Using a Two-Dimensional Array
Listing 5.20 incorporates an initialized two-dimensional array and a nested loop into a program. This time the program reverses the order of the loops, placing the column loop (city index) on the outside and the row loop (year index) on the inside. Also it uses a common C++ practice of initializing an array of pointers to a set of string constants. That is, cities is declared as an array of pointers-to-char. That makes each element, such as cities[0], a pointer-to-char that can be initialized to the address of a string. The program initializes cities[0] to the address of the "Gribble City" string, and so on. Thus, this array of pointers behaves like an array of strings.
// nested.cpp -- nested loops and 2-D array
#include <iostream>
const int Cities = 5;
const int Years = 4;
int main()
{
using namespace std;
const char * cities[Cities] = // array of pointers
{ // to 5 strings
"Gribble City",
"Gribbletown",
"New Gribble",
"San Gribble",
"Gribble Vista"
};
int maxtemps[Years][Cities] = // 2-D array
{
{96, 100, 87, 101, 105}, // values for maxtemps[0]
{96, 98, 91, 107, 104}, // values for maxtemps[1]
{97, 101, 93, 108, 107}, // values for maxtemps[2]
{98, 103, 95, 109, 108} // values for maxtemps[3]
};
cout << "Maximum temperatures for 2008 - 2011
";
for (int city = 0; city < Cities; ++city)
{
cout << cities[city] << ": ";
for (int year = 0; year < Years; ++year)
cout << maxtemps[year][city] << " ";
cout << endl;
}
// cin.get();
return 0;
}
Here is the output for the program in Listing 5.20:
Maximum temperatures for 2008 - 2011
Gribble City: 96 96 97 98
Gribbletown: 100 98 101 103
New Gribble: 87 91 93 95
San Gribble: 101 107 108 109
Gribble Vista: 105 104 107 108
Using tabs in the output spaces the data more regularly than using spaces would. However, different tab settings can cause the output to vary in appearance from one system to another. Chapter 17 presents more precise, but more complex, methods for formatting output.
More awkwardly, you could use an array of arrays of char instead of an array of pointers for the string data. The declaration would look like this:
char cities[Cities][25] = // array of 5 arrays of 25 char
{
"Gribble City",
"Gribbletown",
"New Gribble",
"San Gribble",
"Gribble Vista"
};
This approach limits each of the five strings to a maximum of 24 characters. The array of pointers stores the addresses of the five string literals, but the array of char arrays copies each of the five string literals to the corresponding five arrays of 25 char. Thus, the array of pointers is much more economical in terms of space. However, if you intended to modify any of the strings, the two-dimensional array would be a better choice. Oddly enough, both choices use the same initialization list and the same for loop code to display the strings.
Also you could use an array of string class objects instead of an array of pointers for the string data. The declaration would look like this:
const string cities[Cities] = // array of 5 strings
{
"Gribble City",
"Gribbletown",
"New Gribble",
"San Gribble",
"Gribble Vista"
};
If you intended for the strings to be modifiable, you would omit the const qualifier. This form uses the same initializer list and the same for loop display code as the other two forms. If you want modifiable strings, the automatic sizing feature of the string class makes this approach more convenient to use than the two-dimensional array approach.
Summary
C++ offers three varieties of loops: for loops, while loops, and do while loops. A loop cycles through the same set of instructions repetitively, as long as the loop test condition evaluates to true or nonzero and the loop terminates execution when the test condition evaluates to false or zero. The for loop and the while loop are entry-condition loops, meaning that they examine the test condition before executing the statements in the body of the loop. The do while loop is an exit-condition loop, meaning that it examines the test condition after executing the statements in the body of the loop.
The syntax for each loop calls for the loop body to consist of a single statement. However, that statement can be a compound statement, or block, formed by enclosing several statements within paired curly braces.
Relational expressions, which compare two values, are often used as loop test conditions. Relational expressions are formed by using one of the six relational operators: <, <=, ==, >=, >, or !=. Relational expressions evaluate to the type bool values true and false.
Many programs read text input or text files character-by-character. The istream class provides several ways to do this. If ch is a type char variable, the following statement reads the next input character into ch:
cin >> ch;
However, it skips over spaces, newlines, and tabs. The following member function call reads the next input character, regardless of its value, and places it in ch:
cin.get(ch);
The member function call cin.get() returns the next input character, including spaces, newlines, and tabs, so it can be used as follows:
ch = cin.get();
The cin.get(char) member function call reports encountering the EOF condition by returning a value with the bool conversion of false, whereas the cin.get() member function call reports the EOF by returning the value EOF, which is defined in the iostream file.
A nested loop is a loop within a loop. Nested loops provide a natural way to process two-dimensional arrays.
Chapter Review
1. What’s the difference between an entry-condition loop and an exit-condition loop? Which kind is each of the C++ loops?
2. What would the following code fragment print if it were part of a valid program?
int i;
for (i = 0; i < 5; i++)
cout << i;
cout << endl;
3. What would the following code fragment print if it were part of a valid program?
int j;
for (j = 0; j < 11; j += 3)
cout << j;
cout << endl << j << endl;
4. What would the following code fragment print if it were part of a valid program?
int j = 5;
while ( ++j < 9)
cout << j++ << endl;
5. What would the following code fragment print if it were part of a valid program?
int k = 8;
do
cout <<" k = " << k << endl;
while (k++ < 5);
6. Write a for loop that prints the values 1 2 4 8 16 32 64 by increasing the value of a counting variable by a factor of two in each cycle.
7. How do you make a loop body include more than one statement?
8. Is the following statement valid? If not, why not? If so, what does it do?
int x = (1,024);
What about the following?
int y;
y = 1,024;
9. How does cin>>ch differ from cin.get(ch) and ch=cin.get() in how it views input?
Programming Exercises
1. Write a program that requests the user to enter two integers. The program should then calculate and report the sum of all the integers between and including the two integers. At this point, assume that the smaller integer is entered first. For example, if the user enters 2 and 9, the program should report that the sum of all the integers from 2 through 9 is 44.
2. Redo Listing 5.4 using a type array object instead of a built-in array and type long double instead of long long. Find the value of 100!
3. Write a program that asks the user to type in numbers. After each entry, the program should report the cumulative sum of the entries to date. The program should terminate when the user enters 0.
4. Daphne invests $100 at 10% simple interest. That is, every year, the investment earns 10% of the original investment, or $10 each and every year:
interest = 0.10 × original balance
At the same time, Cleo invests $100 at 5% compound interest. That is, interest is 5% of the current balance, including previous additions of interest:
interest = 0.05 × current balance
Cleo earns 5% of $100 the first year, giving her $105. The next year she earns 5% of $105, or $5.25, and so on. Write a program that finds how many years it takes for the value of Cleo’s investment to exceed the value of Daphne’s investment and then displays the value of both investments at that time.
5. You sell the book C++ for Fools. Write a program that has you enter a year’s worth of monthly sales (in terms of number of books, not of money). The program should use a loop to prompt you by month, using an array of char * (or an array of string objects, if you prefer) initialized to the month strings and storing the input data in an array of int. Then, the program should find the sum of the array contents and report the total sales for the year.
6. Do Programming Exercise 5 but use a two-dimensional array to store input for 3 years of monthly sales. Report the total sales for each individual year and for the combined years.
7. Design a structure called car that holds the following information about an automobile: its make, as a string in a character array or in a string object, and the year it was built, as an integer. Write a program that asks the user how many cars to catalog. The program should then use new to create a dynamic array of that many car structures. Next, it should prompt the user to input the make (which might consist of more than one word) and year information for each structure. Note that this requires some care because it alternates reading strings with numeric data (see Chapter 4). Finally, it should display the contents of each structure. A sample run should look something like the following:
How many cars do you wish to catalog? 2
Car #1:
Please enter the make: Hudson Hornet
Please enter the year made: 1952
Car #2:
Please enter the make: Kaiser
Please enter the year made: 1951
Here is your collection:
1952 Hudson Hornet
1951 Kaiser
8. Write a program that uses an array of char and a loop to read one word at a time until the word done is entered. The program should then report the number of words entered (not counting done). A sample run could look like this:
Enter words (to stop, type the word done):
anteater birthday category dumpster
envy finagle geometry done for sure
You entered a total of 7 words.
You should include the cstring header file and use the strcmp() function to make the comparison test.
9. Write a program that matches the description of the program in Programming Exercise 8, but use a string class object instead of an array. Include the string header file and use a relational operator to make the comparison test.
10. Write a program using nested loops that asks the user to enter a value for the number of rows to display. It should then display that many rows of asterisks, with one asterisk in the first row, two in the second row, and so on. For each row, the asterisks are preceded by the number of periods needed to make all the rows display a total number of characters equal to the number of rows. A sample run would look like this:
Enter number of rows: 5
....*
...**
..***
.****
*****