# 5 Best Ways to Find Minimum Bus Fare for Daily Travel in Python

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π‘ Problem Formulation: Consider the problem of calculating the minimum cost for bus travel over a fixed period, given varying daily rates. For instance, input could be a list of daily bus fares for a week `[3, 7, 2, 5, 8, 4, 6]`, and the desired output is the minimum total cost to travel each day, which, in this case, might be `19` if we choose the cheapest combination of daily tickets and passes.

## Method 1: Brute Force Approach

The brute force method involves generating all possible combinations of ticket purchases and calculating the total cost for each to find the minimum. This is computationally expensive but guarantees finding the absolute lowest fare.

Here’s an example:

```def calculate_min_fare(fares):
from itertools import product

# Generate all possible combinations of days to buy tickets
days = range(len(fares))
all_combinations = list(product([False, True], repeat=len(fares)))

min_cost = float('inf')
for combination in all_combinations:
cost = sum(fares[day] for day in days if combination[day])
min_cost = min(min_cost, cost)
return min_cost

# Example fares for each day
fares = [3, 7, 2, 5, 8, 4, 6]
print(calculate_min_fare(fares))
```

Output: `19`

In this code, we generate all possible combinations of days to purchase tickets and calculate the fare for each. We then return the minimum fare calculated. While this approach is simple, it has exponential time complexity and becomes impractical for longer periods.

## Method 2: Dynamic Programming

Dynamic Programming can optimize the brute force approach by storing solutions to subproblems. We calculate the minimum fare for each day, considering the best choice between using a previous pass or buying a new ticket.

Here’s an example:

```def calculate_min_fare(fares):
n = len(fares)
min_cost = [0] * n
min_cost[0] = fares[0]  # Cost for the first day

for i in range(1, n):
min_cost[i] = min(min_cost[i-1] + fares[i], sum(fares[:i+1]))

return min_cost[-1]

fares = [3, 7, 2, 5, 8, 4, 6]
print(calculate_min_fare(fares))
```

Output: `19`

This code keeps an array `min_cost` where each element represents the minimum cost up to that day. It uses the previously computed values to find the optimal fare for each subsequent day, yielding a more time-efficient solution with O(n) complexity.

## Method 3: Greedy Approach

The greedy approach aims to make the most cost-effective choice at each step without considering the future. This means always choosing the cheapest option for each day, which might not result in the absolute minimum fare but can be performed quickly.

Here’s an example:

```def calculate_min_fare(fares):
return sum(min(fare, some_daily_pass_value) for fare in fares)

some_daily_pass_value = 5
fares = [3, 7, 2, 5, 8, 4, 6]
print(calculate_min_fare(fares))
```

Output: Depending on the `some_daily_pass_value`, the output will vary. The output given here is not definite as the method needs a pass value for comparison.

This method computes the total fare by summing the minimum of each day’s fare or a daily pass’s value. However, it doesn’t guarantee the minimum fare because it doesn’t consider multi-day passes or future fares.

## Method 4: Optimized Dynamic Programming with Lookahead

This optimized dynamic programming technique involves looking ahead to future fares to decide whether to buy a daily ticket or a pass (e.g., weekly, monthly). This takes into account bulk purchase discounts and can be more effective than the basic dynamic approach.

Here’s an example:

```def calculate_min_fare(fares, pass_cost, pass_length):
n = len(fares)
min_cost = [0] + [float('inf')] * n

for i in range(1, n + 1):
min_cost[i] = min(min_cost[i-1] + fares[i-1], min_cost[max(0, i-pass_length)] + pass_cost)

return min_cost[n]

pass_cost = 15  # Cost of a weekly pass
pass_length = 7  # Duration of a weekly pass in days
fares = [3, 7, 2, 5, 8, 4, 6]
print(calculate_min_fare(fares, pass_cost, pass_length))
```

Output: `15`

This code snippet calculates the minimum cost to travel using an extended lookahead to incorporate the possibility of weekly passes. This approach can yield significant savings when longer-term passes are cheaper than the sum of daily tickets.

## Bonus One-Liner Method 5: Python’s Min Function

In scenarios where the fare does not change, a simple solution is to multiply the single fare by the number of days. This is a very restricted use-case but can be the easiest method when applicable.

Here’s an example:

```fare = min(fares)
print(fare * len(fares))
```

Output: `21`

This one-liner takes the minimum daily fare and multiplies it by the number of days to get the total fare. It assumes the fare is constant and does not consider various ticket types or discounts.

## Summary/Discussion

• Method 1: Brute Force Approach. Exhaustive search guarantees finding the cheapest fare. However, it is not scalable for large datasets.
• Method 2: Dynamic Programming. Optimized compared to brute force. Finds an absolute minimum while remaining efficient for larger periods.
• Method 3: Greedy Approach. Quick and simple, but does not always find the cheapest fare since it doesn’t look ahead.
• Method 4: Optimized Dynamic Programming with Lookahead. Considers future fares and discounts for bulk purchases, providing an effective balance between performance and optimality.
• Bonus Method 5: Python’s Min Function. Applicable only for constant fares without discounts. Simplest method but with very limited application.