# Python Programming/Loops/Solutions

1. *Create a program to count by prime numbers. Ask the user to input a number, then print each prime number up to that number.*

For this program we will make a list of possibilities up to the user's input, then test each one using Fermat's little theorem, verifying that it is prime, then adding the prime to a list of primes. Finally we print the list of primes.

Comment: All primes fulfill Fermat's little theorem, but some returned results are "pseudoprimes" which are not primes (e.g. 341=11*31, in python: (((2**341)-2)%341 == 0)). More information about finding primes & proving primality

```
primes = []
# Creates a list that we will throw our prime numbers into.
user = 1 + int(raw_input("What number would you like to count primes up to? "))
# 1 is added on to the user's number so that their number
# is counted.
list = range(2,user)
# 0 and 1 are not prime, but our algorithm registers them
# as prime, so we start with two
for possibility in list:
if ((2**possibility)-2)%possibility == 0:
# Our algorithm (Fermat's little theorem) states
# that if "i" is an integer and i^p-i divides evenly
# into p, p is prime. We are using 2 in place of i.
# See Python Programming/Basic Math for more info.
isprime = True
test = 2
while test < possibility and isprime:
# Here we verify that the number is in fact prime by
# starting at 2 and dividing it by every number lower
# than the potential prime. See comment above.
if (possibility%test) == 0:
# This ends the while loop causing the next number
# to be tested.
isprime = False
test = test + 1
if isprime:
# We add each prime to the primes list.
primes.append(possibility)
print ("The prime numbers from 0 to", user-1, "are: ")
for possibility in primes:
print possibility
# Print out all the primes!
```

2. Instruct the user to pick an arbitrary number from 1 to 100 and proceed to guess it correctly within seven tries. After each guess, the user must tell whether their number is higher than, lower than, or equal to your guess.

```
print "\nIn your head, pick a number between 1 and 100, \ni'll guess it with some hints:\n"
letsstart = raw_input("ready? input anything...\n")
uppers = 100
lowers = 0
guess = 50
boom = 0
for c in range(1,10):
if boom == 0:
print "guess: ",guess
answer = raw_input("Was that\na. too low\nb. too high\nc. thats right!\n")
if answer == "c":
print "I got it! Thanks for playing.\n"
boom = 1
elif answer == "a":
lowers = guess
blah = int((uppers-guess)/2)
guess = guess + blah
elif answer == "b":
uppers = guess
blash = int((guess-lowers)/2)
guess = guess-blash
```