Python Simple Programs (using for loop) – Part 2
🔢 1. Compute Factorial of N
➜ This Python program calculates the factorial of a positive integer using a for loop.
✨ Factorial Formula
N! = N × (N − 1) × (N − 2) × … × 2 × 1
For example, 5! = 5 × 4 × 3 × 2 × 1 = 120.
💡 How the Program Works
1.
The user enters a positive integer N.
2.
factorial is initialized to 1.
3.
The for loop runs from 1 to N.
4.
Each value of i is multiplied into factorial.
5.
The final factorial value is displayed using print().
🔎 Example: N = 5
factorial = 1
1 × 2 × 3 × 4 × 5 = 120
▶ Sample Output
Enter value of N: 5
Factorial of N is: 120
📌 Quick Note:
The factorial of 0 is also defined as
0! = 1. In this program, the initial value
factorial = 1 supports that definition.
➕ 2. Sum of First Five Natural Numbers
➜ This Python program uses a for loop to find the sum of the first five natural numbers.
➕ How the Sum Builds Up
0 + 1 = 1
1 + 2 = 3
3 + 3 = 6
6 + 4 = 10
10 + 5 = 15
💡 How the Program Works
1.
sum is initialized to 0.
2.
range(1, 6) generates the numbers 1 through 5.
3.
Each number is added to sum.
4.
print() displays the final sum.
▶ Sample Output
Sum of 1st 5 numbers is: 15
📌 Quick Note:
In range(1, 6), the starting value is included,
but the ending value 6 is excluded. Therefore,
the loop runs for 1, 2, 3, 4, 5.
➕ 3. Sum of Integers from 1 to 100
➜ This Python program uses a for loop to calculate the sum of all integers from 1 to 100.
🔢 Key Concept
The variable sum acts as an
accumulator. Each number generated by the loop
is added to it.
sum = sum + i
💡 How the Program Works
1.
sum starts with the value 0.
2.
range(1, 101) generates numbers from 1 through 100.
3.
Each number is added to sum.
4.
After the loop finishes, sum contains the final result.
🔎 Why range(1, 101)?
In range(1, 101), 1 is included but 101 is excluded.
Therefore, the loop processes 1 to 100.
▶ Sample Output
Sum of integers from 1 to 100 is: 5050
📌 Quick Note:
The program uses repeated addition. The final result is
5050. The same accumulator technique can be used
to calculate the sum of other ranges of numbers.
➕ 4. Sum of Integers from 1 to 100
➜ This Python program uses a for loop to calculate the sum of all integers from 1 to 100.
🔢 Key Concept
The variable sum acts as an
accumulator. Each number generated by the loop
is added to it.
sum = sum + i
💡 How the Program Works
1.
sum starts with the value 0.
2.
range(1, 101) generates numbers from 1 through 100.
3.
Each number is added to sum.
4.
After the loop finishes, sum contains the final result.
🔎 Why range(1, 101)?
In range(1, 101), 1 is included but 101 is excluded.
Therefore, the loop processes 1 to 100.
▶ Sample Output
Sum of integers from 1 to 100 is: 5050
📌 Quick Note:
The program uses repeated addition. The final result is
5050. The same accumulator technique can be used
to calculate the sum of other ranges of numbers.
🔢 5. Sum of Squares of N Natural Numbers
➜ This Python program calculates the sum of the squares of the first N natural numbers using a for loop.
📐 Concept
1² + 2² + 3² + … + N²
For N = 5:
1² + 2² + 3² + 4² + 5² = 55
💡 How the Program Works
1.
The user enters the value of N.
2.
sum is initialized to 0.
3.
range(1, n + 1) generates the numbers from 1 to N.
4.
Each number is squared using i * i and added to sum.
5.
The final sum is displayed using print().
🔎 Example: N = 5
1² + 2² + 3² + 4² + 5²
= 1 + 4 + 9 + 16 + 25 = 55
▶ Sample Output
Enter value of n: 5
Sum of squares of integers up to n is: 55
📌 Quick Note:
The expression i * i calculates the square of
each number. The variable sum keeps a running total
throughout the loop.
➕ 6. Sum of All Even Numbers up to N
➜ This Python program uses a for loop to calculate the sum of all even numbers up to a given value of N.
🔢 Understanding range()
range(0, n + 1, 2)
0 is the starting value,
n + 1 sets the upper limit, and
2 is the step value.
For N = 10, the loop generates:
0, 2, 4, 6, 8, 10.
💡 How the Program Works
1.
The user enters the value of N.
2.
sum is initialized to 0.
3.
range(0, n + 1, 2) generates even numbers.
4.
Each even number is added to sum.
5.
The final sum is displayed using print().
🔎 Example: N = 10
0 + 2 + 4 + 6 + 8 + 10 = 30
Since adding 0 does not change the result, the sum is
30.
▶ Sample Output
Enter value of n: 10
Sum of even numbers up to n is: 30
📌 Quick Note:
The third argument of range() is the
step. Using a step of 2 lets the
loop move directly from one even number to the next.
