Initial Value in Fold Algorithms

If the initial value provided to fold_left() is not the identity of the operation, it can affect the result of the fold in ways that might not be immediately obvious.

Understanding how the initial value interacts with the folding process is crucial to ensuring correct behavior.

The identity of an operation is a value that, when used in the operation, does not change the other operand. For example, 0 is the identity for addition, and 1 is the identity for multiplication.

When the initial value is not the identity for the operation, it becomes an active part of the folding process and influences the result. This can be useful or problematic depending on your intention.

Let's see what happens when the initial value for addition is not 0:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 4, 5};
7
8  // Using non-identity initial value 10
9  int result = std::ranges::fold_left(
10    numbers, 10, std::plus<>());
11  std::cout << "Result: " << result;
12}

1Result: 25

Explanation

• The initial value 10 is added to the sum of the elements in the numbers vector.
• The result is $10 + (1 + 2 + 3 + 4 + 5) = 25$.

Now, let's see the effect of a non-identity initial value for multiplication:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 4, 5};
7
8  // Using non-identity initial value 2
9  int result = std::ranges::fold_left(
10    numbers, 2, std::multiplies<>());
11  std::cout << "Result: " << result;
12}

1Result: 240

Explanation

• The initial value 2 multiplies with the product of the elements in the numbers vector.
• The result is $2 \times (1 \times 2 \times 3 \times 4 \times 5) = 240$.

• Identity Value: Use the identity value of the operation when you want the fold to reflect the natural combination of elements (e.g., 0 for addition, 1 for multiplication).
• Non-Identity Value: Use a non-identity initial value when you need to adjust the result of the fold by a specific factor or offset.

• Providing an inappropriate initial value can lead to unexpected results or logical errors in your program.
• Always ensure that the initial value aligns with the intended outcome of your operation.

Example of Logical Error

Consider the following example where the initial value leads to an unintended result:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 4, 5};
7
8  // Incorrect initial value for subtraction
9  int result = std::ranges::fold_left(
10    numbers, 10, std::minus<>());
11  std::cout << "Result: " << result;
12}

1Result: -5

Explanation

• The initial value 10 starts the subtraction chain, leading to an unexpected result.
• The result is $10 - 1 - 2 - 3 - 4 - 5 = -5$.

In summary, using a non-identity initial value in fold_left() can significantly influence the result.

It's essential to choose the initial value carefully, ensuring it aligns with the intended operation and outcome.

Understanding the role of the initial value helps in leveraging fold algorithms effectively.