Rust: checked_sub & unwrap_or

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Just a quick note for these two functions I learned today.

checked_sub is used to check if the abstraction of an integer overflows. For example:

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let mut num_u32: u32 = 0;
num_u32 -= 1; // overflow

let mut num_i32 = i32::MIN; // -2147483648
num_i32 -= 1; // overflow

To prevent it, we can use the combo of checked_sub and unwrap_or. checked_sub will return Option<self> which means if there is no overflow occurs, we get Some(substraction_result), else we will get None. unwarp_or can then give the result a default value if we get None or just unwrap Some(substraction_result) to return substraction_result. Let’s look at the code.

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let mut num_i32 = i32::MIN + 1; // -2147483648 + 1

num_i32 = num_i32.checked_sub(1).unwrap_or(i32::MIN); // -2147483648

num_i32 = num_i32.checked_sub(1).unwrap_or(100); // 100
// Since it overflows, it will return the default value 
// that we give to unwrap_or which is 100 for this example

That’s it for today!