Loops - Repetition in Python

The for-loop:

This type of loop is useful when you know how many times to repeat your instructions. For example.

for i in range(3):
    print “Hello everyone!”
print “Loop finished.”

produces the output:

Hello everyone!
Hello everyone!
Hello everyone!
Loop finished

The function range(3) causes the loop to repeat 3 times. Changing it to range(5) would repeat the loop 5 times. The variable i takes on the values 0, 1, and 2 during each repetition of the loop. For example,

for i in range(3):
    print str(i) +“. Hello everyone!”

produces the output:

0. Hello everyone!
1. Hello everyone!
2. Hello everyone!

The variable i can be used as a type of counter in the following way:

count = 0
for i in range(3):
    count = count + i
    print count, i

produces the output:

0 0
1 1
3 2

The range(3) function can be modified in the following ways:

for i in range(1,4):
    print str(i) +“. Hello everyone!”

produces the output:

1. Hello everyone!
2. Hello everyone!
3. Hello everyone!

Notice how the variable i now starts at a value of 1 but only goes up to the maximum value (in this case 4) but does not take on a value of 4. Also,

for i in range(1,7,2):
    print str(i) +“. Hello everyone!”

produces the output:

1. Hello everyone!
3. Hello everyone!
5. Hello everyone!

In this case, the variable i takes on values in steps of 2. We can also count backwards by doing the following:

for i in range(4,1,-1):
    print str(i) +“. Hello everyone!”

produces the output:

4. Hello everyone!
3. Hello everyone!
2. Hello everyone!

Again, the variable i does not take on the minimum value of 1, but only goes down to it.

The for-loop - without the range() function:

The for-loop can be used without the range() function in a couple ways. It can be used to pass through the characters in a string as follows:

name =“Ada”
for ch in name:
    print( ch )

produces the output:

A
d
a

It can also be used to pass through the items in a list as follows:

names = [“Ada”, “Python”, “Pyret”]
for language in names:
    print language

produces the output:

Ada
Python
Pyret

The while-loop:

The while-loop is used when you don't know the exact number of times that a loop needs to repeat. It only repeats as long as a given condition is true. Here is an example, where the loop will end only when the user enters a negative value.

n = 1
while n > 0:
  n = input("Enter a number: ")

Here is an example of a while-loop that will end when the user enters a 'secret' number.

secret = 42
guess = -1
while guess != secret:
    guess = int( input("Guess the secret number: ") )

You can write a while-loop to repeat a fixed number of times by using an accumulator variable (counter). This is similar to how a for-loop works.

counter = 1
while counter <= 10:
    print("Hello")
    counter = counter + 1

This code will produce the following output.

Hello
Hello
Hello
Hello
Hello
Hello
Hello
Hello
Hello
Hello