Computers and coding for classes 9 and 10

Let us begin

The big idea

When you open a web page, it appears in a moment. Behind that moment, many machines in different places pass messages to each other. Let us see how it works, step by step.

The internet is a huge network of networks. Millions of computers, phones and other machines are linked by cables, radio waves and satellites. A router connects the devices in your home to the wider network.

Your device is the client. It asks for something, such as a web page. A server is a powerful computer that stores web pages and sends them when asked. This is the client-server model.

Every device on the internet has an address called an IP address. People prefer names, so we use domain names such as the name of a school website. A system called DNS works like a phone book: it turns the name into the number address.

Data does not travel in one piece. It is cut into small parts called packets. Each packet has the sender, the receiver and its number. Packets may take different routes and are put back in order at the end.

The web (websites) is only one service that runs on the internet, just as shops run on roads. Email and video calls also use the internet. A padlock in the address bar means the connection is encrypted, so others cannot read your data on the way.

Worked examples

See how it works

1. Opening a web page

   Step 1: you type the website name. Step 2: DNS finds the IP address. Step 3: your device sends a request to that server. Step 4: the server sends back the page in packets. Step 5: your browser puts them together and shows the page.

2. Counting packets

   A file of 1,000 KB is sent in packets of 250 KB each. Number of packets = 1,000 / 250 = 4. If one packet goes missing, the receiver can ask for just that one again.

3. Internet or web?

   Sending an email uses the internet but is not a web page. A news website is on the web, and the web runs on the internet. So the internet is the roads and the web is one kind of shop on those roads.

Try it at home

Packet post game

Let us begin

The big idea

If you need the same five steps in three places, you could write them three times. Programmers do better: they write the steps once, give the block a name, and call it whenever they need it. That named block is a function.

A function has a name, a list of parameters (the values it needs), a body (the steps) and often a return value (the answer it gives back). In pseudocode: FUNCTION square(n): RETURN n * n. The name is square, the parameter is n, and the body returns n times n.

Defining a function does nothing by itself. The steps run only when the function is called, for example square(7). The value 7 is called the argument; inside the function, n becomes 7, the body runs, and 49 is returned to the place where it was called.

Functions can take more than one parameter, such as FUNCTION area(length, breadth): RETURN length * breadth. Each call can pass different arguments: area(3, 4) returns 12 and area(10, 2) returns 20. Variables made inside a function belong to that function only; this is called local scope.

Why bother? Three reasons. Reuse: write once, call many times. Fewer mistakes: fix a bug in one place and every call is fixed. Readability: a line like total = average(marks) says what it does without showing how.

Worked examples

See how it works

1. Tracing a call

   FUNCTION double(x): RETURN x * 2. Main program: a = 5, b = double(a), c = double(b). Trace: a = 5. Call double(5): x is 5, returns 10, so b = 10. Call double(10): x is 10, returns 20, so c = 20.

2. A function with two parameters

   FUNCTION perimeter(l, b): RETURN 2 * (l + b). Call perimeter(7, 3): l is 7, b is 3, 7 + 3 = 10, 2 * 10 = 20. The function returns 20.

3. A function that calls another

   FUNCTION circleArea(r): RETURN 3.14 * square(r). Call circleArea(10): square(10) returns 100, then 3.14 * 100 = 314. Reusing square keeps circleArea short.

4. Why reuse matters

   A shop program works out a bill with tax in six places. If the tax rate changes and the sum is copied six times, you must change six lines and might miss one. With one function withTax(price), you change one line.

Try it at home

Function cards

Let us begin

The big idea

A class has 40 students. Making 40 separate variables for their marks would be painful. Instead, a program keeps all the marks in one list, also called an array. Each value sits in a numbered slot so the program can reach any of them.

A list is a row of values stored under one name. We write it with square brackets: marks = [72, 85, 90, 64]. This list has four items. Its length is 4.

Each slot has an index number. In most programming languages the first slot is index 0, not 1. So marks[0] is 72, marks[1] is 85, marks[2] is 90 and marks[3] is 64. The last index is always one less than the length. Asking for marks[4] is an error because there is no such slot.

You can change one slot: marks[1] = 88 replaces 85 with 88. You can add to the end: append 70 makes the list [72, 88, 90, 64, 70] with length 5.

The real power comes from loops. To add all the marks: total = 0, then FOR i FROM 0 TO length - 1: total = total + marks[i]. To find the largest: biggest = marks[0], then for each item, IF item > biggest THEN biggest = item. The same short loop works for a list of 4 or 4,000.

Worked examples

See how it works

1. Reading by index

   fruits = ["mango", "apple", "banana", "guava"]. fruits[0] is "mango", fruits[2] is "banana", and fruits[3] is "guava". The length is 4 and the highest valid index is 3.

2. Adding up a list

   marks = [72, 85, 90, 64]. Trace total: start 0; i = 0, total = 72; i = 1, total = 157; i = 2, total = 247; i = 3, total = 311. The sum is 311, and the average is 311 divided by 4 = 77.75.

3. Finding the largest

   nums = [14, 3, 27, 9]. biggest = 14. Compare 3: not bigger. Compare 27: bigger, so biggest = 27. Compare 9: not bigger. Final answer 27.

4. Counting matches

   temps = [31, 35, 29, 36, 33]. Count the days above 32: count = 0; 31 no; 35 yes, count 1; 29 no; 36 yes, count 2; 33 yes, count 3. Three hot days.

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An egg-tray array

Let us begin

The big idea

Computers spend a lot of their time putting things in order and finding things. A phone contact list, a leaderboard and a dictionary all depend on sorting and searching. You can learn the classic methods with nothing but a pencil and a row of numbers.

Bubble sort puts a list in order by comparing neighbours. Go along the list; whenever the left item is bigger than the right item, swap them. After one full pass, the largest value has "bubbled" to the end. Repeat the passes until a whole pass makes no swaps; then the list is sorted.

Linear search finds a value by checking the items one by one from the start until it is found or the list ends. It works on any list, sorted or not, but in the worst case it looks at every item: 1,000 items could mean 1,000 checks.

Binary search works only on a sorted list, but it is much faster. Look at the middle item. If it is the one you want, stop. If the item you want is smaller, throw away the right half; if bigger, throw away the left half. Repeat on the remaining half. Each step halves the list, so 1,000 items need at most about 10 checks, and 1,000,000 need about 20.

This is why sorting matters: a dictionary is sorted so you can open it near the middle and jump, instead of reading from page one. The number of steps a method needs as the list grows is called its efficiency, and choosing an efficient method is a big part of computer science.

Worked examples

See how it works

1. Bubble sort trace

   Sort [5, 2, 4, 1]. Pass 1: compare 5,2 swap to [2, 5, 4, 1]; compare 5,4 swap to [2, 4, 5, 1]; compare 5,1 swap to [2, 4, 1, 5]. Pass 2: 2,4 no swap; 4,1 swap to [2, 1, 4, 5]; 4,5 no swap. Pass 3: 2,1 swap to [1, 2, 4, 5]; then no more swaps. Pass 4 makes no swaps, so the list is sorted.

2. Linear search

   Find 7 in [3, 9, 7, 1, 8]. Check 3: no. Check 9: no. Check 7: yes, found at index 2 after 3 checks. To find 8 would take 5 checks; to find 4, which is absent, also takes 5 checks before giving up.

3. Binary search

   Find 23 in the sorted list [4, 8, 15, 16, 23, 42, 50]. Middle is index 3, value 16. 23 is bigger, so keep the right half [23, 42, 50]. Middle is 42. 23 is smaller, so keep [23]. Middle is 23: found in 3 checks. Linear search would have needed 5.

Try it at home

Sort and search with cards

Let us begin

The big idea

When you type an address and a page appears, a short conversation has happened between two computers: your browser (the client) and a server somewhere else. This lesson follows that conversation, from the URL you type to the page you see.

A URL (Uniform Resource Locator) is the address of a page. Take the address www.example.in/science/water-cycle.html with https written in front of it. The scheme, https, says which rules to use; it is followed by a colon and two slashes. The domain name, www.example.in, names the server. The path, /science/water-cycle.html, names the page on that server. Some URLs add a query after a question mark, such as ?q=rain, to pass extra information.

The browser first asks the DNS (Domain Name System) to turn the domain name into an IP address, because computers find each other by number, not by name. Then it sends an HTTP request to that server. HTTP (HyperText Transfer Protocol) is the agreed set of rules for asking for and sending web pages. The most common request is GET, which means "please send me this page".

The server finds or builds the page and sends back an HTTP response: a status code and the content. Code 200 means OK, here it is. Code 404 means not found. Code 500 means the server had an error. The content is usually HTML, the language that describes the page structure, along with images, styles and scripts the browser then fetches with more requests.

HTTPS is HTTP with encryption. The data between you and the server is scrambled so no one in between can read or change it. The padlock in the address bar shows the connection is encrypted. Always prefer https pages, especially when logging in.

Worked examples

See how it works

1. Taking a URL apart

   Take the address www.ncert.nic.in/textbook.php?class=8, typed with the scheme https in front. It has scheme https, domain www.ncert.nic.in, path /textbook.php and query class=8. The query passes the value 8 for the name class to the server.

2. One page load, step by step

   You type www.example.in/about. The browser asks DNS for the IP address of www.example.in and gets one, say 203.0.113.5. It sends GET /about to that address. The server replies 200 OK with the HTML. The browser reads the HTML, sees it needs a picture, sends another GET for the picture, and draws the page.

3. Reading a status code

   You follow an old link and the page says 404 Not Found. The server was reached fine, but there is no page at that path; perhaps it was moved. If instead you see 500, the page exists but the server program failed while building it.

Try it at home

Play the browser and the server

Let us begin

The big idea

A password is a secret that proves you are you. Encryption is a way to scramble information so only someone with the key can read it. Together with a second check called two-factor authentication, they keep accounts and messages safe. You can practise all three on paper.

A strong password is long, hard to guess and used for only one account. Length matters most: each extra character multiplies the number of guesses an attacker must try. A short phrase of unrelated words with a number and a symbol, such as "mango!tractor7cloud", is both long and easy to remember. Never use your name, birthday, "123456" or "password", and never share a password, even with a friend.

Encryption turns readable text (plaintext) into scrambled text (ciphertext) using a rule and a key. The oldest example is the Caesar cipher: shift every letter forward by a fixed number. With key 3, A becomes D, B becomes E, and Z wraps round to C. To decrypt, shift back by the same key. Anyone can know the rule; the key is the secret.

A Caesar cipher is easy to break because there are only 25 useful keys, so an attacker can simply try them all. Modern encryption, such as the kind behind HTTPS, uses keys that are 128 bits or more long (a string of 128 or more zeros and ones), so trying every key would take longer than the age of the universe. The idea is the same: a public rule, a secret key.

Two-factor authentication (2FA) asks for two different kinds of proof: something you know (the password) and something you have (your phone, which receives a one-time code, or an authenticator app). If someone steals your password, they still cannot log in without your phone. Turn it on for important accounts, with an adult's help.

Worked examples

See how it works

1. Encrypting with Caesar, key 3

   Plaintext HELLO. H plus 3 is K, E plus 3 is H, L plus 3 is O, L plus 3 is O, O plus 3 is R. Ciphertext: KHOOR. To decrypt KHOOR, shift each letter back 3 and HELLO returns.

2. Wrapping round the alphabet

   Encrypt XYZ with key 4. X plus 4: X, Y, Z, then wrap to A, B, so X becomes B. Y becomes C. Z becomes D. Ciphertext: BCD.

3. Comparing passwords

   "rahul2010" is weak: it is a name plus a birth year, both easy to find out. "Tj7" is short, so a computer can try every three-character combination in seconds. "purple!river42kite" is 18 characters of unrelated words, so it is strong and still memorable.

4. How 2FA stops a thief

   Someone learns Meera's email password from a leaked website. They type it in, but the site then asks for the six-digit code sent to Meera's phone. The thief does not have her phone, so the login fails, and Meera gets a warning to change her password.

Try it at home

Secret messages with a paper cipher wheel