C++ Primer Section 1-4

Section 1-4 Basic Flow of Control

The while Statement

Basic Structure

while (condition)
    statement

Rule for while

The body part(statement part) will be executed repeatedly until the condition turns into false.

Exercises

Exercise 1-9

Write a program that uses a while to sum the numbers from 50 to 100.

Solution

In 1-9.cpp

Exercise 1-10

In addition to the ++ operator that adds 1 to its operand, there is a decrement operator (--) that subtracts 1. Use the decrement operator to write a while that prints the numbers from ten down to zero.

Solution

In 1-10.cpp

Exercise 1-11

Write a program that prompts the user for two integers. Print each number in the range specified by those two integers.

Solution

In 1-11.cpp

Example Code

In WhileStatement.cpp

The for Statement

Basic Structure

for (init; cond; incre(statement))
    statement;

init for initialization; run the body statement until the cond is false; incre usually alternates the variable to make the cond false and the program terminate.

Main Purpose

Describe the fixed pattern of iteration.

Exercises

Exercise 1-12

What does the following for loop do? What is the final value of sum ?

int sum = 0;
for (int i = -100; i <= 100; ++i)
    sum += i;

Solution

Calculate the sum from -100 to 100, which is 0.

Exercise 1-13

Rewrite the exercises from 1.4.1 using for loop.

Solution

In 1-13-1-9.cpp, 1-13-1-10.cpp and 1-13-1-11.cpp

Exercise 1-14

Compare and contrast the loops that used a for with those using a while. Are there advantages or disadvantages to using either form?

Solution

for statement is clearer. However, the while statement can be more flexible. Although these two patterns are equivalent, using the alternative in the situation suited for the other will be confusing.

Exercise 1-15

Write programs that contain the common errors discussed in the box on page 16.

Familiarize yourself with the messages the compiler generates.

Solution

// error: missing ) in parameter list for main
int main ( {
    // error: used colon, not a semicolon, after all
    std::cout << "Read each file." << std::endl:
    // error: missing quotes around string literal
    std::cout << Update master. << std::endl;
    // error: second output operator is missing
    std::cout << "Write new master." std::endl;
    // error: missing; on return statement
    return 0
}

will generate the compiler messages:

int main ( {
^
1-15-1.cpp:6:48: error: found ‘:’ in nested-name-specifier, expected ‘::’
std::cout << “Read each file.” << std::endl:
^
1-15-1.cpp:6:44: error: ‘std::endl’ is not a class or namespace
std::cout << “Read each file.” << std::endl:
^
1-15-1.cpp:8:18: error: ‘Update’ was not declared in this scope
std::cout << Update master. << std::endl;
^
1-15-1.cpp:8:25: error: expected ‘}’ before ‘master’
std::cout << Update master. << std::endl;
^
1-15-1.cpp:8:25: error: expected ‘)’ before ‘master’
1-15-1.cpp:10:10: error: ‘cout’ in namespace ‘std’ does not name a type
std::cout << “Write new master.” std::endl;
^
1-15-1.cpp:12:5: error: expected unqualified-id before ‘return’
return 0
^
1-15-1.cpp:13:1: error: expected declaration before ‘}’ token
}
^

#include<iostream>
int main()
{
    int v1 = 0, v2 = 0;
    std::cin >> v >> v2;    // error: uses "v" not "v1"
    // error: cout not defined; should be std::cout
    cout << v1 + v2 << std::endl;
    return 0;
}

will generate the compiler messages:

1-15-2.cpp: In function ‘int main()’:
1-15-2.cpp:5:17: error: ‘v’ was not declared in this scope
std::cin >> v >> v2; // error: uses “v” not “v1”
^
1-15-2.cpp:7:5: error: ‘cout’ was not declared in this scope
cout << v1 + v2 << std::endl;
^
1-15-2.cpp:7:5: note: suggested alternative:
In file included from 1-15-2.cpp:1:0:
/usr/include/c++/5/iostream:61:18: note: ‘std::cout’
extern ostream cout; /// Linked to standard output
^

Example Code

In ForStatement.cpp

Read Unknown Number of numbers from input

Since we have learned about while and for statements, we can explore this topic.

Both of the statements can fulfill the purpose.

Key to success is the return value of cin . When tested, the return value will be true as long as the read-in value is legal. If the read-in value does not suit the type of the variable or turns out to be EOF(End Of File), the return value will be false.

Exercise

Exercise 1-16

Write your own version of a program that prints the sum of a set of integers read from cin.

Solution

In 1-16.cpp, I use for statement instead of while statement in the example.

Example Code

In ReadUntilEOF.cpp

Enter EOF From Your Keyboard

On Windows, enter ctrl+z. On Max OS X or Linux, enter ctrl+d.

if Statement

Basic Structure

if (cond0)
    statement0;
else if (cond1)
    statement1;
...
else if (condn)
    statementn;
else
    statement;

Rules for if Statement

• Whenever the condition is true, the corresponding statement will be executed.
• If all the conditions are false, the else-part will be executed.
• All the parts except the first cond0 can be omitted. And once a part is omitted, they will not be executed. So there is a chance that the if statement does nothing.

Exercises

Exercise 1-17

What happens in the program(IfStatement.cpp) presented in this section if the input values are all equal? What if there are no duplicated values?

Solution

In Both situations, this program will work well and give correct answer.

But in the first situation, with all values equal, the else-part of if statement will never be executed. Correspondingly, in the second situation, with all values distinct, the if-part of if statement will never be executed.

Exercises 1-18

Compile and run the program from this section giving it only equal values as input. Run it again giving it values in which no number is repeated.

Solution

Give the identical input:

1 1 1 1 1 1 1

Corresponding output:

1 occurs 7 times

Give the distinct input:

1 2 3 4 5 6 7

Corresponding output:

1 occurs 1 times
2 occurs 1 times
3 occurs 1 times
4 occurs 1 times
5 occurs 1 times
6 occurs 1 times
7 occurs 1 times

Exercises 1-19

Revise the program you wrote for the exercises in 1.4.1 that printed a range of numbers so that it handles input in which the first number is smaller than the second.

Solution

When I wrote it for the first time in 1-11.cpp and 1-13-1-11.cpp, I designed it to be able to handle this situation.

Common Errors Presented by Compilers

1. Syntax Errors
2. Type Errors
   Assigning a value of wrong type to a variable
3. Declaration Errors
   Using a variable before declaring it or mistakenly spelling an identifier