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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.