  o  o
B  o  o  o  o  o  o  o  o  o  o
C  o  o  o  o  o  o  o  o  o  o
D  o  o  o  o  o  o  o  o  o  o
E  o  o  o  o  o  o  o  o  o  o
F  o  o  o  o  o  o  o  o  o  o
G  o  o  o  o  o  o  o  o  o  o
H  o  o  o  o  o  o  o  o  o  o
I  o  o  o  o  o  o  o  o  o  o
J  o  o  o  o  o  o  o  o  o  o

Since it’s a lot easier to print top to bottom, we’ll print the board row by row.

for (int i = 1; i <= 10; i++)
{
    cout << i << "  ";
}

for (int i = 0; i < 10; i++)
{
    cout << "o  ";
}

for (int i = 0; i < 10; i++)
{
    for (int j = 0; j < 10; j++)
    {
        cout << "o  ";
    }
    cout << endl;
}

// print rows of holes, with letters in leftmost column
for (int i = 0; i < 10; i++)
{
    cout << (char) ('A' + i) << "  ";

    // print 10 holes
    cout << endl;
}

const int BOARD_SIZE = 10;

Here’s the full solution:

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#include <iostream>
using namespace std;

int main()
{
    const int BOARD_SIZE = 10;

    // print top row of numbers
    cout << "   ";
    for (int i = 1; i <= BOARD_SIZE; i++)
    {
        cout << i << "  ";
    }
    cout << endl;

    // print rows of holes, with letters in leftmost column
    for (int i = 0; i < BOARD_SIZE; i++)
    {
        // letter
        cout << (char) ('A' + i) << "  ";

        // holes
        for (int j = 0; j < BOARD_SIZE; j++)
        {
            cout << "o  ";
        }
        cout << endl;
    }
}

Think of strings as of sequences of characters. For example, the string "hello" is a sequence of five characters:

You can get the length of the string by calling the length function on that string. For example,

string name = "Grace";
cout << name.length() << endl;

5

You can access individual characters of a string by using bracket (subscript) notation. This code accesses the first character of name:

string name = "Grace";
cout << name[0] << endl;

G

Be careful not to access the string an an invalid index! For example, the valid indices for the string "hello" are only 0, 1, 2, 3 and 4. So doing something like this is bad and might cause your program to crash:

string name = "Grace";
cout << name[5] << endl;

Write a program that asks the user for a string and than prints that string’s characters, one per line. The program should behave per the output below, wherein red underlined text represents some user’s input:

Give me a string: Lumos
L
u
m
o
s

#include <iostream>
using namespace std;

int main()
{
    // get a string from the user
    cout << "Give me a string: ";
    string s;
    getline(cin, s); (1)

    // print characters in s, one per line
    for (int i = 0; i < s.length(); i++) (2)
    {
        cout << s[i] << endl; (3)
    }
}

Pass by value copies the value from a variable or a literal into a parameter.
The original variable’s value is not changed.

Pass by reference gives the parameter a reference to the variable that was passed as an argument. The original variable’s value may be changed.

Arrays are data structures that allow us to store data of the same type in contiguous memory locations.

As an anology, think of the mailboxes in your dorm. An array is simply a block of contiguous space in memory (the mail center) that has been partitioned into identically-sized chunks (mailboxes). Each chunk can store a certain amount of data (mail) that can be accessed by an index number (mailbox number).

Note that array indices in C always start at 0!

Declare an array by specifying the data type it will store, its name, as well as its size. Here, we declare an array of 3 ints called numbers and load it with values.

int numbers[3];

numbers[0] = 1;
numbers[1] = 2;
numbers[2] = 3;

int numbers[] = {1, 2, 3};

Because arrays store data contiguously in memory, array size is fixed after array declaration. You are effectively asking the operating system to reserve the appropriate number of contiguous chunks of memory for the array’s elements. There’s no guarantee that more memory adjacent to your array will be available for use later, so arrays cannot easily grow.

Remember that the first valid index of an array is 0 and the last valid index its size - 1. Be careful not to access invalid indices of the array (< 0 or ≥ size).

int numbers[3] = 4;