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C++: How do I use an output file as in input file in the same program?

  • Hey! I have to take a set of numbers from one file, another set of numbers from another file, and then list the total numbers in ascending order in the output file. What I decided was that I will just simply list all the numbers in an intermediary output file, and then use that file as an input file. I'll take the numbers, put them through a simple loop to arrange them in ascending order, and then output them on a second output file. The issue is that once I declared the first output file as ifstream, I can't redefine it as ofstream in the same program. Is there any way around this? Keep in mind I only know basic C++, like loops, functions and i/o file stream. Not strings, arrays, or anything. Thanks!

  • Answer:

    hi buddy , you do read and write to a single file at any point of time. a little complex but once you get on to the right track its easy and simple. use fstream filestr; filestr.open ("test.txt", fstream::in | fstream::out ); the above will open the test.txt in read and write mode. i guess you know array sorting . where you will be tracking array positions using index .. right.. here u have to use file pointer locations. . heard of seeg,seekp,tellg, and tellp. .. if not learn it from the link i provide. not a big deal so simple. now you have to implement the same array sorting logic in to the file. same as array , in file also having your number at different positions ( i expect one number at a line ) . so array[0] will be same as filepointer+0 , array[8] is same as filepointer+8. got it . and my suggestion is you should go for selection sort. once you got the concept of all the above. code your program. i am writing the algorithm here ok. you have to program. if you stuck with code. then let us know 1. open file in Input-output mode. 2. read num1 from file 3. num1loc = filepointer.tellg(); 3. if file pointer NOT EOF 3.1 read num2 3.2 num2loc = filepointer.tellg() 3.3 if ( num1 > num2 ) 3.3.1 seekp(num1loc,ios::beg) 3.3.2 write num2 to file 3.3.3 seekp(num2loc,ios::beg) 3.3.4 write num1 to file the algorithm is not complete. but it can help you . i am not having a c++ compiler in this system. an i am not supposed to install one. thats why. i cant provide you a sample working code.. i do code testing through codepad.org. and file handling is a dream over there.

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You can't start writing a sorted list until you've read all the values from at least one of your files. So, even if your goal was to simply read one file and write the sorted numbers to another file you cannot do that without intermediate storage in some type of collection like array, array list, vector, etc. In other words, you cannot sort until you have all the values read into your program. Working with file i/o usually comes after familiarity with arrays, etc...

modulo_function

You should learn to use an array. It will make this a very simple problem, store inputfile1 values, store inputfile2 values, sort array, write output, profit :)

coachabower

You can not do the operation on the same file, atleast in your problem. By the way,, does both the files contents numbers in ascending order? If so, you can follow merging of merge sort and save the total numbers in another file in ascending order without storing in arrays and taking one element from the files at a time.

James Bond

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