LOOP + DECISION CHALLENGE

How Many Prime Numbers?

Use loops, operators and decision statements to discover prime numbers between 1 and 1000.

PROBLEM

Prime Number

How many prime numbers are there between 1 and 1000?

EXAMPLE

2, 3, 5, 7, 11, 13, 17, …

EXPECTATION

Prime count: 168

PSEUDOCODE

How to test one number

Start
Prime numbers begin at 2.

Choose a candidate
Check each number from 2 through 1000.

Try divisors
Test whether the candidate can be divided evenly.

Use modulo (%)
If candidate % divisor == 0, it has another factor.

Decide
No divisor found → it is prime.

Count and repeat
Add 1 to the prime counter, then test the next candidate.

MENTAL MODEL

7 % 2 ≠ 0

7 % 3 ≠ 0

No divisor → 7 is PRIME ✓

8 % 2 = 0 → NOT PRIME ✗

○ Not completed

INSTRUCTOR SOLUTION

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number = 1000
resultList = [2]

for i in range(3, number + 1):
    isPrime = True

    for n in resultList:
        if i % n == 0:
            isPrime = False
            break

    if isPrime:
        resultList.append(i)

print("There are", len(resultList),
      "prime numbers between 1 -", number)
print(resultList)

Instructor note: This solution reuses already-discovered prime numbers as divisors. The key concepts are nested loops, %, decision statements, break, and a result list.

💻 Prime Number Lab

Translate the pseudocode into Python

PYTHON CONSOLE

Ready.

CONCEPT CONNECTION

Variable
number

Operator
%

Decision
if

Loop
for