rustlings 学习笔记 -- exercises/03_if

东离的学习笔记
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exercises/03_if

if1.rs - 基础 if 条件判断

问题代码

fn bigger(a: i32, b: i32) -> i32 {
    // TODO: Complete this function to return the bigger number!
    // If both numbers are equal, any of them can be returned.
    // Do not use:
    // - another function call
    // - additional variables
}

fn main() {
    // You can optionally experiment here.
}

// Don't mind this for now :)
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn ten_is_bigger_than_eight() {
        assert_eq!(10, bigger(10, 8));
    }

    #[test]
    fn fortytwo_is_bigger_than_thirtytwo() {
        assert_eq!(42, bigger(32, 42));
    }

    #[test]
    fn equal_numbers() {
        assert_eq!(42, bigger(42, 42));
    }
}

解题要点

  • 错误原因: 函数体为空，需要实现比较两个数大小的逻辑
  • 解决方法: 使用 if 表达式比较两个参数，返回较大值

正确写法

fn bigger(a: i32, b: i32) -> i32 {
    if a > b {
        a
    } else {
        b
    }
}

fn main() {
    // You can optionally experiment here.
}

// Don't mind this for now :)
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn ten_is_bigger_than_eight() {
        assert_eq!(10, bigger(10, 8));
    }

    #[test]
    fn fortytwo_is_bigger_than_thirtytwo() {
        assert_eq!(42, bigger(32, 42));
    }

    #[test]
    fn equal_numbers() {
        assert_eq!(42, bigger(42, 42));
    }
}

if2.rs - if 表达式类型一致性

问题代码

// TODO: Fix the compiler error on this function.
fn picky_eater(food: &str) -> &str {
    if food == "strawberry" {
        "Yummy!"
    } else {
        1
    }
}

fn main() {
    // You can optionally experiment here.
}

// TODO: Read the tests to understand the desired behavior.
// Make all tests pass without changing them.
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn yummy_food() {
        // This means that calling `picky_eater` with argument "strawberry" should return "Yummy!".
        assert_eq!(picky_eater("strawberry"), "Yummy!");
    }

    #[test]
    fn neutral_food() {
        assert_eq!(picky_eater("potato"), "I guess I can eat that.");
    }

    #[test]
    fn default_disliked_food() {
        assert_eq!(picky_eater("broccoli"), "No thanks!");
        assert_eq!(picky_eater("gummy bears"), "No thanks!");
        assert_eq!(picky_eater("literally anything"), "No thanks!");
    }
}

解题要点

  • 错误原因: if 表达式的两个分支返回不同类型（字符串 vs 整数），Rust 要求类型一致
  • 解决方法: 根据测试用例，所有分支都应返回字符串类型

正确写法

fn picky_eater(food: &str) -> &str {
    if food == "strawberry" {
        "Yummy!"
    } else if food == "potato" {
        "I guess I can eat that."
    } else {
        "No thanks!"
    }
}

fn main() {
    // You can optionally experiment here.
}

// TODO: Read the tests to understand the desired behavior.
// Make all tests pass without changing them.
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn yummy_food() {
        // This means that calling `picky_eater` with argument "strawberry" should return "Yummy!".
        assert_eq!(picky_eater("strawberry"), "Yummy!");
    }

    #[test]
    fn neutral_food() {
        assert_eq!(picky_eater("potato"), "I guess I can eat that.");
    }

    #[test]
    fn default_disliked_food() {
        assert_eq!(picky_eater("broccoli"), "No thanks!");
        assert_eq!(picky_eater("gummy bears"), "No thanks!");
        assert_eq!(picky_eater("literally anything"), "No thanks!");
    }
}

if3.rs - if 表达式与类型推断

问题代码

fn animal_habitat(animal: &str) -> &str {
    // TODO: Fix the compiler error in the statement below.
    let identifier = if animal == "crab" {
        1
    } else if animal == "gopher" {
        2.0
    } else if animal == "snake" {
        3
    } else {
        "Unknown"
    };

    // Don't change the expression below!
    if identifier == 1 {
        "Beach"
    } else if identifier == 2 {
        "Burrow"
    } else if identifier == 3 {
        "Desert"
    } else {
        "Unknown"
    }
}

fn main() {
    // You can optionally experiment here.
}

// Don't change the tests!
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn gopher_lives_in_burrow() {
        assert_eq!(animal_habitat("gopher"), "Burrow")
    }

    #[test]
    fn snake_lives_in_desert() {
        assert_eq!(animal_habitat("snake"), "Desert")
    }

    #[test]
    fn crab_lives_on_beach() {
        assert_eq!(animal_habitat("crab"), "Beach")
    }

    #[test]
    fn unknown_animal() {
        assert_eq!(animal_habitat("dinosaur"), "Unknown")
    }
}

解题要点

  • 错误原因: if 表达式的分支返回不同类型（整数、浮点数、字符串），无法推断统一类型
  • 解决方法: 所有分支返回相同类型，根据后续使用场景选择整数类型

正确写法

fn animal_habitat(animal: &str) -> &str {
    let identifier = if animal == "crab" {
        1
    } else if animal == "gopher" {
        2
    } else if animal == "snake" {
        3
    } else {
        0  // 使用整数 0 表示未知
    };

    // Don't change the expression below!
    if identifier == 1 {
        "Beach"
    } else if identifier == 2 {
        "Burrow"
    } else if identifier == 3 {
        "Desert"
    } else {
        "Unknown"
    }
}

fn main() {
    // You can optionally experiment here.
}

// Don't change the tests!
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn gopher_lives_in_burrow() {
        assert_eq!(animal_habitat("gopher"), "Burrow")
    }

    #[test]
    fn snake_lives_in_desert() {
        assert_eq!(animal_habitat("snake"), "Desert")
    }

    #[test]
    fn crab_lives_on_beach() {
        assert_eq!(animal_habitat("crab"), "Beach")
    }

    #[test]
    fn unknown_animal() {
        assert_eq!(animal_habitat("dinosaur"), "Unknown")
    }
}

知识点汇总

if 基础语法

  • if 条件判断: 使用 if condition { } else { } 进行条件分支
  • if 表达式: if 可以作为表达式使用，返回值
  • else if 链: 使用 else if 处理多个条件分支

if 表达式特性

  • 类型一致性: if 表达式的所有分支必须返回相同类型
  • 返回值: 最后一个表达式的值作为整个 if 表达式的返回值
  • 类型推断: 编译器根据所有分支推断统一类型

实际应用

  • 比较操作: 使用 >, <, == 等比较运算符
  • 字符串比较: 使用 == 比较字符串字面量
  • 条件赋值: 使用 if 表达式进行条件赋值操作
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