how to put a real number into a file

[liangchenen]

if I have a variable mark. and it is 98.123123

and I want to put this real number into a file called "mark.out" with 2 decimals places, can anybody tell me how to do this in C++?

[wtd]

c++:

#include <fstream> #include <iomanip> int main() {    std::ofstream outputFile("mark.out");    double mark(98.123123);    outputFile << setprecision(2) << mark << std::endl;    outputFile.close();    return 0; }

[liangchenen]

the setprecision part

it gives the significant digits

for example, if my number is 98.41234e1234

and if I have setprecision(3), the output will be 98.4

it is not exactly the decimals.

[wtd]

If all else fails, you could truncate the number like so:

c++:

#include <cmath> double truncate(double number, int digits) {    int factor(pow(10, digits));    return static_cast<double>(static_cast<int>(number * factor)) / factor; }

c++:

outputFile << truncate(mark, 3) << std::endl;

[liangchenen]

do you know the following:

printf("%0.2f",mark);

this outputs the real number mark onto the screen with 2 decimals places.

how do I put that into a file?

[liangchenen]

cout << setiosflags(ios::fixed) << setprecision(2) << x;

just use this.

use setiosflas(iso::fixed), then it will outputs the decimals to 2 digits instead of the whole thing.

[wtd]

liangchenen wrote:

do you know the following:

printf("%0.2f",mark);

this outputs the real number mark onto the screen with 2 decimals places.

how do I put that into a file?

That's C. Avoid it at all costs in C++ code.

[bugzpodder]

how about we do it the C++ way?

cout<<setiosflags(ios::fixed)<<setprecision(2)<<myfloat<<endl;

[liangchenen]

However the setprecision thing does do the rounding

for example, we have 0.666666..
when I take setprecision 5, it would give me 0.66667

But I do not want it to do the rounding, instead, I want it to trunctate the decimals, what I want is 0.66666

what should I do then?

[wtd]

liangchenen wrote:

However the setprecision thing does do the rounding

for example, we have 0.666666..
when I take setprecision 5, it would give me 0.66667

But I do not want it to do the rounding, instead, I want it to trunctate the decimals, what I want is 0.66666

what should I do then?

Write a truncate function that does the work. Here's a thought... if you need it truncated to 2 decimal places, multiply by 100 (10 ^ 2) to move the decimal point two places to the right, cast to int to remove the fractional part, re-cast to a floating point number, then divide by 100.

[bugzpodder]

that way would work. but it may cause overflow.
alternatively, use a stringstream and output one more digit than necessary. truncate the last digit of the output.

stringstream ss;
ss<<setiosflags(ios::fixed)<<setprecision(6)<<myValue；
ss>>str;
cout<<str.substr(0,str.length()-1)<<endl;