Conditions and Loops solved by 30106

Dec. 7, 2012, 6:43 p.m. by Rosalind Team

Topics: Introductory Exercises, Programming

Conditions and Loops

If you need Python to choose between two actions, then you can use an if/else statement. Try running this example code:

a = 42
if a < 10:
  print 'the number is less than 10'
  print 'the number is greater or equal to 10'

Note the indentation and punctuation (especially the colons), because they are important.

If we leave out an else, then the program continues on. Try running this program with different initial values of a and b:

if a + b == 4:
  print 'printed when a + b equals four'
print 'always printed'

If you want to repeat an action several times, you can use a while loop. The following program prints Hello once, then adds 1 to the greetings counter. It then prints Hello twice because greetings is equal to 2, then adds 1 to greetings. After printing Hello three times, greetings becomes 4, and the while condition of greetings <= 3 is no longer satisfied, so the program would continue past the while loop.

greetings = 1
while greetings <= 3:
  print 'Hello! ' * greetings
  greetings = greetings + 1

Be careful! If you accidentally create an infinite loop, your program will freeze and you will have to abort it. Here's an example of an infinite loop. Make sure you see why it will never exit the while loop:

greetings = 1
while greetings <= 3:
  print 'Hello! ' * greetings
  greetings = greetings + 0 # Bug here

If you want to carry out some action on every element of a list, the for loop will be handy

names = ['Alice', 'Bob', 'Charley']
for name in names:
  print 'Hello, ' + name

And if you want to repeat an action exactly $n$ times, you can use the following template:

n = 10
for i in range(n):
  print i

In the above code, range is a function that creates a list of integers between $0$ and $n$, where $n$ is not included.

Finally, try seeing what the following code prints when you run it:

print range(5, 12)

More information about loops and conditions can be found in the Python documentation.


Given: Two positive integers $a$ and $b$ ($a < b < 10000$).

Return: The sum of all odd integers from $a$ through $b$, inclusively.

Sample Dataset

100 200

Sample Output



You can use a % 2 == 1 to test if a is odd.

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