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1. Print "Hello, World!"2. Add Two Numbers3. Swap Two Variables4. Find ASCII Value of a Character5. Check Even or Odd6. Find Largest of Two Numbers7. Find Largest of Three Numbers8. Check Positive, Negative, or Zero9. Convert Kilometers to Miles10. Convert Celsius to Fahrenheit11. Calculate Area of a Triangle12. Calculate Simple Interest13. Calculate Compound Interest14. Calculate Square Root15. Convert Seconds to Hours, Minutes, and Seconds16. Calculate Body Mass Index (BMI)17. Check Leap Year18. Find Absolute Value19. Calculate Age from Birth Year20. Sum of First N Natural Numbers21. Calculate Power Without Using pow()22. Print Multiplication Table23. Convert Decimal to Binary24. Convert Binary to Decimal25. Calculate Electricity Bill

Convert Binary to Decimal in Python

Left to right: double, add the bit. 1010 → 10. Reject anything that isn't 0 or 1.

BeginnerBasic Python ProgramsExample 24 of 25
convert-binary-to-decimal.py
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1# Program to convert binary to decimal without using int(x, 2)
2

3binary_str = input("Enter a binary number: ")
4

5if not all(ch in '01' for ch in binary_str):
6    print("Invalid binary number.")
7else:
8    decimal_value = 0
9    for ch in binary_str:
10        decimal_value = decimal_value * 2 + int(ch)
11

12    print(f"Decimal value of {binary_str} is {decimal_value}")

Output

Enter a binary number: 1010
Decimal value of 1010 is 10

In this one

  • double, add bit, repeat
  • reject characters that aren't 0 or 1

What's going on

Start at 0. For each bit: double what you have, add the bit.

1010:

0×2+1=1

1×2+0=2

2×2+1=5

5×2+0=10

int(binary_str, 2) does this. The loop is the same algorithm with the lid off.

all(ch in '01' for ch in binary_str) dumps 1021 early. int('1021', 2) would throw. Catching it yourself is nicer.

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