Control Statements

Question 1

Write a program that reads characters from the keyboard until a period is received. Have the program count the number of spaces. Report the total at the end of the program.

Answer 1

// Count spaces.
class Spaces {
public static void main(String[] args)
throws java.io.IOException {

char ch;
int spaces = 0;

System.out.println("Enter a period to stop.");

do {
ch = (char) System.in.read();
if(ch == ‘ ‘) spaces++;
} while(ch != ‘.’);

System.out.println("Spaces: " + spaces);   } }

Question 2

Show the general form of the if-else-if ladder.

Answer 2

if(condition)
statement;
else if(condition)
statement;
else if(condition)
statement;
.
.
.
else
statement;

Question 3

Given

if(x < 10)
if(y > 100) {
if(!done) x = z;
else y = z;
}
else System.out.println(“error”); // what if?

to what if does the last else associate?

Answer 3

The last else associates with if(y > 100).

Question 4

Show the for statement for a loop that counts from 1000 to 0 by –2.

Answer 4

for(int i = 1000; i >= 0; i -= 2) // …

Question 5

Is the following fragment valid?

for(int i = 0; i < num; i++)
sum += i;

count = i;

Answer 5

No; i is not known outside of the for loop in which it is declared.

Question 6

Explain what break does. Be sure to explain both of its forms.

Answer 6

A break without a label causes termination of its immediately enclosing loop or switch statement.

A break with a label causes control to transfer to the end of the labeled block.

Question 7

In the following fragment, after the break statement executes, what is displayed?

for(i = 0; i < 10; i++) {
while(running) {
if(x<y) break;
// …
}
System.out.println(“after while”);
}
System.out.println(“After for”);

Answer 7

After break executes, “after while” is displayed.

Question 8

What does the following fragment print?

for(int i = 0; i<10; i++) {
System.out.print(i + “ “);
if((i%2) == 0) continue;
System.out.println();
}

Answer 8

Here is the answer:
0 1
2 3
4 5
6 7
8 9

Question 9

The iteration expression in a for loop need not always alter the loop control variable by a fixed amount. Instead, the loop control variable can change in any arbitrary way.

Using this concept, write a program that uses a for loop to generate and display the progression 1, 2, 4, 8, 16, 32, and so on.

Answer 9

/* Use a for loop to generate the progression

1 2 4 8 16, …
*/
class Progress {
public static void main(String[] args) {

for(int i = 1; i < 100; i += i)
  System.out.print(i + " ");

}
}

Question 10

The ASCII lowercase letters are separated from the uppercase letters by 32. Thus, to convert a lowercase letter to uppercase, subtract 32 from it. Use this information to write a program that reads characters from the keyboard. Have it convert all lowercase letters to uppercase, and all uppercase letters to lowercase, displaying the result. Make no changes to any other character. Have the program stop when the user enters a period. At the end, have the program display the number of case changes that have taken place.

Answer 10

// Change case.
class CaseChg {
public static void main(String[] args)
throws java.io.IOException {
char ch;
int changes = 0;

System.out.println("Enter period to stop.");

do {
  ch = (char) System.in.read();
  if(ch >= 'a' & ch <= 'z') {
    ch -= 32;
    changes++;
    System.out.println(ch);
  }
  else if(ch >= 'A' & ch <= 'Z') {
    ch += 32;
    changes++;
    System.out.println(ch);
  }
} while(ch != '.');
System.out.println("Case changes: " + changes);   } }

Question 11

What is an infinite loop?

Answer 11

An infinite loop is a loop that runs indefinitely

Question 12

When using break with a label, must the label be on a block that contains the break?

Answer 12

Yes.