11.1.2.7 use nested loops when solving problems
11.1.2.6 debug a program

Python. Nested loops

A nested loop has one loop inside of another. 
These are typically used for working with two dimensions such as printing stars in rows and columns as shown below.

When a loop is nested inside another loop, the inner loop runs many times inside the outer loop. In each iteration of the outer loop, the inner loop will be re-started. The inner loop must finish all of its iterations before the outer loop can continue to its next iteration.

Example

Output

Trace nested loops

Task 1. Output the next stars.

n = int(input())
m = int(input())
for i in range(n): 
    for j in range(m):   # nested loop
        print('*', end='')
    print()

Task 2. Output multiplication table in line for number n. (n < 10)

k = int(input())
for i in range(1, 10):
    print(i, '*', k, '=', k * i, sep='', end='\t')

Input:

5

Output:

1*5=5       2*5=10    3*5=15     4*5=20   5*5=25   6*5=30   7*5=35   8*5=40   9*5=45

Task 3. How to output a full multiplication table?

for i in range(1, 10):                      # start of outer loop        
    for j in range(1, 10):                  # start of inner loop
        print(i, '*', j, '=', i * j, end='\t’) # output the multiplication table
    print()                                       # newline

Task 3. The second way of solving

i = 1               # the initial value of the first factor
while i < 10:       # condition for the first factor
    j = 1           # the initial value of the second multiplier
    while j < 10:   # the condition for the second multiplier 
        print(i, '*', j, '=', i * j, end='\t') # output
        j += 1      # the change of the second multiplier in the inner loop
    i += 1          # change the first multiplier in the outer loop
    print()         # output new line

Commands break and continue in nested loops

while True:             # start of external loop
    while True:         # the start of the inner loop 
         if [condition]:
            continue    # the transition to the next iteration in the inner loop
         if [condition]:
            break       # the output of the inner loop 
     if [condition]:
        continue        # the transition to the next iteration in the outer loop
    if [condition]:
        break           # the output of the outer loop

Questions:

Exercises: 

Ex. 1 Write a program to print the following start pattern using the for loop

Ex. 2 In number theory, a perfect number is a positive integer that is equal to the sum of its positive divisors, excluding the number itself. For instance, 6 has divisors 1, 2 and 3 (excluding itself), and 1 + 2 + 3 = 6, so 6 is a perfect number.
Write a program to print all perfect numbers from 1 to 100

Tasks:

Tasks on Stepik.org course "Python Programming for NIS"