Informatics Practices · Class 11 Optional

Ch 3Brief Overview of Python — Class 11 Informatics Practices, concept-first.

A computer, by itself, does nothing. It acts only when it is given instructions — and those instructions must be arranged in a definite order, because the machine executes them one after another. An ordered set of instructions or commands given to a computer for execution is called a program.

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Key concepts

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Variable Type Identification

You walk into a room. You see a chair, a table, a cup of water, and a person. Without thinking, you know: the chair is for sitting, the cup holds liquid, the person can talk.

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>>> quantity = 10 >>> type(quantity) <class 'int'> >>> Price = -1921.9 >>> type(price) <class 'float'>Preview

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

3.1

Introduction to Python

A computer, by itself, does nothing. It acts only when it is given instructions — and those instructions must be arranged in a definite order, because the machine executes them one after another.

3.1.1

Working with Python

Writing a Python program is only half the job — something must read those statements and make the computer carry them out. That something is the Python interpreter.

3.1.2

Execution Modes

The Python interpreter can be put to work in two distinct ways, and choosing between them is the first practical decision every programmer makes.

3.2

Python Keywords

Every language — human or programming — has a small set of words whose meaning is fixed in advance. In Python, these are called keywords: reserved words that carry a specific, built-in meaning for the…

3.3

Identifiers

A program constantly refers to things — a variable that holds marks, a function that computes an average, or some other entity defined in the code.

3.4

Variables

A variable is an identifier whose value can change during the life of a program. The name stays fixed; what it refers to may vary.

3.5

Data Types

Every value that appears in a Python program — a number, a name, a price, a list of cities — belongs to a specific data type.

3.5.1

Number

The Number data type is Python's home for numerical values — it stores numbers and nothing else. Within this one family, Python distinguishes three different types, because whole numbers, decimal numb…

+Worked Examples1 question

Example 3.1>>> quantity = 10 >>> type(quantity) <class 'int'> >>> Price = -1921.9 >>> type(price) <class 'float'>Preview

3.5.2

Sequence

2 Q

A sequence in Python is an ordered collection of items in which each item is indexed by an integer value. Both halves of that definition matter.

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Example 3.2#To create a list >>> list1 = [5, 3.4, "New Delhi", "20C", 45] #print the elements of the list list1 >>> list1 [5, 3.4, 'New Delhi', '20C',…Preview

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Example 3.3#create a tuple tuple1 >>> tuple1 = (10, 20, "Apple", 3.4, 'a') #print the elements of the tuple tuple1 >>> print(tuple1) (10, 20, "Apple",…Preview

3.5.3

Mapping

Sequences store items by position; a mapping stores items by name. Mapping is an unordered data type in Python — its items are not kept at numbered positions the way string, list and tuple elements ar…

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Example 3.4#create a dictionary >>> dict1 = {'Fruit':'Apple', 'Climate':'Cold', 'Price(kg)':120} >>> print(dict1) {'Fruit': 'Apple', 'Climate': 'Cold',…Preview

3.6

Operators

Every program ultimately does two things: it stores values, and it does something with them. The "doing something" part is the job of operators.

3.6.1

Arithmetic Operators

Arithmetic operators are the workhorses of everyday computation. Python supports operators for the four basic arithmetic operations — addition, subtraction, multiplication and division — and adds thre…

3.6.2

Relational Operators

Arithmetic operators produce a new value; relational operators produce a verdict. A relational operator compares the values of the operands on its two sides and determines the relationship between the…

3.6.3

Assignment Operators

A variable is only useful if you can put a value into it — and change that value later. That is exactly what an assignment operator does: it assigns a value to the variable on its left, or changes the…

3.6.4

Logical Operators

Relational operators give single verdicts — True or False. Real conditions, though, are rarely single: "the marks are above 90 AND attendance is above 75", "the day is Saturday OR Sunday".

3.6.5

Membership Operators

Very often a program needs to ask a simple question about a collection: is this value inside it, or not? Membership operators answer exactly that.

3.7

Expressions

Constants store fixed values, variables store changeable ones, and operators act on them — an expression is where all three come together.

3.7.1

Precedence of Operators

3 Q

When an expression contains more than one operator, Python must decide which operation to perform first — and the answer changes the result.

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Example 3.5How will Python evaluate the following expression? 20 + 30 * 40Preview

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Example 3.6How will Python evaluate the following expression? (20 + 30) * 40Preview

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Example 3.7How will the following expression be evaluated? 15.0 / 4.0 + (8 + 3.0)Preview

3.8

Input and Output

3 Q

A program becomes genuinely useful the moment it can talk to its user — accept data while it runs and show results back.

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Example 3.8>>> fname = input("Enter your first name: ") Enter your first name: Arnab >>> age = input("Enter your age: ") Enter your age: 19Preview

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Example 3.9#function int() to convert string to integer >>> age = int(input("Enter your age: ")) Enter your age: 19 >>> type(age) <class 'int'>Preview

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Example 3.10| Statement | Output | |---|---| | print("Hello") | Hello | | print(10*2.5) | 25.0 |Preview

3.9

Debugging

No matter how carefully a program is written, mistakes creep in — and a mistake, or error, can hurt a program in two distinct ways.

3.9.1

Syntax Errors

Every programming language, Python included, has rules that determine how a program must be written — which symbols go where, what a valid statement looks like.

3.9.2

Logical Errors

A logical error — also called a bug, or a semantic error — is the sneakiest of the three error types, because it does not stop the program at all.

3.9.3

Runtime Error

A runtime error occupies the middle ground between a syntax error and a logical one: the statement is perfectly correct syntactically, so the program starts executing — but at some point the interpret…

3.10

Functions

When the same task has to be performed again and again in a program, writing the same statements repeatedly is wasteful and error-prone.

3.11

if..else Statements

2 Q

Ordinarily the statements of a program execute one after another, top to bottom. But real problems often present a choice: depending on the outcome of some condition, the program should do one thing r…

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Example 3.11age = int(input("Enter your age ")) if age >= 18: # use ':' to indicate end of condition. print("Eligible to vote")Preview

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Example 3.12Check whether a number is positive, negative, or zero. ```python number = int(input("Enter a number: ")) if number > 0: print("Number is pos…Preview

3.12

For Loop

Programs frequently need to do the same thing many times over. Think of a program that has to display the attendance of every student of a class: the print statement must execute once for each student…

3.12.1

The range() Function

A for loop is only as useful as the sequence it walks through, and Python supplies a built-in function whose whole job is to manufacture sequences of integers on demand: range().

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Example 3.13>>> list(range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] #start value is given as 2 >>> list(range(2, 10)) [2, 3, 4, 5, 6, 7, 8, 9] #step value is…Preview

3.13

Nested Loops

A loop may itself contain another loop inside its body. A loop written inside another loop is called a nested loop.

Summary

- Python is an open-source, high-level, interpreter-based language, suitable for both scientific and non-scientific computing tasks.

Exercises

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Ch 3Brief Overview of Python — Class 11 Informatics Practices, concept-first.

Key concepts

Variable Type Identification

Chapter contents

Introduction to Python

Working with Python

Execution Modes

Python Keywords

Identifiers

Variables

Data Types

Number

Sequence

Mapping

Operators

Arithmetic Operators

Relational Operators

Assignment Operators

Logical Operators

Membership Operators

Expressions

Precedence of Operators

Input and Output

Debugging

Syntax Errors

Logical Errors

Runtime Error

Functions

if..else Statements

For Loop

The range() Function

Nested Loops

Summary

Exercises

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