Iteration is the process of repeatedly executing a block of code. The ability to execute a segment of code multiple times is necessary in complex programs. It helps minimize copying and pasting code — instead of writing the same lines of code over and over again, we can simply put those lines inside a loop. Almost every programming language (including Python) offers ways to loop over a part of code.
The simplest form of iteration is the
while statement. A
while statement consists of a condition and a suite of statements:
while condition: # suite
while statement executes its suite as long as
condition evaluates to a
condition is checked each time before executing the suite again. If
condition evaluates to a
False value, execution continues with the statements after the while loop.
If the suite does not modify the variables being tested in
condition, the loop will run forever. This is called an infinite loop. In the Python interpreter, press <Control>-C to terminate the program.
The following while loop prints the numbers 0 – 5:
>>> i = 0 >>> while i <= 5: ... print(i) ... i += 1 ... 0 1 2 3 4 5
Example: First 10 Fibonacci numbers
One day Rohin challenges Andrew to find a way to write out first 10 Fibonacci numbers. The Fibonacci sequence is a mathematical sequence whose first two numbers are 1 and 1, and the following numbers are obtained by adding previous two numbers. Andrew can start with the first two values of the sequence, add those two numbers to create the 3rd number, add the 2nd and 3rd numbers to create the 4th, add the 3rd and 4th to create the 5th, and so on. He realizes that the problem can be solved iteratively, so he uses a
i = 10
previous = 0
current = 1
while i > 0:
previous, current = current, previous + current
i = i - 1
Recall that in the assignment
previous, current = current, previous + current (line 5), the right side of the equals sign is evaluated first before making any assignment to
current. In this case, we evaluate
current, and add
current, and then set
previous to the value we got from evaluating
current and set
current to the result of the addition.
i = i - 1 (line 6) decrements
i to make the while loop stop when
i is 0 (i.e., make it execute 10 times).
As in many other programming languages, the
for statement is used to iterate over a piece of code multiple times. However, unlike many other languages (such as Java and C), in Python, the
for statement creates a loop over elements of the provided sequence and iterates over a piece of code inside the statement.
Generally we use a
while statement to execute a piece of code n times. The for loop is generally used to iterate over a sequence and apply our code on each item until we reach the end of the sequence. The syntax is:
for <name> in <expression>: <suite>
To execute a
- Evaluate the <expression> which must yield an iterable (a fancy way of saying that it must return a valid sequence of items)
- For each element in that iterable (in order):
- Bind <name> to that value in the local environment.
- Execute the <suite>.
for statement is equivalent to:
iterator = iter(<expression>) while True: try: <name> = next(iterator) except StopIteration: break <suite>
Example: Iterating over a string
Lets assume that Andrew wants to get back at Rohin so he dares him to pronounce the longest english word, Pneumonoultramicroscopicsilicovolcanoconiosis, without using a vowel. Rohin thinks that if he had a list of all the characters of the word he can go over each one and write out only the consonants thus making it easier when he is pronouncing. He immediately comes up with a solution that involves a for loop. He provides the fallowing code:
vowels = ["a", "e", "i", "o", "u"] word = "pneumonoultramicroscopicsilicovolcanoconiosis" for character in list(word): if character not in vowels: print (character, end=" ") # optional 2nd argument is to print every character in one line
As you can see Rohin creates a list of all the characters of the word by
list(word), goes over each character one by one and prints out only the ones that are not vowels. To make things more interesting when we are using while or for statements to iterate, we can make nested loops by putting multiple for statements or while statements inside each other. This nested loop structure helps us iterate over a list of items inside another list of items.