First occurrence of True number in Python

In this article, we will find the first occurrence of a non-zero (True) number in a given list. In Python, zero evaluates to False while non-zero numbers evaluate to True, making this a common programming task.

Using enumerate() with next()

The enumerate() function provides both index and value, while next() returns the first matching element ?

Example

numbers = [0, 0, 13, 4, 17]

# Given list
print("Given list:", numbers)

# Using enumerate to get index of first non-zero number
result = next((i for i, j in enumerate(numbers) if j), None)

# Printing result
print("The first non-zero number is at index:", result)

Given list: [0, 0, 13, 4, 17]
The first non-zero number is at index: 2

Using next() with filter()

The filter() function with a lambda expression filters out zero values, then index() finds the position ?

Example

numbers = [0, 0, 13, 4, 17]

# Given list
print("Given list:", numbers)

# Using next, filter and lambda
first_nonzero = next(filter(lambda i: i != 0, numbers))
result = numbers.index(first_nonzero)

# Printing result
print("The first non-zero number is at index:", result)
print("The first non-zero number is:", first_nonzero)

Given list: [0, 0, 13, 4, 17]
The first non-zero number is at index: 2
The first non-zero number is: 13

Using a Simple Loop

A straightforward approach using a for loop to find the first non-zero number ?

Example

numbers = [0, 0, 13, 4, 17]

# Given list
print("Given list:", numbers)

# Using simple loop
for i, num in enumerate(numbers):
    if num != 0:
        print("The first non-zero number is at index:", i)
        print("The first non-zero number is:", num)
        break
else:
    print("No non-zero number found")

Given list: [0, 0, 13, 4, 17]
The first non-zero number is at index: 2
The first non-zero number is: 13

Comparison

Conclusion

Use enumerate() with next() for a concise solution when you only need the index. For better readability and when you need both value and index, a simple loop is often the best choice.