Writing Programs with Scanner Input and Tracing Their Outputs (ICSE Class 9 Java)

Understand Writing Programs with Scanner Input and Tracing Their Outputs

Writing Programs with Scanner Input and Tracing Their Outputs: key facts

Order of parts

• •comment
• •import
• •class header
• •main()
• •declarations
• •Scanner object
• •input
• •statements
• •output

The CISCE structure.

Average of three

• •(a + b + c) / 3.0
• •divide by a decimal
• •or store in a double

Integer division otherwise drops the fraction.

Hours and minutes

• •hours = total / 60
• •minutes = total % 60
• •total in minutes

Integer division and remainder.

Swap using a third variable

• •t = a
• •a = b
• •b = t

Three assignments in this order.

How to answer Writing Programs with Scanner Input and Tracing Their Outputs questions

1. 1Decide the inputs, outputs and the formula.
2. 2Choose a type for each variable and the matching Scanner method.
3. 3Write the structure: import, class, main(), declarations, Scanner object.
4. 4Prompt with print() and read each value.
5. 5Write the statements and print the result with a label; check integer division.

Writing Programs with Scanner Input and Tracing Their Outputs in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Writing Programs with Scanner Input and Tracing Their Outputs

Short memory aids. Say each one aloud once, then use it on a question the same day.

• I-P-O

  Input, Process, Output: the three jobs of a simple program.

  Use it when: Planning a program.

• Type decides the method

  int uses nextInt(), double uses nextDouble().

  Use it when: Choosing a Scanner method.

• Make one operand decimal

  Divide by 3.0 to keep the fraction.

  Use it when: Averages.

Writing Programs with Scanner Input and Tracing Their Outputs: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

import java.util.*;
class Calc
{
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
int a = sc.nextInt();
int b = sc.nextInt();
int s = a + b, d = a - b, m = a * b;
double q = (double) a / b;
System.out.println(s + " " + d + " " + m + " " + q);
}
}

int total = sc.nextInt();
int h = total / 60;
int m = total % 60;
System.out.println(h + " h " + m + " min");

double price = sc.nextDouble();
int qty = sc.nextInt();
double amount = price * qty;
double discount = amount * 10 / 100;
double net = amount - discount;
System.out.println(amount + " " + discount + " " + net);

int a = 12, b = 20, t;
t = a;
a = b;
b = t;
System.out.println(a + " " + b);

Where students lose marks in Writing Programs with Scanner Input and Tracing Their Outputs

The slips examiners see most often, and what to write instead.

• ✗Mistake: Wrong Scanner method

  The type of the variable is ignored.

  Wrong working

  double p = sc.nextInt();

  Right working

  double p = sc.nextDouble();

  ✓Do this: Match the method to the type.

  Usually costs: 1 mark

• ✗Mistake: Integer division in an average

  Both operands are int.

  Wrong working

  (a + b + c) / 3 gives a whole number

  Right working

  (a + b + c) / 3.0 keeps the fraction

  ✓Do this: Divide by a decimal.

  Usually costs: 1 to 2 marks

• ✗Mistake: No prompt before input

  The user does not know what to type.

  Wrong working

  int n = sc.nextInt();

  Right working

  System.out.print("Enter n: "); int n = sc.nextInt();

  ✓Do this: Always print a prompt first.

  Usually costs: 1 mark

Practice questions on Writing Programs with Scanner Input and Tracing Their Outputs

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which statement creates the keyboard input object?
   (a) Scanner sc = new Scanner(System.in); (b) Scanner sc = Scanner(System.in); (c) new Scanner sc(System.in); (d) sc = Scanner;

   Hint

   Use new.

   Answer

   (a) Scanner sc = new Scanner(System.in);

2. Q2 · 2 marks · Very short answerBoard level

   Write statements to read two integers x and y from the keyboard, with prompts.

   Hint

   Prompt with print().

   Answer

   Scanner sc = new Scanner(System.in);
   System.out.print("Enter x: ");
   int x = sc.nextInt();
   System.out.print("Enter y: ");
   int y = sc.nextInt();

3. Q3 · 2 marks · Short answerHarder board level

   Give the output:

   int m1 = 78, m2 = 85, m3 = 92;
   double avg = (m1 + m2 + m3) / 3.0;
   System.out.println(avg);

   Hint

   Add first, then divide by a decimal.

   Answer

   85.0 (78 + 85 + 92 = 255 and 255 / 3.0 = 85.0)

4. Q4 · 3 marks · Short answerHarder board level

   A program has int avg = (m1 + m2) / 2; and for m1 = 75 and m2 = 80 it shows 77 while the true average is 77.5. Explain and correct.

   Hint

   Which types are involved?

   Answer

   Both operands are int, so 155 / 2 drops the fraction and gives 77, and avg is an int anyway. Correct: double avg = (m1 + m2) / 2.0; which gives 77.5.

5. Q5 · 4 marks · Short answerHOTS / competency

   Write a program that reads the price of 1 kg of rice (double) and a weight in grams (int) and prints the cost. For the input 80 and 250, state the output if the program prints only the cost.

   Hint

   Cost = price * grams / 1000.

   Answer

   import java.util.*;
   class RiceCost
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Price per kg: ");
   double price = sc.nextDouble();
   System.out.print("Weight in grams: ");
   int g = sc.nextInt();
   double cost = price * g / 1000;
   System.out.println(cost);
   }
   }
   For 80 and 250 the output is 20.0 (80.0 x 250 = 20000.0 and 20000.0 / 1000 = 20.0).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which line makes Scanner available?Show answer

   import java.util.*;

2. 2.Method to read an int?Show answer

   nextInt()

3. 3.Method to read a decimal into a double?Show answer

   nextDouble()

4. 4.Which expression gives whole hours from minutes m?Show answer

   m / 60

5. 5.Which expression gives the leftover minutes?Show answer

   m % 60

6. 6.How to avoid integer division in an average?Show answer

   Divide by a decimal such as 3.0.

Writing Programs with Scanner Input and Tracing Their Outputs: frequently asked questions