Loops in python 3

Loops in python 3

The looping statements allow a set of instructions to be performed repeatedly until a certain condition is fulfilled. The looping statements are also called as iterations statements.

Python provides 2 kinds of loops:

• For loop in python 3
• While loop

for loop in python 3 | for loop in python shell

 The for loop in Python is designed to process the items of any sequence, such as a list or a string one by one.

Syntax:

for variable-name in sequence:

#statements

For example:

for a in (1,4,7):

       Print(a)

       Print(a*a)

Here, the loop variable a. Variable a will be assigned each value of list one by one, for the first time ‘a’ will be 1 then 4 and then 7.

Firstly, the loop variable ‘a’ will be assigned first value of list 1 and the statements in the body of the loop will be executed with this value of ‘a’. Hence value 1 will be printed with statement print(a) value 1 will again be printed with print(a*a).

Next, ‘a’ will be assigned next value in the list 4 and loop body executed. Thus 4 as result of print(a) and 16 as result of print(a*a) are printed.

Next, ‘a’ will be assigned next value in the list 7 and loop body executed. Thus 7 as result of print(a) and 49 as result of print(a*a) are printed.

All the values in the list are executed hence loop ends.

Output

1

1

4

16

7

49

Let’s take another example of for loop

for ch in ‘calm’:

print(ch)

The above loop will produce output as:

c

a

l

m

Here variable ch given values as ‘c’, ’a’, ‘l’, ‘m’ from the string ’calm’.

But what if we have to iterate through a very large list such as containing natural numbers between 1 to 100? Writing such a big list is neither easy nor good on readability. Then we use range () function to represent a list.

For example:

for val in range (2,16):

                print (val)

Here, range (2,16) will first generate a list [2, 3, 4, 5, . . .., 14,15] with which for loop will work.

python for loop example

Question: Write a program to print a table of 5 number?

Answer:

 Question: Write a program to print sum of natural numbers between 1 to 7. Print the sum after adding each natural number and print sum so far?

Answer:

While loop in  python

A while loop is a conditional loop that will repeat the instructions within itself as long as a conditional remains true.

Python while loop syntax:

while <condition> :

                #statement

The loop iterates while the condition evaluates to true. When the condition becomes false, the program control passes to the line after the loop-body.

For example:

a=5

while a>0:

                print (“Hello Python”, a)

                a=a-3

print (“Loop is over!!”)

Output:

Hello Python 5

Hello Python 2

Loop is over!!

Here, the condition is tested. If it is true, the loop body is executed.

The condition a>5 is evaluated. If the condition is true then all statements in the loop’s body are executed. If the condition is false then control moves to the next statement after the body of the loop.

Let’s take another example

n=1

while n<5 :

                print (“Square of”, n, “is”, n*n)

                n=n+1

print (“Thank you”)

Output:

Square of 1 is 1

Square of 2 is 4

Square of 3 is 9

Square of 4 is 16

Thank you

Thus, you see that as long as the condition n<5 is true above, the while loop’s body is executed. After exiting from the loop, the statement following the while loop is executed that prints Thank you.

While loop has 4 control elements

1. Initialisation Expressions (Starting): Before entering in a while loop, its loop variable must be initialised. The initialization of the loop variable takes place under initialization expressions. The initialization expression gives the loop variable their first value. The initialization expression for a while loop are outside the while loop before it starts.

2.Test Expression (Repeating or stopping): The test expression whose truth value decides whether the loop body will be executed or not. If the test expression evaluates to true, the loop body gets executed, other wise the loop is terminated.

In a while loop, the test expression is evaluated before entering into a loop.

3.The Body of the Loop (Doing): The statements that are executed repeatedly from the body of the loop. In a while loop, before every iteration the test expression is evaluated and if it is true, the body of the loop is executed; if the test expression evaluates to false, the loop is terminated.

4.Update Expression (Changing): The update expression changes the value of loop variable. The update expression is given as a statement inside the body of while loop.

It is also called entry-controlled loop because it controls the entry of the loop by testing a condition.

Python does not offer any exit-controlled loop.

Python while loop questions

Question: Write a program to accept transaction made in a day and items sold in day for a week and then print average sales made per transaction?

Answer:

Question: Write a program to calculate total amount raised in a charity camp where people can buy donated items or even make monetary donations. The charity camp ran for 3 days. Also, calculate if they were able to raise target amount of Rs 2,00,000.

Answer: