Change a file name including char and float

Question

I'm working on a C++ code, but I'm having some problems since I have never used this language before.

The structure of my project (.pro) file contains different .ccp files, including:

IOFile.cpp : where the file name I want to change is;

CModel.cpp : where there is the model of my project and, particularly, the parameter (float), chosen by user each time, that I would like to use for my file name.

So, for now the output name is something like: filename.txt, but I would like to rename it in order to obtain in somthing like filename_parameter.txt (for example: file_0.998.txt).

The void of my IOFile.cpp is written like:

void IOFile::saveIntensityMap(const char* fileName, vector<float>* intensityMap)
{
   std::ofstream file(fileName, std::fstream::out|std::fstream::trunc);

}

whereas the parameter (ray_factor) is initialized in the CModel.cpp file:

ray_factor= 0.99f

So, how could I pass the ray_factor, that is in CModel, to the IOfile? And how can I write a filename that includes a string and a float? Sorry for the silly question, but I can't find a solution.

Possible duplicate of [Convert float to std::string in C++](http://stackoverflow.com/questions/2125880/convert-float-to-stdstring-in-c) — Dialecticus, Nov 30 '16 at 16:17
Welcome to SO. Your post doesn't provide enough information to solicit constructive answers. For example, you don't show how or where `saveIntensityMap` is actually called, nor how or where `filename` is constructed. When asking a question about a problem caused by your code, you will get much better answers if you provide code people can use to reproduce the problem. See [SO help here](http://stackoverflow.com/help/mcve). — CAB, Nov 30 '16 at 16:19

score 2 · Accepted Answer · answered Nov 30 '16 at 16:19

This is not a silly question, and there's a ton of bad advice on this particular subject out there.

First, to get the ray_factor into saveIntensityMap, simply pass it as another argument:

void IOFile::saveIntensityMap(const char* fileName,
                              float ray_factor,
                              vector<float>* intensityMap)
{
   ...
}

Second, forget you ever heard of formatted iostreams output, it is enormously more trouble than it's worth. (This is the part where there's lots of bad advice on the 'net. I personally never use iostreams at all; I think the old-fashioned <stdio.h> API is a much more ergonomic interface. Writing a floating-point number into a string with proper control over the output format is one of the things that iostreams are especially bad at.)

To construct the filename you want, use snprintf to format the number and concatenation of std::string objects to assemble the name, like this: (note that I am assuming the fileName argument to saveIntensityMap does not have the .txt extension already - it will probably be easier to change that than to take the string apart and put it back together; C++ is not really very good at strings)

char ray_factor_buf[10];
snprintf(ray_factor_buf, sizeof ray_factor_buf, "%.2f", ray_factor);

string full_filename = fileName;
full_filename += "_";
full_filename += ray_factor_buf;
full_filename += ".txt";

std::ofstream file(full_filename, std::fstream::out|std::fstream::trunc);

If your C++ library has not yet been updated to C++11, that last line will need to be

std::ofstream file(full_filename.c_str(), std::fstream::out|std::fstream::trunc);

If going with `snprintf` then why not skip `std::string` altogether? `snprintf(.., "%s_%.2f.txt",..)` — Dialecticus, Dec 01 '16 at 12:11
@Dialecticus Because then you have to allocate memory by hand. If `asprintf` is available, that's not a problem, but otherwise it's fiddly and easy to get wrong. — zwol, Dec 01 '16 at 15:52

Change a file name including char and float

1 Answers1