Loops 2¶

Quick revision

Step 1: Initialization of a loop variable.

Step 2: Write while with condition.

Step 3: Write the task you want to repeat.

Step 4: Updation of loop variable.

Syntax of while loop¶

initialize
while(condition){ // loop stops when the condition fails
    // task to be repeated
    // updation
}

Flow chart of while loop:

Question:

How to find the last digit of a number N?

Answer: Use the modulus operator as N%10.

Give an example

Question: How to delete the last digit of N?

Answer:

N = N / 10;
SOP(N);

title: Printing all digits

description: Print all the digits of that number from right to left

duration: 480

Ques: Given a integer number, print all the digits of that number from right to left.

Example, if n = 6397 the correct output should be 7 9 3 6

Give the students, an intuition to solve the problem as follows:

Warning

Please take some time to think about the solution approach on your own before reading further.....

Approach:

Find the last digit.
Print the digit.
Remove last digit.

Code:

int n = scn.nextInt();
while(n > 0){
    int digit = n % 10;
    SOPln(digit);
    n = n / 10;
}

To figure out the condition in the while loop expression (i.e., n > 0), give the students an intuition as follows:

How to handle negative numbers?

Ans: Convert negative numbers to positive numbers.

Take an example of a negative number, dry run the code. Tell the students that the code exits from the while loop condition since n < 0. Then give the solution.

The updated code is as follows:

int n = scn.nextInt();
if(n < 0){
    n = n * -1;
}
while(n > 0){
    int digit = n % 10;
    SOPln(digit);
    n = n / 10;
}

Next corner test case: What if n == 0?

In this case, the output should be \(0\), but according to the code this will print nothing. So we need to handle this case as well.

The updated code is as follows:

int n = scn.nextInt();
if(n < 0){
    n = n * -1;
    return;
}
else if(n == 0){
    SOPln(0);
    return;
}
while(n > 0){
    int digit = n % 10;
    SOPln(digit);
    n = n / 10;
}

Dry Run:

title: Find sum of digits of a given number

description: Take examples to explain how to use while loops

duration: 900

Find Sum of Digits of A Given Number¶

Question:

Find the sum of digits of a given number.

Give examples -> 1274, 1004, -512

1274 -> 1 + 2 + 7 + 4 = 14
1004 -> 1 + 0 + 0 + 4 = 5
-512 -> 5 + 1 + 2 = 8

Note: Negative sign (-) is not a digit.

Warning

Please take some time to think about the solution approach on your own before reading further.....

Approach:

To discuss the approach take an example.

// Initialization
n = 2748
s = 0

n	n > 0	d = n % 10	s = s + d	n = n / 10
2748	true	8	8	274
274	true	4	12	27
27	true	7	19	2
2	true	2	21	0
0	false	-	-	-

int n = scn.nextInt();

if (n < 0) {
    n = n * - 1;
}

int s = 0;
while (n > 0) {
    int d = n % 10;
    s = s + d;
    n = n / 10;
}

SOPln(s);

title: Add a given digit to the back of a given number N.

description: Take examples to explain how to use while loops

duration: 800

Example 1¶

Question:

Given a positive integer N and a single digit d, add d to the back of N.

Example:

Formula to add d to the back of N:

n = n * 10 + d;

title: Find the reverse of a given number

description: Take examples to explain how to use while loops

duration: 1100

Example 2¶

Question:

Given a number N, store it's reverse in another variable and print it.

Examples:

Idea/Approach:

Initialize a variable rev = 0 and one by one take the last digit of N and add it to rev as shown below.

Steps:

Get last digit
Add last digit to the back of rev
Remove last digit
Repeat the above three steps till the number is greater than zero

Dry run:

n	n > 0	d = n % 10	rev = rev * 10 + d	n = n / 10
1456	true	6	6	145
145	true	5	65	14
14	true	4	654	1
1	true	1	6541	0
0	false	-	-	-

int n = scn.nextInt();
int copy = n;

if (n < 0) {
    n = n * - 1;
}

int rev = 0;
while (n > 0) {
    int d = n % 10;
    rev = rev * 10 + d;
    n = n / 10;
}

if (copy < 0) {
    rev = rev * - 1;
}

SOPln(s);

Dry run with n = 2400 and show that the output will be 42 and not 0042.

Tell them that if you want to print 0042, print the digits of n from right to left. It is not possible for an integer variable to store 0042.

Show dry run with - 417 as n.

title: Check if a given number is palindrome or not

description: Take examples to explain how to use while loops

duration: 720

Example 3¶

Question: .

Given a number N, check if number if palindrome or not. A number is said to a palindrome if it remains the same when its digits are reversed. Ex- 1221, 1551, 131, etc.

Exercise for students:

Warning

Please take some time to think about the solution approach on your own before reading further.....

Approach:

Find the reverse of the number using what we discussed in the last quiz and compare it with the original number. It it is the same, then the number is palindromic, otherwise not.

int n = scn.nextInt();
int copy = n;

if (n < 0) {
    n = n * - 1;
}

int rev = 0;
while (n > 0) {
    int d = n % 10;
    rev = rev * 10 + d;
    n = n / 10;
}

if (copy < 0) {
    rev = rev * - 1;
}

if (rev == copy) {
    SOPln("PALINDROME");
}
else {
    SOPln("NOT PALINDROME")
}

title: For loop basics

description: Quick recap of the syntax and flow of for loops

duration: 120

For loop Basics¶

Every for loop question can be done using a while loop. The difference lies in the syntax.

Syntax:

for(Initialization; Condition; update) {
    // Statements to be executed
}

Explain the syntax.

Flow:

title: Print numbers from 1 to 5 using for loops

description: Take examples to explain how to use for loops

duration: 180

Example 4¶

Question:

Print all numbers from 1 to 5.

for(int i = 1; i <= 5; i ++ ) {
    SOPln(i);
}

Explain the logic behind initialization, condition and update statements.

Dry Run:

i	i <= 5	print(i)	i++
1	true	1	2
2	true	2	3
3	true	3	4
4	true	4	5
5	true	5	6
6	false	-	-

title: Quiz 1

description: Quiz 1

duration: 60

Question¶

Expected output for following code :

for(int i = 1; i <= 10; i = i + 2) {
    System.out.println(i);
}

Choices¶

All Numbers from 1 to 10
All Even Numbers from 1 to 10
All Odd Numbers from 1 to 10
All Numbers from 1 to 9

title: Explain the quiz answer

description: Perform a dry run to explain the quiz question

duration: 240

Explaination¶

Dry Run:

i	i <= 10	print(i)	i += 2
1	true	1	3
3	true	3	5
5	true	5	7
7	true	7	9
9	true	9	11
11	false	-	-

title: Print the count of digits of a number

description: Take examples to explain how to use for loops

duration: 600

Example 5¶

Question:

Given a positive number, print the count of digits.

Give examples such as 5164, 121700, 9, etc.

Approach/Intuition:

int count = 0;
for(int i = n; i > 0; i = i / 10) {
    count += 1;
}

SOPln(count);

Show that the above code does not work for n = 0 and make the following change.

int count = 0;
if (n == 0) count = 1;

for(int i = n; i > 0; i = i / 10) {
    count += 1;
}

SOPln(count);

title: Read 5 numbers and for every number print last digit of the number.

description: Explain the need of for loops

duration: 780

Example 6¶

Question:

Read 5 numbers and for every number print last digit of the number.

Example:

Input: 34 45 378 980 456

Output:

4 5 8 0 6

Warning

Please take some time to think about the solution approach on your own before reading further.....

Approach 1:

int a = scn.nextInt();
int b = scn.nextInt();
int c = scn.nextInt();
int d = scn.nextInt();
int e = scn.nextInt();

SOPln(a % 10);
SOPln(b % 10);
SOPln(c % 10);
SOPln(d % 10);
SOPln(e % 10);

Approach 2:

for(int i = 0; i < 5; i ++ ) {
    int n = scn.nextInt();
    SOPln(n % 10);
}

title: Read T numbers and for every number print last digit of the number.

description: Show examples to explain how to use for loops

duration: 360

Example 7¶

Question:

Read T numbers and for every number print the last digit.

Input Format:

1^st Line: Contains T Followed by T lines containing the T numbers

int T = scn.nextInt();

for(int i = 0; i < T; i ++ ) {
    int n = scn.nextInt();
    SOPln(n % 10);
}

title: Read T numbers and for every number print the sum of digits of the number.

description: Show examples to explain how to use for loops

duration: 420

Example 8¶

Question:

Read T numbers and for each number, print the sum of digits of the number.

Input:

3 566 4130 162

Output:

17 8 9

int T = scn.nextInt();

for(int i = 0; i < T; i ++ ) {
    int n = scn.nextInt();

    if (n < 0) {
        n = n * - 1;
    }

    int s = 0;
    while (n > 0) {
        int d = n % 10;
        s = s + d;
        n = n / 10;
    }

    SOPln(s);
}

Show dry run for the example above.

Same question using for loop -

int T = scn.nextInt();

for(int i = 0; i < T; i ++ ) {
    int n = scn.nextInt();

    if (n < 0) {
        n = n * -1;
    }

    int s = 0;
    for(int x = n; x > 0; x ++ ) {
        int d = x % 10;
        s = s + d;
    }

    SOPln(s);
}