7. Control flow.md

Control Flow: If-Else and Recursion

In this section, we would be talking about if-else statements, Recursions and Loops for repetitive tasks.

If-else Statements

The concept of conditional statements is general and quite straightforward but in comparison to Cairo 0.x, Cairo 1.0 came with a new option for conditionals: else if.

let status = true;

if (status == true){
    ...
} else if (status == false){
    ...
} else {
    ...
}

Recursions

Recursions were the only option for repetitive tasks in Cairo 0.x.

To demonstrate how they work, we'll create a function that sums up the first ten natural numbers using recursion:

use array::ArrayTrait;
use option::OptionTrait;

// `start` should be 1_u32 and `sum` be 0_32
fn sum(start: u32, sum: u32) -> Option<u32> {
    match gas::withdraw_gas() {
        Option::Some(_) => {},
        Option::None(_) => {
            let mut data = ArrayTrait::new();
            data.append('Out of gas');
            panic(data);
        },
    }
    // Terminator
    if(start == 11_u32){
        return Option::Some(sum);
    }

    // Sum
    let currentSum = sum + start;
    return sum(start + 1_u32, currentSum);
}

Loops

Loops are useful for creating logic that executes repetitively while a specific condition holds. The loop keyword in Cairo will repeatedly execute a block of code until stopped by the break keyword.

use debug::PrintTrait;
fn main() {
    let mut i: usize = 0;
    loop {
        if i > 10 {
            break;
        }
        'again'.print();
        i += 1;
    }
}

• use array::ArrayTrait; allows us use arrays
• use option::OptionTrait; gives us the Option enum which we will talk about in chapter 12.
• From line 32 to line 39 is just a boilerplate code we should always add when writing recursive functions in Cairo 1.0 for gas management.
• From line 42 to line 44, we have the terminator that makes sure the function stops at some point.
• let currentSum = sum + start; does the increamenting
• And, return sum(start + 1_u32, currentSum) calls the function again with the start value plus 1 and the current summation of the values.