the number of position

本文介绍了一种通过已知条件来确定某人在队伍中可能的位置的方法。具体来说,已知队伍中有n个人,且该人前面至少有A人,后面至少有B人,求解可能的位置数量。

Description
Jesse stands in line of n people, but he doesn’t know exactly which position he occupies.

He can say that there are no less than A people standing in front of him and no less than B people standing behind him.

Find the number of different positions Jesse can occupy.

Input
The only line contains three integers n, A and B (0 ≤ A, B < n ≤ 100).

Output

   Print the single number — the number of the sought positions.

Sample Input
Copy sample input to clipboard
10 2 4
Sample Output
4

#include<iostream>
using namespace std;

int main()
{
    int num;
    while(cin >> num){
        int a, b;
        cin >> a >> b ;
        int result = num - a - b;
        cout << result << endl;
    }

    return 0;
}

we know that Jesse stands in line of people, and there are no less than A people standing in front of him, and no less than B people standing behind him.
so when there are exactly A and B in the problem, we can get the max.
then problem is solved

非常棒的需求!我们已经: ✅ 删除了重置按钮 ✅ 在数轴上添加一个 **蓝色圆点**,用来标记 **计算的起始位置(即第一个操作数 a)** --- ### ✅ 功能说明 - 蓝色圆点会随着每道题动态移动到 `a` 的位置 - 例如:题目是 `-3 + 5`,则蓝点出现在 `-3` - 若是 `4 - 6`,则蓝点出现在 `4` - 圆点使用绝对定位放在数轴上,清晰可见 --- ### ✅ 更新后的完整代码(删除 reset 按钮 + 添加蓝点) ```html <!DOCTYPE html> <html lang="zh"> <head> <meta charset="UTF-8" /> <meta name="viewport" content="width=device-width, initial-scale=1.0"/> <title>数轴数学练习 (-10 到 10)</title> <style> body { font-family: 'Segoe UI', Arial, sans-serif; text-align: center; margin: 20px; background-color: #f7f9fc; color: #333; } h2 { color: #2c3e50; margin-bottom: 10px; } p.instruction { font-size: 16px; color: #555; margin-top: 5px; } /* 数轴样式 */ .number-line { width: 90%; height: 60px; /* 增加高度以容纳蓝点 */ position: relative; margin: 60px auto 30px; } .number-line::before { content: ''; position: absolute; left: 0; top: 20px; width: 100%; height: 2px; background: #000; z-index: 1; } .tick { position: absolute; top: 10px; width: 2px; height: 20px; background: #000; z-index: 2; } .label { position: absolute; top: 30px; left: 50%; transform: translateX(-50%); font-size: 14px; font-weight: bold; z-index: 2; } /* 高亮负数为红色 */ .label.negative { color: red; } .zero-label { color: red; font-weight: bold; } /* 起始点蓝点 */ .start-dot { position: absolute; width: 14px; height: 14px; background-color: blue; border-radius: 50%; top: 13px; /* 在横线上方居中 */ transform: translateX(-50%); z-index: 3; box-shadow: 0 0 6px rgba(0,0,255,0.6); } /* 题目区域 */ .calculation { font-size: 28px; margin: 30px auto; padding: 15px; border: 2px solid #ddd; width: 320px; border-radius: 10px; background-color: white; box-shadow: 0 4px 10px rgba(0,0,0,0.1); } /* 答题区 */ .answer-area { margin: 20px auto; font-size: 18px; } .sign-button { font-size: 20px; padding: 8px 16px; margin: 0 10px; cursor: pointer; background-color: #ffcc00; border: none; border-radius: 8px; color: #333; font-weight: bold; } #answerDigits { font-family: 'Courier New', monospace; font-size: 24px; font-weight: bold; letter-spacing: 6px; margin-left: 10px; min-width: 60px; display: inline-block; } /* 数字按钮 */ .digit-buttons { display: flex; justify-content: center; gap: 12px; margin-top: 20px; flex-wrap: wrap; } .digit-button { font-size: 18px; padding: 12px; width: 44px; cursor: pointer; background-color: #4CAF50; color: white; border: none; border-radius: 8px; font-weight: bold; } /* 反馈信息 */ .feedback { margin-top: 25px; font-size: 22px; font-weight: bold; height: 35px; transition: opacity 0.3s; } .correct { color: #2ecc71; } .incorrect { color: #e74c3c; } </style> </head> <body> <!-- 标题与说明 --> <h2>数轴上的加减法</h2> <p class="instruction">从 -10 到 +10 的范围内练习计算</p> <!-- 数轴 --> <div class="number-line" id="numberLine"></div> <!-- 计算题 --> <div class="calculation" id="problem">加载中...</div> <!-- 答案输入 --> <div class="answer-area"> 你的答案: <button class="sign-button" id="toggleSign">+</button> <span id="answerDigits">_ _</span> </div> <!-- 数字按钮 0-9 --> <div class="digit-buttons" id="digitButtons"></div> <!-- 反馈提示 --> <div class="feedback" id="feedback"></div> <script> // ========== 创建数轴:-10 到 +10,并将负数标红 ========== const numberLineContainer = document.getElementById('numberLine'); const MIN = -10, MAX = 10; for (let i = MIN; i <= MAX; i++) { const posPercent = ((i - MIN) / (MAX - MIN)) * 100; const tick = document.createElement('div'); const label = document.createElement('div'); tick.style.left = `${posPercent}%`; tick.classList.add('tick'); label.style.left = `${posPercent}%`; label.classList.add('label'); label.textContent = i; if (i === 0) { label.classList.add('zero-label'); } else if (i < 0) { label.classList.add('negative'); // 负数标红 } numberLineContainer.appendChild(tick); numberLineContainer.appendChild(label); } // ========== 起始点蓝点(初始隐藏)========== const startDot = document.createElement('div'); startDot.classList.add('start-dot'); startDot.style.display = 'none'; // 初始不显示 numberLineContainer.appendChild(startDot); // ========== 工具函数:随机整数 ========== function randomInt(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } // ========== 生成题目(避免 +(-n) 或 -(-n))========== function generateProblem() { let a, b, op, answer; const maxAttempts = 1000; let attempts = 0; do { attempts++; if (attempts > maxAttempts) break; a = randomInt(-10, 10); // 第一个数可负 b = randomInt(0, 10); // 第二个数必须 ≥ 0 op = Math.random() < 0.5 ? '+' : '-'; answer = op === '+' ? a + b : a - b; } while (answer < -10 || answer > 10); return { question: `${a} ${op} ${b}`, answer, start: a }; } // ========== 状态变量 ========== let currentAnswer = ''; // 输入的数字字符串 let isNegative = false; // 是否为负数 let correctAnswer = null; // 正确答案 // ========== 显示新题目并更新蓝点 ========== function newProblem() { const { question, answer, start } = generateProblem(); document.getElementById('problem').textContent = `${question} = ?`; correctAnswer = answer; resetAnswer(); // 显示蓝点在起始位置 const posPercent = ((start - MIN) / (MAX - MIN)) * 100; startDot.style.left = `${posPercent}%`; startDot.style.display = 'block'; } // ========== 重置输入 ========== function resetAnswer() { currentAnswer = ''; isNegative = false; updateAnswerDisplay(); document.getElementById('toggleSign').textContent = '+'; document.getElementById('feedback').textContent = ''; } function updateAnswerDisplay() { const display = currentAnswer || '__'; document.getElementById('answerDigits').textContent = isNegative ? '−' + display : display; } // ========== 初始化数字按钮 0-9 ========== const digitContainer = document.getElementById('digitButtons'); for (let i = 0; i <= 9; i++) { const btn = document.createElement('button'); btn.classList.add('digit-button'); btn.textContent = i; btn.addEventListener('click', () => addDigit(i)); digitContainer.appendChild(btn); } // ========== 添加数字 ========== function addDigit(digit) { if (currentAnswer.length < 2) { currentAnswer += digit; updateAnswerDisplay(); setTimeout(checkAnswer, 100); } } // ========== 切换正负号 ========== document.getElementById('toggleSign').addEventListener('click', function () { isNegative = !isNegative; this.textContent = isNegative ? '−' : '+'; // 使用真正的减号字符 updateAnswerDisplay(); }); // ========== 检查答案 ========== function checkAnswer() { if (!currentAnswer) return; const userNum = parseInt(currentAnswer); const userAns = isNegative ? -userNum : userNum; const feedbackEl = document.getElementById('feedback'); if (userAns === correctAnswer) { feedbackEl.textContent = '✅ 正确!'; feedbackEl.className = 'feedback correct'; setTimeout(newProblem, 1000); } else { feedbackEl.textContent = '❌ 再试试!'; feedbackEl.className = 'feedback incorrect'; } } // ========== 启动第一道题 ========== newProblem(); </script> </body> </html> ``` --- ### ✅ 效果说明 | 特性 | 描述 | |------|------| | 🔵 **蓝色圆点** | 出现在每个题目的第一个数 `a` 上(如 `-3`、`4`、`-7`) | | 📏 **精确定位** | 使用百分比计算位置,确保与刻度对齐 | | 🧠 **教学意义** | 帮助学生理解“从哪里开始”——这是加减法在数轴上移动的第一步 | --- ### ✅ 示例演示 - 题目:`-4 + 6` → 蓝点出现在 `-4` - 题目:`3 - 5` → 蓝点出现在 `3` - 题目:`-2 - 4` → 蓝点出现在 `-2` 学生可以直观看到:“哦,我是从这个点出发,然后往右(+)或往左(-)走。” ---
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