String Programs in Java: Palindrome, Vowels, Words and Sorting Names (ICSE Class 10)

Understand String Programs: Palindrome, Words and Sorting Names

for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    // test or use c
}

if (a[j].compareTo(a[j + 1]) > 0) {
    String t = a[j]; a[j] = a[j + 1]; a[j + 1] = t;
}

String Programs: Palindrome, Words and Sorting Names: key facts

Loop over a string

for i from 0 to s.length() - 1 | c = s.charAt(i)

Indices start at 0.

Palindrome test

Build r by looping from the last index to 0 | Palindrome if s.equals(r)

equalsIgnoreCase ignores case.

Words in a sentence

Words = number of spaces + 1 | Trim the sentence first

Valid for single spaces between words.

Sorting names

Ascending: swap when a[j].compareTo(a[j + 1]) is greater than 0 | Descending: when less than 0

Capital letters sort before small letters.

How to answer String Programs: Palindrome, Words and Sorting Names questions

1. 1Write the string with indices; decide whether to loop forwards or backwards.
2. 2Initialise the counter (0) or the result string (empty) before the loop.
3. 3Inside the loop test or transform each charAt(i) and update the counter or result.
4. 4After the loop, print the result with the labels the question asks for.
5. 5For arrays of strings, use compareTo for order and equals for equality.

String Programs: Palindrome, Words and Sorting Names 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 String Programs: Palindrome, Words and Sorting Names

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

• Backwards loop reverses

  Looping from length - 1 down to 0 and adding each char builds the reverse.

  Use it when: Reverse and palindrome.

• Spaces plus one

  Words = spaces + 1, just as 2 cuts make 3 pieces of string.

  Use it when: Counting words.

• equals for contents

  Compare contents with equals or compareTo, never with ==.

  Use it when: Searching strings.

String Programs: Palindrome, Words and Sorting Names: solved examples

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

String s = "Education";
int v = 0;
for (int i = 0; i < s.length(); i++) {
    char c = Character.toLowerCase(s.charAt(i));
    if ("aeiou".indexOf(c) >= 0)
        v++;
}
System.out.println(v);

String s = "Radar";
String r = "";
for (int i = s.length() - 1; i >= 0; i--)
    r = r + s.charAt(i);
System.out.println(r + " " + s.equals(r) + " " + s.equalsIgnoreCase(r));

String[] n = {"Mohan", "Anil", "Zoya", "Dev"};
for (int i = 0; i < n.length - 1; i++)
    for (int j = 0; j < n.length - 1 - i; j++)
        if (n[j].compareTo(n[j + 1]) > 0) {
            String t = n[j]; n[j] = n[j + 1]; n[j + 1] = t;
        }
for (int i = 0; i < n.length; i++)
    System.out.print(n[i] + " ");

String s = "the quick fox";
int w = 1;
String r = "";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    if (c == ' ')
        w++;
    if (i == 0 || s.charAt(i - 1) == ' ')
        r += Character.toUpperCase(c);
    else
        r += c;
}
System.out.println(r + " " + w);

String[] a = {"red", "blue", "green"};
String[] keys = {"blue", "pink"};
for (int k = 0; k < keys.length; k++) {
    int pos = -1;
    for (int i = 0; i < a.length; i++)
        if (a[i].equals(keys[k])) { pos = i; break; }
    System.out.print(pos + " ");
}

Where students lose marks in String Programs: Palindrome, Words and Sorting Names

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

• ✗Mistake: Using == to compare strings

  == compares references, not contents.

  Wrong working

  if (a[i] == key)

  Right working

  if (a[i].equals(key))

  ✓Do this: Use equals or compareTo.

  Usually costs: 1 mark

• ✗Mistake: Looping to length() instead of length() - 1

  charAt(length()) is outside the string and causes StringIndexOutOfBoundsException.

  Wrong working

  for (i = 0; i <= s.length(); i++)

  Right working

  for (i = 0; i < s.length(); i++)

  ✓Do this: Use < length().

  Usually costs: 1 mark

• ✗Mistake: Ignoring case in a palindrome test

  equals treats R and r as different.

  Wrong working

  "Radar" is not a palindrome with equals

  Right working

  Use equalsIgnoreCase when case should not matter

  ✓Do this: Read the question for case rules.

  Usually costs: 1 mark

Practice questions on String Programs: Palindrome, Words and Sorting Names

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

   What does "abc".compareTo("abd") return?
   (a) 1 (b) -1 (c) 0 (d) true

   Hint

   First differing characters: c and d.

   Answer

   (b) -1, since 'c' minus 'd' is -1.

2. Q2 · 1 mark · Very short answerBoard level

   Write the expression that gives the index of the last character of a String s.

   Hint

   Length minus one.

   Answer

   s.length() - 1

3. Q3 · 2 marks · Short answerHarder board level

   Write the output:

   String s = "AbC";
   String r = "";
   for (int i = 0; i < s.length(); i++) {
       char c = s.charAt(i);
       if (Character.isUpperCase(c))
           r += Character.toLowerCase(c);
       else
           r += Character.toUpperCase(c);
   }
   System.out.println(r);

   Hint

   Toggle each letter.

   Answer

   aBc

4. Q4 · 4 marks · Short answerHarder board level

   Write a program fragment that counts the words in a sentence s whose words are separated by single spaces, and give the count for "to be or not".

   Hint

   Count the spaces.

   Answer

   int w = 1;
   for (int i = 0; i < s.length(); i++)
   if (s.charAt(i) == ' ')
   w++;
   System.out.println(w);
   For "to be or not" there are 3 spaces, so w = 4.

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): A string is a palindrome if it equals its reverse.
   Reason (R): A palindrome reads the same from left to right and from right to left.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Does R define the term?

   Answer

   (a) Both are true and R explains A.

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.How do you reverse a string with a loop?Show answer

   Loop from the last index down to 0 and add each char to a new string.

2. 2.How many words in a sentence with 4 spaces?Show answer

   5 (single spaces).

3. 3.Which method sorts order for strings?Show answer

   compareTo.

4. 4.What does a positive compareTo result mean?Show answer

   The left string comes after the right string.

5. 5.Which search is asked for string arrays?Show answer

   Linear search.

6. 6.What is a palindrome?Show answer

   A string that reads the same forwards and backwards.

String Programs: Palindrome, Words and Sorting Names: frequently asked questions