Flujo de control
La capacidad de ejecutar cierto código dependiendo de si una condición es verdadera y de ejecutar código repetidamente mientras una condición es verdadera son bloques de construcción básicos en la mayoría de los lenguajes de programación. Las construcciones más comunes que le permiten controlar el flujo de ejecución del código en Cairo son las expresiones if y los bucles.
Expresiones if
Una expresión if le permite ramificar su código según condiciones. Proporciona una condición y luego establece: "Si se cumple esta condición, ejecute este bloque de código. Si no se cumple la condición, no ejecute este bloque de código".
Create a new project called branches in your cairo_projects directory to explore the if expression. In the src/lib.cairo file, input the following:
fn main() {
let number = 3;
if number == 5 {
println!("condition was true and number = {}", number);
} else {
println!("condition was false and number = {}", number);
}
}
Todos las expresiones if comienzan con la palabra clave if, seguido de una condición. En este caso, la condición verifica si la variable number tiene un valor igual a 5. Colocamos el bloque de código a ejecutar si la condición es true inmediatamente después de la condición dentro de llaves.
Opcionalmente, también podemos incluir una expresión else, que elegimos hacer aquí, para dar al programa un bloque de código alternativo para ejecutar si la condición se evalúa como false. Si no proporciona una expresión else y la condición es false, el programa simplemente omitirá el bloque if y pasará al siguiente fragmento de código.
Intente ejecutar este código; debería ver la siguiente salida:
$ scarb cairo-run
Compiling no_listing_24_if v0.1.0 (listings/ch02-common-programming-concepts/no_listing_27_if/Scarb.toml)
Finished `dev` profile target(s) in 4 seconds
Running no_listing_24_if
condition was false and number = 3
Run completed successfully, returning []
Intentaré cambiar el valor de number por uno que haga que la condición sea true para ver qué sucede:
let number = 5;
$ scarb cairo-run
condition was true and number = 5
Run completed successfully, returning []
It’s also worth noting that the condition in this code must be a bool. If the condition isn’t a bool, we’ll get an error. For example, try running the following code:
fn main() {
let number = 3;
if number {
println!("number was three");
}
}
The if condition evaluates to a value of 3 this time, and Cairo throws an error:
$ scarb build
Compiling no_listing_28_bis_if_not_bool v0.1.0 (listings/ch02-common-programming-concepts/no_listing_28_bis_if_not_bool/Scarb.toml)
error: Mismatched types. The type `core::bool` cannot be created from a numeric literal.
--> listings/ch02-common-programming-concepts/no_listing_28_bis_if_not_bool/src/lib.cairo:4:18
let number = 3;
^
error: could not compile `no_listing_28_bis_if_not_bool` due to previous error
The error indicates that Cairo inferred the type of number to be a bool based on its later use as a condition of the if statement. It tries to create a bool from the value 3, but Cairo doesn't support instantiating a bool from a numeric literal anyway - you can only use true or false to create a bool. Unlike languages such as Ruby and JavaScript, Cairo will not automatically try to convert non-Boolean types to a Boolean. If we want the if code block to run only when a number is not equal to 0, for example, we can change the if expression to the following:
fn main() {
let number = 3;
if number != 0 {
println!("number was something other than zero");
}
}
Running this code will print number was something other than zero.
Manejando de múltiples condiciones con else if
You can use multiple conditions by combining if and else in an else if expression. For example:
fn main() {
let number = 3;
if number == 12 {
println!("number is 12");
} else if number == 3 {
println!("number is 3");
} else if number - 2 == 1 {
println!("number minus 2 is 1");
} else {
println!("number not found");
}
}
Este programa tiene cuatro posibles caminos que puede seguir. Después de ejecutarlo, debería ver la siguiente salida:
$ scarb cairo-run
Compiling no_listing_25_else_if v0.1.0 (listings/ch02-common-programming-concepts/no_listing_30_else_if/Scarb.toml)
Finished `dev` profile target(s) in 4 seconds
Running no_listing_25_else_if
number is 3
Run completed successfully, returning []
When this program executes, it checks each if expression in turn and executes the first body for which the condition evaluates to true. Note that even though number - 2 == 1 is true, we don’t see the output number minus 2 is 1 nor do we see the number not found text from the else block. That’s because Cairo only executes the block for the first true condition, and once it finds one, it doesn’t even check the rest. Using too many else if expressions can clutter your code, so if you have more than one, you might want to refactor your code. Chapter 6 describes a powerful Cairo branching construct called match for these cases.
Using if in a let Statement
Because if is an expression, we can use it on the right side of a let statement to assign the outcome to a variable.
fn main() {
let condition = true;
let number = if condition {
5
} else {
6
};
if number == 5 {
println!("condition was true and number is {}", number);
}
}
$ scarb cairo-run
Compiling no_listing_26_if_let v0.1.0 (listings/ch02-common-programming-concepts/no_listing_31_if_let/Scarb.toml)
Finished `dev` profile target(s) in 4 seconds
Running no_listing_26_if_let
condition was true and number is 5
Run completed successfully, returning []
The number variable will be bound to a value based on the outcome of the if expression, which will be 5 here.
Repetición con bucles
It’s often useful to execute a block of code more than once. For this task, Cairo provides a simple loop syntax, which will run through the code inside the loop body to the end and then start immediately back at the beginning. To experiment with loops, let’s create a new project called loops.
Cairo has three kinds of loops: loop, while, and for. Let’s try each one.
Repetición de Código con loop
La palabra clave loop le dice a Cairo que ejecute un bloque de código una y otra vez para siempre o hasta que le digas explícitamente que pare.
Como ejemplo, cambie el archivo src/lib.cairo en tu directorio loops para quede de la siguiente manera:
fn main() {
loop {
println!("again!");
}
}
When we run this program, we’ll see again! printed over and over continuously until either the program runs out of gas or we stop the program manually. Most terminals support the keyboard shortcut ctrl-c to interrupt a program that is stuck in a continual loop. Give it a try:
$ scarb cairo-run --available-gas=20000000
Compiling loops v0.1.0 (file:///projects/loops)
Finished release target(s) in 0 seconds
Running loops
again!
again!
again!
^Cagain!
The symbol ^C represents where you pressed ctrl-c. You may or may not see the word again! printed after the ^C, depending on where the code was in the loop when it received the interrupt signal.
Note: Cairo prevents us from running program with infinite loops by including a gas meter. The gas meter is a mechanism that limits the amount of computation that can be done in a program. By setting a value to the
--available-gasflag, we can set the maximum amount of gas available to the program. Gas is a unit of measurement that expresses the computation cost of an instruction. When the gas meter runs out, the program will stop. In the previous case, we set the gas limit high enough for the program to run for quite some time.
It is particularly important in the context of smart contracts deployed on Starknet, as it prevents from running infinite loops on the network. If you're writing a program that needs to run a loop, you will need to execute it with the
--available-gasflag set to a value that is large enough to run the program.
Now, try running the same program again, but this time with the --available-gas flag set to 200000 instead of 2000000000000. You will see the program only prints again! 3 times before it stops, as it ran out of gas to keep executing the loop.
Fortunately, Cairo also provides a way to break out of a loop using code. You can place the break keyword within the loop to tell the program when to stop executing the loop.
fn main() {
let mut i: usize = 0;
loop {
if i > 10 {
break;
}
println!("i = {}", i);
i += 1;
}
}
The continue keyword tells the program to go to the next iteration of the loop and to skip the rest of the code in this iteration. Let's add a continue statement to our loop to skip the println! statement when i is equal to 5.
fn main() {
let mut i: usize = 0;
loop {
if i > 10 {
break;
}
if i == 5 {
i += 1;
continue;
}
println!("i = {}", i);
i += 1;
}
}
Al ejecutar este programa, no se imprimirá el valor de i cuando sea igual a 5.
Retorno de Valores de Loops
Uno de los usos de un loop es volver a intentar una operación que sabes que podría fallar, como verificar si una operación ha tenido éxito. También es posible que necesites pasar el resultado de esa operación fuera del bucle para el resto de tu código. Para hacer esto, puedes agregar el valor que deseas devolver después de la expresión break que utilizas para detener el bucle; ese valor se devolverá fuera del bucle para que puedas utilizarlo, como se muestra aquí:
fn main() {
let mut counter = 0;
let result = loop {
if counter == 10 {
break counter * 2;
}
counter += 1;
};
println!("The result is {}", result);
}
Antes del bucle, declaramos una variable llamada counter y la inicializamos en 0. Luego declaramos una variable llamada result para mantener el valor devuelto por el bucle. En cada iteración del bucle, comprobamos si counter es igual a 10, y añadimos 1 a la variable counter. Cuando se cumple la condición, utilizamos la palabra clave break con el valor counter * 2. Después del bucle, usamos un punto y coma para terminar la sentencia que asigna el valor a result. Finalmente el valor en result, que en este caso es 20.
Conditional Loops with while
A program will often need to evaluate a condition within a loop. While the condition is true, the loop runs. When the condition ceases to be true, the program calls break, stopping the loop. It’s possible to implement behavior like this using a combination of loop, if, else, and break; you could try that now in a program, if you’d like. However, this pattern is so common that Cairo has a built-in language construct for it, called a while loop.
In Listing 2-2, we use while to loop the program three times, counting down each time after printing the value of number, and then, after the loop, print a message and exit.
fn main() {
let mut number = 3;
while number != 0 {
println!("{number}!");
number -= 1;
};
println!("LIFTOFF!!!");
}
Listing 2-2: Using a while loop to run code while a condition holds true.
This construct eliminates a lot of nesting that would be necessary if you used loop, if, else, and break, and it’s clearer. While a condition evaluates to true, the code runs; otherwise, it exits the loop.
Looping Through a Collection with for
You can also use the while construct to loop over the elements of a collection, such as an array. For example, the loop in Listing 2-3 prints each element in the array a.
fn main() {
let a = [10, 20, 30, 40, 50].span();
let mut index = 0;
while index < 5 {
println!("the value is: {}", a[index]);
index += 1;
}
}
Listing 2-3: Looping through each element of a collection using a while loop
Here, the code counts up through the elements in the array. It starts at index 0, and then loops until it reaches the final index in the array (that is, when index < 5 is no longer true). Running this code will print every element in the array:
$ scarb cairo-run
Compiling no_listing_45_iter_loop_while v0.1.0 (listings/ch02-common-programming-concepts/no_listing_45_iter_loop_while/Scarb.toml)
Finished `dev` profile target(s) in 4 seconds
Running no_listing_45_iter_loop_while
the value is: 10
the value is: 20
the value is: 30
the value is: 40
the value is: 50
Run completed successfully, returning []
All five array values appear in the terminal, as expected. Even though index will reach a value of 5 at some point, the loop stops executing before trying to fetch a sixth value from the array.
However, this approach is error prone; we could cause the program to panic if the index value or test condition is incorrect. For example, if you changed the definition of the a array to have four elements but forgot to update the condition to while index < 4, the code would panic. It’s also slow, because the compiler adds runtime code to perform the conditional check of whether the index is within the bounds of the array on every iteration through the loop.
As a more concise alternative, you can use a for loop and execute some code for each item in a collection. A for loop looks like the code in Listing 2-4.
fn main() {
let a = [10, 20, 30, 40, 50].span();
for element in a {
println!("the value is: {element}");
}
}
Listing 2-4: Looping through each element of a collection using a for loop
When we run this code, we’ll see the same output as in Listing 2-3. More importantly, we’ve now increased the safety of the code and eliminated the chance of bugs that might result from going beyond the end of the array or not going far enough and missing some items.
Using the for loop, you wouldn’t need to remember to change any other code if you changed the number of values in the array, as you would with the method used in Listing 2-3.
The safety and conciseness of for loops make them the most commonly used loop construct in Cairo. Even in situations in which you want to run some code a certain number of times, as in the countdown example that used a while loop in Listing 2-2. Another way to run code a certain number of times would be to use a Range, provided by the core library, which generates all numbers in sequence starting from one number and ending before another number.
Here’s how you can use a Range to count from 1 to 3:
fn main() {
for number in 1..4_u8 {
println!("{number}!");
};
println!("Go!!!");
}
This code is a bit nicer, isn’t it?
Equivalence Between Loops and Recursive Functions
Loops and recursive functions are two common ways to repeat a block of code multiple times. The loop keyword is used to create an infinite loop that can be broken by using the break keyword.
fn main() -> felt252 {
let mut x: felt252 = 0;
loop {
if x == 2 {
break;
} else {
x += 1;
}
};
x
}
Loops can be transformed into recursive functions by calling the function within itself. Here is an example of a recursive function that mimics the behavior of the loop example above.
fn main() -> felt252 {
recursive_function(0)
}
fn recursive_function(mut x: felt252) -> felt252 {
if x == 2 {
x
} else {
recursive_function(x + 1)
}
}
In both cases, the code block will run indefinitely until the condition x == 2 is met, at which point the value of x will be displayed.
In Cairo, loops and recursions are not only conceptually equivalent: they are also compiled down to similar low-level representations. To understand this, we can compile both examples to Sierra, and analyze the Sierra Code generated by the Cairo compiler for both examples. Add the following in your Scarb.toml file:
[lib]
sierra-text = true
Then, run scarb build to compile both examples. You will find the Sierra code generated by for both examples is extremely similar, as the loop is compiled to a recursive function in the Sierra statements.
Note: For our example, our findings came from understanding the statements section in Sierra that shows the execution traces of the two programs. If you are curious to learn more about Sierra, check out Exploring Sierra.
Resumen
¡Lo has conseguido! Este fue un capítulo considerable: aprendiste sobre variables, tipos de datos, funciones, comentarios, expresiones if y bucles. Para practicar con los conceptos discutidos en este capítulo, intenta construir programas para hacer lo siguiente:
- Generar el n-th número de Fibonacci.
- Calcular el factorial de un número n.
Ahora, revisaremos los tipos de colección comunes en Cairo en el próximo capítulo.