How to check if a string starts with another string in C? Ask Question 68. Is there something like startsWith(stra, strb) in the standard C library? It should take pointers to two strings that end with nullbytes, and tell me whether the first one also appears completely at the beginning of the second one.
Strings, Byte Arrays, and VariantsQString, QByteArray, and QVariant are three classes that have many things in common with containers and that can be used as alternatives to containers in some contexts. Also, like the containers, these classes use implicit sharing as a memory and speed optimization.We will start with QString. Every GUI program uses strings, not only for the user interface but often also as data structures. C natively provides two kinds of strings: traditional C-style '0'-terminated character arrays and the std::string class. Unlike these, QString holds 16-bit Unicode values.
Unicode contains ASCII and Latin-1 as a subset, with their usual numeric values. But since QString is 16-bit, it can represent thousands of other characters for writing most of the world's languages.
See Chapter 18 for more information about Unicode.When using QString, we don't need to worry about such arcane details as allocating enough memory or ensuring that the data is '0'-terminated. Conceptually, QStrings can be thought of as a vector of QChars. A QString can embed '0' characters.
The length function returns the size of the entire string, including embedded '0' characters.QString provides a binary + operator to concatenate two strings and a += operator to append one string to another. Because QString automatically preallocates memory at the end of the string data, building up a string by repeatedly appending characters is very fast. Here's an example that combines + and +=: QString str = 'User: ';str += userName + 'n';There is also a QString::append function that does the same thing as the += operator: str = 'User: ';str.append(userName);str.append('n');A completely different way to combine strings is to use QString's sprintf function: str.sprintf('%s%.1f%%', 'perfect competition', 100.0);This function supports the same format specifiers as the C library's sprintf function. In the preceding example, str is assigned 'perfect competition 100.0%'.Yet another way to build a string from other strings or from numbers is to use arg: str = QString('%1%2 (%3s-%4s)').arg('permissive').arg('society').arg(1950).arg(1970);In this example, '%1' is replaced by 'permissive', '%2' is replaced by 'society', '%3' is replaced by '1950', and '%4' is replaced by '1970'.
The result is 'permissive society (1950s-1970s)'. There are arg overloads to handle various data types. Some overloads have extra parameters for controlling the field width, the numerical base, or the floating-point precision. In general, arg is a much better solution than sprintf, because it is type-safe, fully supports Unicode, and allows translators to reorder the '% n' parameters.QString can convert numbers into strings using the QString::number static function: str = QString::number(59.6);Or using the setNum function: str.setNum(59.6);The reverse conversion, from a string to a number, is achieved using toInt, toLongLong, toDouble, and so on. For example: bool ok;double d = str.toDouble(&ok);These functions accept an optional pointer to a bool variable and set the variable to true or false depending on the success of the conversion.
If the conversion fails, these functions return zero.Once we have a string, we often want to extract parts of it. The mid function returns the substring starting at a given position (the first argument) and of up to a given length (the second argument). For example, the following code prints 'pays' to the console: QString str = 'polluter pays principle';qDebug.
I am having issues where the file is being uploaded twice to the server.I am using the QFileSystemWatcher class from C Qt on Windows XP to send a file when the folder changes The files are small (1-12kb).The application sends the files by scanning the folder whenever it changes (on directoryChanged signal), loop through the files and send the one that I need. The server responds with an xml file that is returned into the same folder for another application to processes it.Apparently what’s happening is that on some systems there are 2 very fast directoryChanged signals at nearly the same time and there are two very fast file uploads happening.The server is running Apache and PHP, and there’s a simple MUTEX in place on the PHP side, but I just wanted to get to the root of the problem which seems to be on the Qt side. I'm open to using another class, another library or straight C.Here is some code, I stripped all the irrelevant content.