C++ - Tutorial 005. Pointers, Arrays and Loops

C++, pointer, arrays, loops

An array of elements of type char can be declared like this:

char v[6];  // array of 6 characters

Similarly, a pointer can be declared like this:

char* p; // pointer to character

In declarations, [] means ‘‘array of’’ and *means ‘‘pointer to.’’ All arrays have0as their lower bound, so v has six elements, v[0] to v[5] . The size of an array must be a constant expression. A pointer variable can hold the address of an object of the appropriate type:

char∗ p = &v[3]; // p указывает на четвёртый элемент массива v
char x = ∗p; // *p является объектом, на который указывает p

In an expression, prefix unary ∗ means ‘‘contents of’’ and prefix unary & means ‘‘address of.’’ We can represent the result of that initialized definition graphically:

Consider copying ten elements from one array to another:

void copy_fct()
{
    int v1[10] = {0,1,2,3,4,5,6,7,8,9};
    int v2[10];  // to become a copy of v1

    for (auto i=0; i!=10; ++i) // copy elements
        v2[i]=v1[i];
}

This for -statement can be read as ‘‘set i to zero; while i is not 10 , copy the i th element and increment i.’’ When applied to an integer variable, the increment operator, ++ , simply adds 1 . C++11 also offers a simpler for -statement, called a range-for-statement, for loops that traverse a sequence in the simplest way:

void print()
{
    int v[] = {0,1,2,3,4,5,6,7,8,9};
    for (auto x : v)
        cout << x << '\n'; // for each x in v

    for (auto x : {10,21,32,43,54,65})
        cout << x << '\n';
}

The first range-for-statement can be read as ‘‘for every element of v , from the first to the last, place a copy in x and print it.’’ Note that we don’t hav e to specify an array bound when we initialize it with a list. The range-for-statement can be used for any sequence of elements.

If we didn’t want to copy the values from v into the variable x, but rather just have x refer to an element, we could write:

void increment()
{
    int v[] = {0,1,2,3,4,5,6,7,8,9};
    for (auto& x : v)
        ++x;
}

In a declaration, the unary suffix & means ‘‘reference to.’’ A reference is similar to a pointer, except that you don’t need to use a prefix * to access the value referred to by the reference.

T a[n];  // array of n Ts
T∗ p;    // pointer to T
T& r;    // reference to T
T f(A);  // function taking an argument of type A returning a result of type T

We try to ensure that a pointer always points to an object, so that dereferencing it is valid. When we don’t hav e an object to point to or if we need to represent the notion of ‘‘no object available’’ (e.g., for an end of a list), we give the pointer the value nullptr (‘‘the null pointer’’). There is only one nullptr shared by all pointer types:

double∗ pd = nullptr;
Link<Record>∗ lst = nullptr; // pointer to a Link to a Record
int x = nullptr; // error : nullptr is a pointer not an integer

It is often wise to check that a pointer argument that is supposed to point to something, actually points to something:

int count_x(char∗ p, char x)
    // count the number of occurrences of x in p[]
    // p is assumed to point to a zero-ter minated array of char (or to nothing)
{
    if (p==nullptr) return 0;
    int count = 0;
    for (; ∗p!=0; ++p)
        if (∗p==x)
            ++count;
    return count;
}

Note how we can move a pointer to point to the next element of an array using ++ and that we can leave out the initializer in a for-statement if we don’t need it.

In older code, 0 or NULL is typically used instead of nullptr. However, using nullptr eliminates potential confusion between integers (such as 0 or NULL ) and pointers (such as nullptr ).

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TT
June 13, 2019, 7:01 p.m.
Taimoor Tanweer

C++ - Test 001. The first program and data types

  • Result:66points,
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TT
June 13, 2019, 6:51 p.m.
Taimoor Tanweer

C++ - Test 002. Constants

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ВМ
June 13, 2019, 12:30 p.m.
Ваня Мороз

C++ - Test 001. The first program and data types

  • Result:100points,
  • Rating points10
Last comments
i
June 17, 2019, 6:10 a.m.
ingenfly

Только по осям xAxis2, уAxis2 значения начинаются с 0. Почему-то xAxis2 и xAxis не синхронизированы по данным. Ну и QCustomPlot последний.
June 16, 2019, 8:21 p.m.
Евгений Легоцкой

Добрый день. Ну точно также добавляете ту же самую информацию на ось xAxis2, только добавляете другое форматирование customPlot->xAxis2->setDateTimeFormat("hh:mm"); если я ...
EF
June 14, 2019, 1:56 p.m.
Egor Fomin

Спасибо за ваш ответ, у меня получилось реализовать это. Тем не менее появилась другая проблема, поэтому опять надеюсь на вашу помощь. Скажем, я уже выставил точки и они соеденены. Когда я нач...
d
June 13, 2019, 2:47 p.m.
damix

Можно классу, который описывает точку, добавить сигнал, который подавать (emit), когда точка перемещается (переопределить mouseMoveEvent или mouseReleaseEvent). Так вот эти сигналы у каждой из...
i
June 13, 2019, 2:09 p.m.
ingenfly

Здравствайте! Подскажите, пожалуйста: customPlot->xAxis2->setTickLabels(true); //Здесь включается отображение данных на оси xAxis2. а можно как-то продублировать информацию cus...
Now discuss on the forum
I
June 19, 2019, 1:41 p.m.
Intruder

Всем добрый день. При разборе XML файла наткнулся на тег вот такого плана: <TagName attribute1="value1" attribute2="value2" /> При попытке проверить на наличие такого элеме...
June 19, 2019, 12:55 p.m.
Михаиллл

Скажите пожалуйста, как его в таком случае перемещать и удалять?
June 18, 2019, 7:50 p.m.
Дмитрий

Большое спасибо! SDK заработал.К сожалению удалось продвинутся только на один шаг. При сборке чистого проекта NDK выдаёт следующие ошибки C:\Android\ndk-bundle/toolchains/arm-linux-andr...
June 18, 2019, 4:59 p.m.
Михаиллл

Добрый день.В этом учебнике представлен код INSTALLED_APPS = ( ... 'rest_framework', 'snippets.apps.SnippetsConfig',) На строчке 'snippets.apps.SnippetsConf...
June 18, 2019, 2:24 p.m.
Михаиллл

Спасибо, работает.Послушаю вашего совета.
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