[PPT] - Computer Programming Dr. Deepak B Phatak Dr. Supratik Chakraborty PowerPoint Presentation

SLIDE 1

IIT Bombay

Computer Programming

Dr. Deepak B Phatak
Dr. Supratik Chakraborty

Department of Computer Science and Engineering IIT Bombay Session: Loops and Assignment Expressions

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Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

SLIDE 2

IIT Bombay

Iteration idioms in programming
Necessity and convenience of iteration
“while …”, “do … while …” and “for …” loops in C++
Use of “break” statements in loops

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Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

Quic ick Recap of f Rele levant Topics

SLIDE 3

IIT Bombay

Closer look at “for” loops

Use of assignment expressions and its variants Use of “,” separated expressions

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Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

Overv rview of f Th This is Le Lecture

SLIDE 4

IIT Bombay

“for …” St Statement: Our Si Simple Vie iew

Part of program before iteration for ( iteration initialization ; loop condition ; instructions to execute at end of every iteration) { Block of statements (“for” loop body ) } Part of program after iteration

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Semi-colons not to denote end of executable statements But to separate three parts inside for ( ….. ) Note absence of semi-colon

SLIDE 5

IIT Bombay

“for …” St Statement: C++ ++ St Standard Vie iew

Part of program before iteration for (iteration initialization ; loop condition ; instructions to execute at end of every iteration) { Block of statements (“for” loop body ) } Part of program after iteration

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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initialization expression update expression

SLIDE 6

IIT Bombay

Appears Nonsensical?

We needed assignment statements to initialize variables

before entering loop

We needed assignment statements to update after each

iteration Is it meaningful to have initialization expression and update expression? What if I write a + b*c for initialization/update expression? Which variable is initialized/updated here?

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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SLIDE 7

IIT Bombay

Assignment as An Operator

C++ allows “=“ (assignment) to be viewed as an operator in

an expression, with side effects Assignment: x = (y + z) As a statement: x = (y + z) ; Assign the value of expression y+z to x As an operator: x = (y + z) Side effect: Value of expression (y+z) is stored in x Type and value: Same as those of (y + z) … RHS of “=“

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Semi-colon present Semi-colon absent

SLIDE 8

IIT Bombay

“for …” St Statement: Our Enhanced Vie iew

Part of program before iteration for ( count = 1.0 ; loop condition ; count = (count + 1)) { Block of statements (“for” loop body ) } Part of program after iteration

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Expressions with side-effects

SLIDE 9

IIT Bombay

More on Assignment as An Operator

Need operator precedence

What is (a = b + c) as an expression ?

Precedence of = lower than that of arithmetic and logical
perators we have seen so far

(a = b + c) as an expression is (a = (b + c)) An expression with side-effect: a is assigned the value of b+c Type and value of (a = b + c) is same as that of (b + c)

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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SLIDE 10

IIT Bombay

More on Assignment as An Operator

Need associativity

Right-to-left associative (a = b = c = a + 1) is evaluated as (a = (b = (c = (a + 1)))) Type and value same as that of (a + 1)

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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SLIDE 11

IIT Bombay

Sp Special Assignment Operators

Increment

Post-increment: x++ Similar to x = x + 1 But, value is that of x before incrementing

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Value of y: 10 Value of x: 2

y = x++;

Value of y: 2 Value of x: 3 x++ as an expression

SLIDE 12

IIT Bombay

Sp Special Assignment Operators

Increment

Pre-increment: ++x Similar to x = x + 1 Value is that of x after incrementing

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Value of y: 10 Value of x: 2

y = ++x;

Value of y: 3 Value of x: 3 ++x as an expression

SLIDE 13

IIT Bombay

Sp Special Assignment Operators

Decrement

Post-decrement: x-- Similar to x = x - 1 Value is that of x before decrementing

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Value of y: 10 Value of x: 2

y = x--;

Value of y: 2 Value of x: 1 x-- as an expression

SLIDE 14

IIT Bombay

Sp Special Assignment Operators

Decrement

Pre-decrement: --x Similar to x = x - 1 Value is that of x after decrementing

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Value of y: 10 Value of x: 2

y = --x;

Value of y: 1 Value of x: 1

-x as an

expression

SLIDE 15

IIT Bombay

Compound Assignment Operators

Increment/decrement variable by an expression

x += (y + z) same as x = x + (y + z) x -= (2w) same as x = x – (2 w)

Can have similar operators from other arithmetic operators

x = 2 same as x = x2 x /= y same as x = x/y x %= 5 same as x = x/5

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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SLIDE 16

IIT Bombay

In Increment and Decrement Operators

Precedence and associativity:
Post-increment/post-decrement same precedence, left-to-right

associative

Pre-increment/pre-decrement same precedence, right-to-left

associative

Pre-increment/pre-decrement has lower precedence than

post-increment/post-decrement

All have higher precedence than other arithmetic and logical
perators we have seen
Exception: pre-increment/pre-decrement same precedence as ! (lo
+=, -=, /=, %= have lowest precedence (same as that of =), right-to-

left associative

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

16

SLIDE 17

IIT Bombay

In Increment and Decrement Operators

Precedence and associativity:
Post-increment/post-decrement same precedence, left-to-right

associative

Pre-increment/pre-decrement same precedence, right-to-left

associative

Pre-increment/pre-decrement has lower precedence than

post-increment/post-decrement

All have higher precedence than other arithmetic and logical
perators we have seen
Exception: pre-increment/pre-decrement same precedence as ! (lo
+=, -=, /=, %= have lowest precedence (same as that of =), right-to-

left associative

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Moral of the story: Try not to mix increment/decrement operators with other operators, if possible Convenient idioms for increment and decrement Use them mostly for that purpose

SLIDE 18

IIT Bombay

Use of f “,” Operator

Using side effects of multiple expressions when only one is

allowed

(x++ , y = z+2 , z + 5) is one expression
Component expressions evaluated in left-to-right order
Two side-effects

x is incremented y is assigned the value of z + 2

One type and value: Same as rightmost expression, i.e. z + 5
Often used in initialization and update of “for” loops
Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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SLIDE 19

IIT Bombay

“for …” St Statement: Our Enhanced Vie iew

Part of program before iteration for ( count = 1.0, i = 2 ; loop condition ; count += 5, i-- , j++) { Block of statements (“for” loop body ) } Part of program after iteration

Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Evaluates as a single expression Two side effects

SLIDE 20

IIT Bombay

Su Summary

Assignment as a statement and as an expression
Variants of assignment statements
Use in loops (and other places too) in C++
Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, IIT Bombay

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Computer Programming

Department of Computer Science and Engineering IIT Bombay Session: Loops and Assignment Expressions

Quic ick Recap of f Rele levant Topics

Use of assignment expressions and its variants Use of “,” separated expressions

Overv rview of f Th This is Le Lecture

“for …” St Statement: Our Si Simple Vie iew

Part of program before iteration for ( iteration initialization ; loop condition ; instructions to execute at end of every iteration) { Block of statements (“for” loop body ) } Part of program after iteration

Semi-colons not to denote end of executable statements But to separate three parts inside for ( ….. ) Note absence of semi-colon

“for …” St Statement: C++ ++ St Standard Vie iew

Part of program before iteration for (iteration initialization ; loop condition ; instructions to execute at end of every iteration) { Block of statements (“for” loop body ) } Part of program after iteration

initialization expression update expression

Appears Nonsensical?

before entering loop

iteration Is it meaningful to have initialization expression and update expression? What if I write a + b*c for initialization/update expression? Which variable is initialized/updated here?

Assignment as An Operator

an expression, with side effects Assignment: x = (y + z) As a statement: x = (y + z) ; Assign the value of expression y+z to x As an operator: x = (y + z) Side effect: Value of expression (y+z) is stored in x Type and value: Same as those of (y + z) … RHS of “=“

Semi-colon present Semi-colon absent

“for …” St Statement: Our Enhanced Vie iew

Part of program before iteration for ( count = 1.0 ; loop condition ; count = (count + 1)) { Block of statements (“for” loop body ) } Part of program after iteration

Expressions with side-effects

More on Assignment as An Operator

What is (a = b + c) as an expression ?

(a = b + c) as an expression is (a = (b + c)) An expression with side-effect: a is assigned the value of b+c Type and value of (a = b + c) is same as that of (b + c)

More on Assignment as An Operator

Right-to-left associative (a = b = c = a + 1) is evaluated as (a = (b = (c = (a + 1)))) Type and value same as that of (a + 1)

Sp Special Assignment Operators

Post-increment: x++ Similar to x = x + 1 But, value is that of x before incrementing

Value of y: 10 Value of x: 2

y = x++;

Value of y: 2 Value of x: 3 x++ as an expression

Sp Special Assignment Operators

Pre-increment: ++x Similar to x = x + 1 Value is that of x after incrementing

Value of y: 10 Value of x: 2

y = ++x;

Value of y: 3 Value of x: 3 ++x as an expression

Sp Special Assignment Operators

Post-decrement: x-- Similar to x = x - 1 Value is that of x before decrementing

Value of y: 10 Value of x: 2

y = x--;

Value of y: 2 Value of x: 1 x-- as an expression

Sp Special Assignment Operators

Pre-decrement: --x Similar to x = x - 1 Value is that of x after decrementing

Value of y: 10 Value of x: 2

y = --x;

Value of y: 1 Value of x: 1

expression

Compound Assignment Operators

x += (y + z) same as x = x + (y + z) x -= (2*w) same as x = x – (2* w)

x *= 2 same as x = x*2 x /= y same as x = x/y x %= 5 same as x = x/5

In Increment and Decrement Operators

associative

associative

post-increment/post-decrement

left associative

In Increment and Decrement Operators

associative

associative

post-increment/post-decrement

left associative

Moral of the story: Try not to mix increment/decrement operators with other operators, if possible Convenient idioms for increment and decrement Use them mostly for that purpose

Use of f “,” Operator

allowed

x is incremented y is assigned the value of z + 2

“for …” St Statement: Our Enhanced Vie iew

Part of program before iteration for ( count = 1.0, i = 2 ; loop condition ; count += 5, i-- , j++) { Block of statements (“for” loop body ) } Part of program after iteration

Evaluates as a single expression Two side effects

Su Summary

x += (y + z) same as x = x + (y + z) x -= (2w) same as x = x – (2 w)

x = 2 same as x = x2 x /= y same as x = x/y x %= 5 same as x = x/5