Parameters and Scope - Забони барномасозии C++ барои барномасозии рақобатӣ

Week 7Functions, Parameters and Scope, Program Decomposition and Debugging

Parameters and Scope

Passing by value and by reference, several answers through references, vectors and strings as const references, local variables and their lifetime, and global variables.

Назария
Дарси 3 аз 9
Хониш: 15 дақиқа
Саволҳо: 5

In the last lesson, digitCount divided its n by 10 until nothing was left, and the number in main didn't change. That is no accident. This lesson is about how values get into a function — as a copy or as the original — and about where each variable lives and for how long.

Passing by value: the function gets a copy

An ordinary parameter is a new variable that gets a copy of the argument. Whatever the function does to it stays inside the function:

This is called passing by value, and most of the time it is exactly what you want: the function can use its parameter as a scratch variable, and the caller's variables are safe from it.

Passing by reference: &

Sometimes changing the caller's variable is the whole point. Put & after the type, and the parameter becomes a reference: not a new variable, but another name for the argument itself.

The classic example is swapping two variables. Here are both versions side by side; step through and watch a and b:

A copy, then the original: swapCopy and swapRef

C++

1#include <bits/stdc++.h>

2using namespace std;

3

4void swapCopy(int x, int y) {

5 int t = x;

6 x = y;

7 y = t;

8}

9

10void swapRef(int &x, int &y) {

11 int t = x;

12 x = y;

13 y = t;

14}

15

16int main() {

17 ios_base::sync_with_stdio(false);

18 cin.tie(NULL);

19

20 int a = 3, b = 8;

21 swapCopy(a, b);

22 cout << a << " " << b << "\n";

23 swapRef(a, b);

24 cout << a << " " << b << "\n";

25 return 0;

26}

Start in main()

сатр 16

The program starts in main(). The two functions above it run only when they are called.

стеки даъватҳо

main()

a—b—x—y—t—

1 / 16

навбатӣ

Practice: Parameters and Scope

Eight problems for references and for passing vectors and strings to functions.

Дарси навбатӣ

Дарси пешина Ба харитаи курс

Parameter	What the function gets	Use it when
`vector<int> a`	a copy of the whole vector	almost never: copying 10⁵ numbers at every call is slow, and changes are lost
`vector<int> &a`	the original	the function changes the vector
`const vector<int> &a`	the original, read-only	the function only reads it — the usual choice

#include <bits/stdc++.h>
using namespace std;

void swapCopy(int x, int y) {    // gets copies of the values
    int t = x;
    x = y;
    y = t;
}

void swapRef(int &x, int &y) {   // x and y are other names for main's a and b
    int t = x;
    x = y;
    y = t;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int a = 3, b = 8;
    swapCopy(a, b);
    cout << a << " " << b << "\n";
    swapRef(a, b);
    cout << a << " " << b << "\n";
    return 0;
}

error: 'count' was not declared in this scope

#include <bits/stdc++.h>
using namespace std;

void divide(int a, int b, int &q, int &r) {   // two answers, through references
    q = a / b;
    r = a % b;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int a, b;
    cin >> a >> b;
    int q, r;
    divide(a, b, q, r);         // fills q and r
    cout << a << " = " << b << " * " << q << " + " << r << "\n";
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

vector<int> readVector(int n) {
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    return a;                   // the whole vector goes back to the caller
}

double average(const vector<int> &a) {   // reads a, never copies or changes it
    long long sum = 0;
    for (int x : a) {
        sum += x;
    }
    return (double)sum / a.size();
}

int countAbove(const vector<int> &a, double limit) {
    int count = 0;
    for (int x : a) {
        if (x > limit) {
            count++;
        }
    }
    return count;
}

void addBonus(vector<int> &a, int bonus) {   // & : changes main's vector
    for (int i = 0; i < (int)a.size(); i++) {
        a[i] += bonus;
    }
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    cin >> n;
    vector<int> scores = readVector(n);
    double avg = average(scores);
    cout << avg << " " << countAbove(scores, avg) << "\n";
    addBonus(scores, 5);
    cout << scores[0] << "\n";
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

bool isPalindrome(const string &s) {
    int n = s.size();
    for (int i = 0; i < n / 2; i++) {
        if (s[i] != s[n - 1 - i]) {
            return false;       // one mismatch is enough
        }
    }
    return true;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int q;
    cin >> q;
    for (int i = 0; i < q; i++) {
        string w;
        cin >> w;
        if (isPalindrome(w)) {
            cout << "YES\n";
        } else {
            cout << "NO\n";
        }
    }
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

int total = 0;                  // global: lives as long as the program

int countLocal() {
    int calls = 0;              // local: made again at every call
    calls++;
    return calls;
}

int countGlobal() {
    total++;
    return total;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    for (int i = 0; i < 3; i++) {
        cout << countLocal() << " ";
    }
    cout << "\n";
    for (int i = 0; i < 3; i++) {
        cout << countGlobal() << " ";
    }
    cout << "\n";
    return 0;
}