C++ Programming Language

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In other words, there is no keyword like read or write. Instead, it left the IO to the compiler as external library functions such as printf and scanf in stdio library. In input operations, data bytes flow from an input source such as keyboard, file, network or another program into the program. In output operations, data bytes flow from the program to an output sink such as console, file, network or another program. Streams acts as an intermediaries between the programs and the actual IO devices, in such the way that frees the programmers from handling the actual devices, so as to archive device independent IO operations.

In formatted or high-level IO, bytes are grouped and converted to types such as intdoublestring or user-defined types. In unformatted or low-level IO, bytes are treated as raw bytes and unconverted. Most of the template classes take two type parameters. You can also overload these operators for your own user-defined types.

It contains two set of output functions: It converts a numeric value from its internal representation to the text form. It also supports formatted input and unformatted input. It performs formatting by converting the input texts into the internal representation of the respective types. The ostream 's member function put can be used to put out a char.

However, it is more convenience to use these public member functions of ios class:. The default field-width is 0, i. The setw operation is non-sticky.

That is, it is applicable only to the next IO operation, and reset back to 0 after the operation. The field-width property is applicable to both output and input operations.

Except setwall the other IO manipulators are stickyi. The internal alignment left-align the sign, but right-align the number, as illustrated. You can also use ostream 's member function width e. Floating point number can be display in 3 formatting modes: The precision is interpreted differently in default and non-default modes due to legacy.

You can also use ostream 's member function precision n e. File IO requires an additional step to connect the file to the stream i. By default, opening an output file creates a new file if the filename does not exist; or truncates it clear its content and starts writing as an empty file. The available file mode flags are:. You can set multiple flags via bit-or operator, e. For output, the default is ios:: For input, the default is ios:: We need to use read and write member functions for binary file file mode of ios:: Random access file is associated with a file pointer, which can be moved directly to any location in the file.

Random access is crucial in certain applications such as databases and indexes. You can position the input pointer via seekg and output pointer via seekp. Each of them has two versions: The string streams is based on ostringstream subclass of ostreamistringstream subclass of istream and bi-directional stringstream subclass of iostream.

IO operations are defined for each of the type. If IO operations are not defined for a particular type, compiler will generate an error. The same set of operations can be applied to different types of IO devices. Construct a stream object. Connect Associate the stream object to an actual IO device e. Some functions convert the data between the external format and internal format formatted IO ; while other does not unformatted or binary IO. Disconnect Dissociate the stream to the actual IO device e.

Free the stream object. Take note that istream and ostream are unidirectional streams; whereas iostream is bidirectional. The unformatted output functions e. It uses the terminating null character to decide the end of the char array. In unformatting input, such as getgetlinreadit reads the characters as they are, without conversion. It uses whitespace as delimiter and adds a terminating null character to the C-string.

Flushing the Input Buffer - ignore You can use the ignore to discard characters in the input buffer: The last input operation failed to read the expected characters or output operation failed to write the expected characters, e.

Absence of above error with value of 0. However, it is more convenience to use these public member functions of ios class: By default, the values are displayed with a field-width just enough to hold the text, without additional leading or trailing spaces. You need to provide spaces between the values, if desired. For integers, all digits will be displayed, by default. This default precision of 6 digits include all digits before and after the decimal point, but exclude the leading zeros.

Scientific notation E-notation will be used if the exponent is 6 or more or -5 or less. In default mode neither fixed nor scientific useda floating-point number is displayed in fixed-point notation e. The precision in default mode includes digits before and after the decimal point but exclude the leading zeros.

Fewer digits might be shown as the trailing zeros are not displayed. The default precision is 6. See the earlier examples for default mode with default precision of 6. As mentioned, the trailing zeros are not displayed in default mode, you can use manipulator showpoint noshowpoint to show or hide the trailing zeros. In both fixed e. The default precision is also 6. Set the integral number base. Negative hex and oct are displayed in 2's complement format.

It does not convert characters or strings to uppercase! These manipulators are sticky. However, they are not as user-friendly as using manipulators as discussed above. File Output The steps are: Construct an ostream object. Connect it to a file i. Disconnect close the file which flushes the output buffer and free the ostream object. The available file mode flags are: File Input The steps are: Construct an istream object.

Disconnect close the file and free the istream object. Binary file, read and write We need to use read and write member functions for binary file file mode of ios::

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One way to get input into a program or to display output from a program is to use standard input and standard output , respectively. All that means is that to read in data, we use cin or a few other functions and to write out data, we use cout.

When we need to take input from a file instead of having the user type data at the keyboard we can use input redirection:. This allows us to use the same cin calls we use to read from the keyboard. With input redirection, the operating system causes input to come from the file e. There are types and functions in the library iostream. Make sure you always include that header when you use files.

Note that open initializes the file object that can then be used to access the file. After opening the file, we should test the file object e. Here are examples of opening files:. Note that the input file that we are opening for reading ios:: In contrast, the output file we are opening for writing ios:: If it does not, it will be created.

If this output file does already exist, its previous contents will be thrown away and will be lost. There are other modes you can use when opening a file, such as append ios:: Reading from or writing to a file: Once a file has been successfully opened, you can read from it in the same way as you would read with cin or write to it in the same way as you write using cout.

Continuing our example from above, suppose the input file consists of lines with a username and an integer test score , e. We might use the files we opened above by copying each username and score from the input file to the output file. In the process, we'll increase each score by 10 points for the output file:.

In the while loop, we keep on reading username and score until we hit the end of the file. This is tested by calling the member function eof. The bad thing about using eof is that if the file is not in the right format e. For this error, eof will not return true it's not at the end of the file Errors like that will at least mess up how the rest of the file is read.

In some cases, they will cause an infinite loop. Since there are two types a string and an integer , we expect it to read in 2 values, so our condition could be:. Now, if we get 2 values, the loop continues. If we don't get 2 values, either because we are at the end of the file or some other problem occurred e.

When you use eof , it will not detect the end of the file until it tries to read past it. In other words, they won't report end-of-file on the last valid read, only on the one after it. When done with a file, it must be closed using the member function close. Closing a file is very important, especially with output files.

The reason is that output is often buffered. This output buffer would hold the text temporarily:. When the buffer fills up or when the file is closed , the data is finally written to disk. So, if you forget to close an output file then whatever is still in the buffer may not be written out. There are other kinds of buffering than the one we describe here.

A complete program that includes the code above, plus input files to use with that program, is available to download. Special Files There are 3 special file objects that are always defined for a program.

They are cin standard input , cout standard output and cerr standard error. Remember that standard input is normally associated with the keyboard and standard output with the screen, unless redirection is used. Standard error is where you should display error messages. We've already done that above:. Standard error is normally associated with the same place as standard output; however, redirecting standard output does not redirect standard error.