条件判断：if / elseConditionals: if / else

5.1 机器人也要"看情况"5.1 Robots Need to "Decide"

想象你的机器人正在场上跑，突然电池快没电了。你会希望它怎么做？Imagine your robot is running on the field and the battery is getting low. What would you want it to do?

• 如果电量还够 → 继续跑If there's enough battery → keep going
• 如果电量太低 → 停下来，屏幕显示警告If the battery is too low → stop and show a warning on screen

这就是条件判断 — 让程序根据不同情况，做不同的事。This is a conditional — letting the program do different things based on different situations.

5.2 最简单的 if5.2 The Simplest if

if 的意思是"如果条件成立，就执行大括号里的代码"。if means "if the condition is true, run the code inside the curly braces."

int main() {
    int battery = Brain.Battery.capacity();

    if (battery < 20) {
        Brain.Screen.print("Warning: Low Battery!");
    }

    return 0;
}

int main() {
    int battery = Brain.Battery.capacity();

    if (battery < 20) {
        Brain.Screen.print("Warning: Low Battery!");
    }

    return 0;
}

void opcontrol() {
    int battery = pros::battery::get_capacity();

    if (battery < 20) {
        pros::lcd::set_text(1, "Warning: Low Battery!");
    }
}

解读：Explanation:

• if (...) — 括号里是条件if (...) — The parentheses contain the condition
• { ... } — 大括号里是"条件成立时要做的事"{ ... } — The curly braces contain "what to do if the condition is true"
• 如果电量 ≥ 20，大括号里的代码直接跳过，什么都不会发生If battery ≥ 20, the code inside the braces is skipped entirely — nothing happens

5.3 if-else：二选一5.3 if-else: Choose One of Two

有时候不只是"做或不做"，而是"做 A 还是做 B"。这时候用 else。Sometimes it's not just "do or don't" — it's "do A or do B." That's when you use else.

int main() {
    int battery = Brain.Battery.capacity();

    if (battery < 20) {
        Brain.Screen.print("Battery LOW!");
    } else {
        Brain.Screen.print("Battery OK!");
    }

    return 0;
}

int main() {
    int battery = Brain.Battery.capacity();

    if (battery < 20) {
        Brain.Screen.print("Battery LOW!");
    } else {
        Brain.Screen.print("Battery OK!");
    }

    return 0;
}

void opcontrol() {
    int battery = pros::battery::get_capacity();

    if (battery < 20) {
        pros::lcd::set_text(1, "Battery LOW!");
    } else {
        pros::lcd::set_text(1, "Battery OK!");
    }
}

if-else 就是"二选一"：条件成立走 if，不成立走 else。永远只走其中一个。if-else is "pick one of two": if the condition is true, run the if block; otherwise, run the else block. Only one ever runs.

5.4 多条件：if - else if - else5.4 Multiple Conditions: if - else if - else

如果情况不止两种呢？比如电量分三个等级：What if there are more than two cases? For example, battery with three levels:

int main() {
    int battery = Brain.Battery.capacity();

    if (battery < 20) {
        Brain.Screen.print("DANGER! Almost dead!");
    } else if (battery < 50) {
        Brain.Screen.print("Battery getting low...");
    } else {
        Brain.Screen.print("Battery is good!");
    }

    return 0;
}

int main() {
    int battery = Brain.Battery.capacity();

    if (battery < 20) {
        Brain.Screen.print("DANGER! Almost dead!");
    } else if (battery < 50) {
        Brain.Screen.print("Battery getting low...");
    } else {
        Brain.Screen.print("Battery is good!");
    }

    return 0;
}

void opcontrol() {
    int battery = pros::battery::get_capacity();

    if (battery < 20) {
        pros::lcd::set_text(1, "DANGER! Almost dead!");
    } else if (battery < 50) {
        pros::lcd::set_text(1, "Battery getting low...");
    } else {
        pros::lcd::set_text(1, "Battery is good!");
    }
}

规则很简单：从上往下检查，哪个条件先成立就执行哪个，后面的全跳过。The rule is simple: check from top to bottom. Whichever condition is true first gets executed, and the rest are skipped.

5.5 比较运算符5.5 Comparison Operators

条件里要用到比较，这些是 C++ 的比较符号：Conditions use comparisons. Here are the C++ comparison operators:

5.6 逻辑运算符：组合条件5.6 Logical Operators: Combining Conditions

有时候一个条件不够，需要组合多个条件：Sometimes one condition isn't enough — you need to combine multiple conditions:

例子：机器人只有在电量够而且没被禁用时才能跑：Example: The robot can only run when the battery is sufficient and it's not disabled:

bool disabled = false;
int battery = 80;

if (battery > 20 && !disabled) {
    // 可以跑！
}

if (battery < 10 || disabled) {
    // 电量太低 或者 被禁用了，停下来
}

5.7 动手预测5.7 Predict the Output

看下面这段代码，先在脑子里想想屏幕会显示什么，再点开答案验证。Look at the code below. Think about it first — what will the screen show? Then click to check your answer.

int score = 75;

if (score >= 90) {
    // 显示 "A"
} else if (score >= 80) {
    // 显示 "B"
} else if (score >= 60) {
    // 显示 "C"
} else {
    // 显示 "D"
}

int x = 15;
if (x > 20) {
    // 显示 "big"
} else if (x > 10) {
    // 显示 "medium"
} else {
    // 显示 "small"
}