Break and continue

Break:

The break statement terminates the loop containing it and control of the program flows to the statement immediately after the body of the loop. If break statement is inside a nested loop, break will terminate the innermost loop.

break in for loop

for var in sequence:

# code inside for loop

If condition:

break (if break condition satisfies it jumps to outside loop)

# code inside for loop

# code outside for loop

break in while loop

while test expression

# code inside while loop

If condition:

break (if break condition satisfies it jumps to outside loop)

# code inside while loop

# code outside while loop

Example:

for num in [11, 9, 88, 10, 90, 3, 19]:

print(num)

if(num==88):

print("The number 88 is found")

print("Terminating the loop")

break

Continue:

The continue statement is used to skip the rest of the code inside a loop for the current iteration only. Loop does not terminate but continues on with the next iteration.

continue in for loop:

for var in sequence:

# code inside for loop

If condition:

continue (if break condition satisfies it jumps to outside loop)

# code inside for loop

# code outside for loop

continue in while loop;

while test expression

# code inside while loop

If condition:

continue(if break condition satisfies it jumps to outside loop)

# code inside while loop

# code outside while loop

Example:

# program to display only odd numbers

for num in [20, 11, 9, 66, 4, 89, 44]:

# Skipping the iteration when number is even

if num%2 == 0:

continue

# This statement will be skipped for all even numbers

print(num)

Repetition Structures

A repetition structure causes a statement or set of statements to execute repeatedly. Repetition structure, which is more commonly known as a loop is used to repeat a set of statements.

Two broad categories of loops are: condition-controlled and count-controlled.

A condition-controlled loop uses a true/false condition to control the number of times that it repeats.

A count-controlled loop repeats a specific number of times.

In Python the while statement is used to write a condition-controlled loop, and the for statement is used to write a count-controlled loop.

1. While loop:

• Loops are either infinite or conditional. Python while loop keeps reiterating a block of code defined inside it until the desired condition is met.
• The while loop contains a boolean expression and the code inside the loop is repeatedly executed as long as the boolean expression is true.
• The statements that are executed inside while can be a single line of code or a block of multiple statements.

Syntax:

while(expression):

    Statement(s)

Example:

i=1

while i<=6:

print("TTWRDC")

i=i+1

2. For loop:

Python for loop is used for repeated execution of a group of statements for the desired number of times. It iterates over the items of lists, tuples, strings, the dictionaries and other iterable objects.

Syntax:

for var in sequence:

Statement(s)

Example 1:

numbers = [1, 2, 4, 6, 11, 20]

seq=0

for val in numbers:

seq=val*val

print(seq)

Example 2: Iterating over a Tuple:

tuple = (2,3,5,7)

print ('These are the first four prime numbers ')

#Iterating over the tuple

for a in tuple:

print (a)

Example 3: Iterating over a dictionary:

#creating a dictionary

college = {"MPCs":"block1","BZC":"block2","BCom":"block3"}

#Iterating over the dictionary to print keys

print ('Keys are:')

for keys in college:

print (keys)

#Iterating over the dictionary to print values

print ('Values are:')

for blocks in college.values():

print(blocks)

Example 4: Iterating over a String:

#declare a string to iterate over

college = 'TTWRDC'

#Iterating over the string

for name in college:

print (name)

3. Nested For loop:

When one Loop defined within another Loop is called Nested Loops.

Syntax:

for val in sequence:

for val in sequence:

statements

statements

Example:

for i in range(1,6):

for j in range(0,i):

print(i, end=" ")

Decision Structures

A control structure is a logical design that controls the order in which a set of statements execute.

A sequence structure is a set of statements that execute in the order that they appear.

For example, the following code is a sequence structure because the statements execute from top to bottom.

name = input('What is your name? ')

age = int(input('What is your age? '))

print('Here is the data you entered:')

print('Name:', name)

print('Age:', age)

Although the sequence structure is heavily used in programming, it cannot handle every type of task.

Control structure: one that can execute a set of statements only under certain circumstances. This can be accomplished with a decision structure. Decision structures are also known as selection structures.

In a decision structure’s simplest form, a specific action is performed only if a certain condition exists. If the condition does not exist, the action is not performed.

1. If Statement:

The if statement contains a logical expression using which data is compared and a decision is made based on the result of the comparison.

Syntax:

if expression:

statement(s)

If the boolean expression evaluates to TRUE, then the block of statement(s) inside the if statement is executed. If boolean expression evaluates to FALSE, then the first set of code after the end of the if statement(s) is executed.

Example:

a = 3

if a > 2:

print(a, "is greater")

print("done")

2. Alternative if (If-Else):

An else statement can be combined with an if statement. An else statement contains the block of code (false block) that executes if the conditional expression in the if statement resolves to 0 or a FALSE value.

The else statement is an optional statement and there could be at most only one else Statement following if.

Syntax of if - else :

if test expression:

Body of if 

else:

Body of else 

Example:

a=int(input('enter the number'))

if a>5:

print("a is greater")

else:

print("a is smaller than the input given")

3. Chained Conditional: (If-elif-else):

The elif statement allows us to check multiple expressions for TRUE and execute a block of code as soon as one of the conditions evaluates to TRUE. Similar to the else, the elif statement is optional. However, unlike else, for which there can be at most one statement, there can be an arbitrary number of elif statements following an if.

Syntax of if – elif - else :

If test expression:

Body of if 

elif test expression:

Body of elif 

else:

Body of else

Example:

a=int(input('enter the number'))

b=int(input('enter the number'))

c=int(input('enter the number'))

if a>b:

print("a is greater")

elif b>c:

print("b is greater")

else:

print("c is greater")

Performing Calculations

A programmer’s tools for performing calculations are math operators. Python has numerous operators that can be used to perform mathematical calculations.

A math expression performs a calculation and gives a value. The following is an example of a simple math expression:

12 + 2

The values on the right and left of the operator are called operands.

Example: (simple_math.py)

a=10;

b=20;

c=a+b

print(‘Total is: “, c)

Floating-Point and Integer Division

Python has two different division operators. The / operator performs floating-point division, and the // operator performs integer division.

The difference between them is that the / operator gives the result as a floating-point value, and the // operator gives the result as an integer.

Example:

>>> 5 / 2   

2.5

>>> 5 // 2  

2

The // operator works like this:

• When the result is positive, it is truncated, which means that its fractional part is thrown away.

• When the result is negative, it is rounded away from zero to the nearest integer.

Example:

>>> −5 // 2

−3

Operator Precedence

First, operations that are enclosed in parentheses are performed first. Then the operator with the higher precedence is applied first.

The precedence of the math operators, from highest to lowest, are:

1. Exponentiation: **

2. Multiplication, division, and remainder: * / // %

3. Addition and subtraction: + −

Notice that the multiplication (*), floating-point division (/), integer division (//), and remainder (%) operators have the same precedence. The addition (+) and subtraction (−) operators also have the same precedence. When two operators with the same precedence share an operand, the operators execute from left to right.

Example:

outcome = 12.0 + 6.0 / 3.0

There is an exception to the left-to-right rule. When two ** operators share an operand, the operators execute right-to-left. For example, the expression 2**3**4 is evaluated as 2**(3**4).

Mixed-Type Expressions and Data Type Conversion

When a math operation is performed on two operands, the data type of the result will depend on the data type of the operands. Python follows these rules when evaluating mathematical expressions:

• When an operation is performed on two int values, the result will be an int.

• When an operation is performed on two float values, the result will be a float.

• When an operation is performed on an int and a float, the int value will be temporarily converted to a float and the result of the operation will be a float. (An expression that uses operands of different data types is called a mixed-type expression.)

The first two situations are straightforward: operations on ints produce ints, and operations on floats produce floats. The third situation, which involves mixed-type expressions:

my_number = 5 * 2.0

When this statement executes, the value 5 will be converted to a float (5.0) and then multiplied by 2.0. The result, 10.0, will be assigned to my_number.

The int to float conversion happens implicitly in above calucation.

For explicit conversion, use either the int( ) or float( ) functions.

Reading Input from the Keyboard

Python’s built-in input function is used to read input from the keyboard. The input function reads a piece of data that has been entered at the keyboard and returns that piece of data, as a string, back to the program.

General format:

variable = input(prompt)

In the general format, prompt is a string that is displayed on the screen.

Example:

name = input('What is your name? ')

When the above statement executes, the following things happen:

• The string 'What is your name? ' is displayed on the screen.

• The program pauses and waits for the user to type something on the keyboard and then to press the Enter key.

• When the Enter key is pressed, the data that was typed is returned as a string and assigned to the name variable.

Reading numeric data using input function

The input function always returns the user’s input as a string, even if the user enters numeric data. This can be a problem if you want to use the value in a math operation. Math operations can be performed only on numeric values, not strings.

Python has built-in functions that you can use to convert a string to a numeric type.

Example:

string_value = input('How many hours did you work? ')

hours = int(string_value)

OR

hours = int(input('How many hours did you work? '))

The int( ) and float( ) functions work only if the item that is being converted contains a valid numeric value. If the argument cannot be converted to the specified data type, an error known as an exception occurs.

Displaying Output with the print Function

The most fundamental built-in function of Python is the print function, which displays output on the screen.

Here is an example of a statement that executes the print function:

print('Hello world')

In interactive mode:

>>> print('Hello world')

Hello world

>>> 

When you call the print function, you type the word print, followed by a set of parentheses. Inside the parentheses, you type an argument, which is the data that you want displayed on the screen.

Using Script:

Program to display name and address on the computer screen

program name: output.py

print(“Ashraf”)

print(“TTWRDC,Maripeda”)

More About Data Output

Output can be formatted in different ways.

1. Suppressing the print Function’s Ending Newline

The print function normally displays a line of output.

Example:

print('One')

print('Two')

print('Three')

Each of the statements shown here displays a string and then prints a newline character.

Output:

One

Two

Three

If the print function should not start a new line of output when it finishes displaying its output, pass the special argument end=' ' to the function.

print('One', end=' ')

print('Two', end=' ')

print('Three')

This specifies that the print function should print a space instead of a newline character at the end of its output.

Here is the output of these statements:

One Two Three

To print anything at the end of its output, not even a space. If that is the case, pass the argument end='' to the print function, as shown in the following code:

print('One', end='')

print('Two', end='')

print('Three')

This specifies that the print function should print nothing at the end of its output. Here is the output of these statements:

OneTwoThree

2. Specifying an Item Separator

When multiple arguments are passed to the print function, they are automatically separated by a space when they are displayed on the screen.

>>> print('One', 'Two', 'Three')

One Two Three

If space is not to be printed between the items, pass the argument sep='' to the print function, as shown here:

>>> print('One', 'Two', 'Three', sep='')

OneTwoThree

To specify a character other than the space to separate multiple items in the output use sep=’*’

>>> print('One', 'Two', 'Three', sep='*')

One*Two*Three

3. Escape Characters

An escape character is a special character that is preceded with a backslash (\), appearing inside a string literal. When a string literal that contains escape characters is printed, the escape characters are treated as special commands that are embedded in the string.

Some of Python’s escape characters

Examples:

Ex \n:

print('One\nTwo\nThree')

When this statement executes, it displays

One

Two

Three

Ex \t:

print('Mon\tTues\tWed')

print('Thur\tFri\tSat')

The output is

Mon     Tues     Wed

Thur     Fri        Sat

Ex \' and \":

The \' and \" escape characters can be used to display quotation marks.

print("Your assignment is to read \"Hamlet\" by tomorrow.")

print('I\'m ready to begin.')

These statements display the following:

Your assignment is to read "Hamlet" by tomorrow.

I'm ready to begin.

Ex \\:

The \\ escape character is used to display a backslash, as shown in the following:

print('The path is C:\\temp\\data.')

This statement will display

The path is C:\temp\data.

4. Displaying Multiple Items with the + Operator

When the + operator is used with two strings, it performs string concatenation.

print('This is ' + 'one string.')

This statement will print

This is one string.

5. Formatting Numbers

When a floating-point number is displayed by the print function, it can appear with up to 12 significant digits.

Python provides the built-in format function, which can be used to round the decimal places. The format specifier is a string that contains special characters specifying how the numeric value should be formatted.

format function has two arguments: a numeric value and a format specifier.

format(12345.6789, '.2f ')

The first argument, which is the floating-point number 12345.6789, is the number that we want to format. The second argument, which is the string '.2f', is the format specifier.

Here is the meaning of its contents:

• The .2 specifies the precision. It indicates that we want to round the number to two decimal places.

• The f specifies that the data type of the number we are formatting is a floating-point number.

output: 12345.68

Formatting in Scientific Notation

To display floating-point numbers in scientific notation, use the letter e or the letter E instead of f.

>>> print(format(12345.6789, 'e'))

1.234568e+04

>>> print(format(12345.6789, '.2e'))

1.23e+04

Inserting Comma Separators

If you want the number to be formatted with comma separators, you can insert a comma into the format specifier.

>>> print(format(12345.6789, ',.2f'))

12,345.68

>>> print(format(123456789.456, ',.2f'))

123,456,789.46

Specifying a Minimum Field Width

The format specifier can also include a minimum field width, which is the minimum number of spaces that should be used to display the value.

>>> print('The number is', format(12345.6789, '12,.2f'))

The number is 12,345.68

The number 12,345.68 uses only 9 spaces on the screen, but it is displayed in a field that is 12 spaces wide. When this is the case, the number is right justified in the field. If a value is too large to fit in the specified field width, the field is automatically enlarged to accommodate it.

The Program Development Cycle

The process of creating a program that works correctly typically requires the five phases. The entire process is known as the program development cycle.

1. Design the Program: A program should be carefully designed before the code is actually written.

The process of designing a program can be summarized in the following two steps:

1. Understand the task that the program is to perform.

2. Determine the steps that must be taken to perform the task.

2. Write the Code: After designing the program, the programmer begins writing code in a high-level language such as Python. A language’s syntax rules dictate things such as how key words, operators, and punctuation characters can be used. A syntax error occurs if the programmer violates any of these rules.

3. Correct Syntax Errors: If the program contains a syntax error, or even a simple mistake such as a misspelled key word, the compiler or interpreter will display an error message indicating what the error is. Once all of the syntax errors and simple typing mistakes have been corrected, the program can be compiled and translated into a machine language program (or executed by an interpreter, depending on the language being used).

4. Test the Program: Once the code is in an executable form, it is then tested to determine whether any logic errors exist. A logic error is a mistake that does not prevent the program from running, but causes it to produce incorrect results.

5. Correct Logic Errors If the program produces incorrect results, the programmer debugs the code. This means that the programmer finds and corrects logic errors in the program.

Sometimes during this process, the programmer discovers that the program’s original design must be changed. In this event, the program development cycle starts over and continues until no errors can be found.