Loops (Part 2)

Saw for loops last time

• Use for when you know how long the loop should last

• ​for x in range(a, b)

• for x in {a1, a2, a3, a4}

Indeterminately long loops?

• What if we don't know how many iterations a loop might have?

• Example: Write a program nprime that takes as input a positive integer n, and prints the first n prime numbers.

• Needs us to try indeterminately large sets of numbers, as n grows!​

• But the terminating condition ("Is the number of prime numbers printed so far less than or equal to n?") is easy to check!​

A while loop

def nprime(n) :

.........ct = 0

.........indx = 1​

.........while ​ct < n :

​....................if isprime(indx) :

.................................print(indx)​

.................................ct = ct + 1

....................indx = indx + 1​

Another example

• A while lets us loop on inputs too!

• Can ask the user for input till the input matches some expected pattern ("Your password must contain an uppercase letter, a lowercase letter, a number, a special character, the blood of your firstborn...")​

Nested loops

• We saw two types of loops -- for and while

• We can have nested loops too

• Often very useful for pretty printing​

• Write a program which prints the numbers 1 through 20 in four rows, with five elements on each row, separated by tabs

Nested for loops

def print1to20() :

.........for i in range(1, 5) :

.................for j in range(1, 5) :

............................print(5*(i-1) + j, end='\t')​

.................print(5*i, end='\n')

​