nested loops and their usage in algorithms

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Nested loop is loop code inside another loop. Outer loops and inner loops. They’re very situational.

You have a while loop inside a for loop, for inside a for, while inside a while, etc.

Very useful if:

• you need to loop two different things (arrays) through the same function

Detailed explanation

for x in range(1, 10):  
	print(x)  

which outputs

1  
2  
3  
4  
5  
6  
7  
8  
9  

Print statements end with a new line character, meaning they print each output on a separate line.

If we want to output it all on the same line, add , end="") to the end of the print statement.

for x in range(1, 10):  
	print(x, end="")  

which outputs 123456789

the end will specify what to add at the end of each individual printed statemnt.

If we do end=" " for example, it will add a space between all the characters, because it’s adding an empty space after each number that’s being printed

Example of nested for loop

the outer loop will repeat the inner loop three times.

The inner loop will repeat 9 times (10 is exclusive)

for x in range(3): # outer  
	for x in range(1, 10):  
		print (x, end="")  

the x in this example is our counter. Avoid having the same counter for nested loops. let’s name the second one y

for x in range(3):  
	for y in range(1, 10):  
		print(y, end="")  

prints out 123456789123456789123456789

if we want to print out each iteration of x onto a new line, but keep all the iterations of y on one, we’d need to add a blank print statement at the end of the x for loop

for x in range(3):  
	for y in range(1, 10):  
		print(y, end="")  
	print()  

so now the result reads:

123456789  
123456789  
123456789  

Applied example

Printing a rectangle made up of a particular symbol

Accept some user input, so the user will specify how many rows and how many columns they want to see. We first create variables for rows and columns, and make them inputs. Then we typecast them as integers

rows = int(input("enter the number of rows:))  
columns = int(input("enter the number of columns:))  

We’re going to reuse the nested forloops from above. Outer loop will be in charge of rows, and the inner loop will be in charge of columns.

for x in range(rows):  
	for y in range(columns):  
		print(symbol, end="")  
	print()  

You’ll get an input questions that you’ll have to answer:

• “enter the number of rows:” 5
• “enter the number of columns:” 5
  And the output will look like:

*****  
*****  
*****  
*****  
*****  

references/links

• Nested loops in Python are easy ➿ - YouTube