python-loops.md

Loops

Writing Loops

Loops are exactly what they sound like: pieces of code that runs in a pattern multiple times. Loops are useful for math (brute-force problems), reading data from a file (line-by-line), and much more.

There are two types of loops in Python: a for loop and a while loop.

For Loop

For loops begin with the keyword for. Its basic syntax looks like this:

for element in iterable:

Here's an example of some code that prints Hello, EasyCTF! 5 times:

for i in range(5):
    print "Hello, EasyCTF!"

And the output for this is exactly what you would think it is:

Hello, EasyCTF!
Hello, EasyCTF!
Hello, EasyCTF!
Hello, EasyCTF!
Hello, EasyCTF!

In this example, i is the element, and range(5) is an iterable that literally contains the numbers 0, 1, 2, 3, 4. More about the range() function can be found on the Python documentation.

In another example of a for loop, we will compute and print the sum of all of the integers from 0 to 49,999.

sum = 0
for i in range(50000):
    sum += i
print "The sum is %d" % sum

And our output is:

The sum is 1249975000

In this example, we can see that the value of the variable i changes every iteration. Its value is determined by the position in the iterable object, the range(50000). For example, in the first iteration, i would be 0, and in the second iteration, i would be 1.