no characters at all) then just find the length of the file and see if it's zero: ekg (0, is. If "empty" means that the length of the file is zero (i.e. Note: Some compilers already support the library as a Technical Specification (eg, GCC v5.3). Use peek() function to check if a file is empty. ![]() Namespace fs = std::filesystem // for readability Question: WRITTEN IN C One function one job Reading an Input File What if the input file is empty. Update: From C 17 onward you can use std::filesyatem::file_size: #include ![]() So if tellg() returns 0 it must be empty. The function tellg() returns the read (get) position of the file and we opened the file with the read (get) position at the end using std::ios::ate. I would open the file at the end and see what that position is using tellg(): std::ifstream ifs("myfile", std::ios::ate) // std::ios::ate means open at end The ouput of this will be as follows (assuming the input was abcd):Ĭhar1 = a, char1 = b, char1 = b, char1 = c, char1 = c.Use peek like following if ( inputFile.peek() = std::ifstream::traits_type::eof() ) Peek is used in an assignment statement as it is not a stand-alone statement like get, ignore or putback, the function also has empty parentheses. Syntax for a peek function is as follows: char = cin.peek() The stream function peek looks into the stream and tells you what the next character is without removing it from the input stream. The putback function lets you put the last character extracted from the input stream by the get function back into the input stream. NOTE: When the function ignore is used without parameters it only skips the very next character e.g. The third statement, cin>b, stores 12 (from the next line) in b. The second statement, cin.ignore(100, "\n"), discards all of the remaining numbers in the first line. The first statement, cin>a, stores 25 in a. nNumberOfBytesToRead The maximum number of bytes to be read. The caller must not use this buffer until the read operation is completed. This buffer must remain valid for the duration of the read operation. Here is a working example of the ignore function int a, b Ĭin > a cin.ignore(100, "\n") cin > b A pointer to the buffer that receives the data read from a file or device. So if there is no ‘A’ in the first 100 characters the next input will be character 101 as the first 100 will be discarded, but if character ‘A’ appears at number 64 then only the first 64 characters are discarded and the next input character is number 65.Īs per Example 3-5 in the text Book written by Mr. The integer you insert are the amount of characters that needs to be ignored, meaning if you say cin(100, ‘A’), then when it executes it will ignore the first 100 characters or until the character ‘A’ is found, whichever comes first. The syntax is as follows: cin.ignore(int, char) When you want to process only partial data, you can use the stream function ignore to discard a portion of the input. NOTE: You cannot use the get function to read a int variable. So for the same problem illustrated above the code will be as follows cin.get(char) cin.get(char2) cin > num //this will store A in char, whitespace in char2 and 25 in num. Flecs 3. ![]() The syntax of cin, together with the get function to read a character is as follows: cin.get(char1) I am writing a part of code to add some data to the end of a file that has been opened and read. Now if you wanted to store the A in char, the whitespace in char2 and 25 in num, you cannot use the extraction operator (>). Initially program creates the file and writes This is but later we had reset the write pointer at 7th position from the beginning and used puts () statement. Let us compile and run the above program that will create a file file.txt with the following content. This means that if you say: cin > char > char2 > numĪ 25, then the extraction operator will store A in char, it will skip the whitespace, store 2 in char2 and 5 in num. The following example shows the usage of fseek () function. When you input multiple data the extraction operator skips all the whitespace characters.
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