While loop, For loop

Repeated Steps

Loops (repeated steps) have iteration variables that change each time through a loop. Often these iteration variables on through a sequence of numbers.

n = 5
while n > 0:
		print(n)
		n = n - 1
print('Blastoff!')
print(n)

/*Output
5
4
3
2
1
Blastoff!
0
*/

Breaking Out of a Loop

• The break statement ends the current loop and jumps to the statement immediately following the loop
• It is like a loop test that can happen anywhere in the body of the loop

while True:
		line = input('> ')
		if line == 'done':
				break;
		print(line)
print('Done!') 

> hello there
hello there
> finished
finished
> done
Done! 

Finishing an Iteration with continue

The continue statement ends the current iteration and jumps to the top of the loop and starts the next iteration

while True:
		line = raw_input('> ')
		if line[0] == '#':
				continue
		if line == 'done':
				break
		print(line)
print('Done!')
	

Indefinite Loops

• While loops are called “indefinite loops” because they keep going until a logical condition becomes False
• The loops we have seen so far are pretty easy to examine to see if they will terminate or if they will be “infinite loops”
• Sometimes it is a little harder to be sure if a loop will terminate

---

Definite Loops

• Quite often we have a list of items of the lines in a file - effectively a finite set of things
• We can write a loop to run the loop once for each of the items in a set using the Python for construct
• These loops are called “definite loops” because they execute an exact number of times
• We say that “definite loops iterate through the members of a set”

A Simple Definite Loop

for i in [5, 4, 3, 2, 1]:
		print(i)
print('Blastoff!')

/*Output
5
4
3
2
1
Blastoff!
*/

A Simple Definite Loop

Definite loops (for loops) have explicit iteration variables that change each time through a loop. These iteration variables move through the sequence or set.

Looking at In…

• The iteration variable “iterates” through the sequence (ordered set)
• The block (body) of code is executed once for each value in the sequence
• The iteration variable moves through all of the values in the sequence

Loop Idioms: What We Do in Loops

Note: Even though these examples are simple, the patterns apply to all kinds of loops