Rounding off floats with ostringstream

Question

I have an issue regarding conversion from float to c++ string using ostringstream. Here is my line:

void doSomething(float t)
{
    ostringstream stream; 
    stream << t;
    cout << stream.str();
}

when t has value -0.89999 it is round off to -0.9, but when it's value is 0.0999 or lesser than this say 1.754e-7, it just prints without round off. what can be the solution for this.

@In silico, actually i want to use that value at some point in my code. — boom, Sep 20 '10 at 04:50
That's why it's a comment, not an answer. :-) I didn't know the `cout` is for debugging purposes. — In silico, Sep 20 '10 at 06:13

score 17 · Accepted Answer · edited Aug 28 '13 at 17:33

17

You need to set the precision for ostringstream using precision

e.g

stream.precision(3);
stream<<fixed;    // for fixed point notation
//cout.precision(3); // display only
stream << t;

cout<<stream.str();

edited Aug 28 '13 at 17:33

answered Sep 20 '10 at 04:51

yadab

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i got solution for 0.0999, but for the number of 7.9e-08 i am still getting the same. so what can be the solution for this. – boom Sep 20 '10 at 05:01
you need to set precision to the stream itself. i.e. stream.precision(3); – yadab Sep 20 '10 at 05:11
i am doing the same. here is my code:ostringstream stream; stream.precision(1); stream << t; cout << endl<<"Value:"<< stream.str(); – boom Sep 20 '10 at 05:20
okay, use a fixed precision. e.g, stream.precision(8); stream << fixed; stream << t; cout << endl<<"Value:"<< stream.str(); – yadab Sep 20 '10 at 05:31
What is the default precision for ostringstream? – stephanmg Feb 11 '20 at 00:44

score 7 · Answer 2 · edited Sep 20 '10 at 05:39

7

If you want a particular number of significant figures displayed try using setprecision(n) where n is the number of significant figures you want.

#include <iomanip>

void doSomething(float t)
{
    ostringstream stream; 
    stream << std::setprecision(4)  << t;
    cout <<  stream.str();
}

edited Sep 20 '10 at 05:39

sbi

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answered Sep 20 '10 at 04:49

shuttle87

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score 2 · Answer 3 · answered Sep 20 '10 at 05:25

2

If you want fixed-point instead of scientific notation, use std::fixed:

stream << std::fixed << t;

Additionally you might want to set the precision as mentioned.

answered Sep 20 '10 at 05:25

Georg Fritzsche

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score 0 · Answer 4 · answered Sep 20 '10 at 04:57

0

Use setprecision:

stream << setprecision(5) <<t ;

Now, your string stream.str() will be of the required precision.

answered Sep 20 '10 at 04:57

Jacob

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i got solution for 0.0999, but for the number of 7.9e-08 i am still getting the same. so what can be the solution for this. – boom Sep 20 '10 at 05:03

Rounding off floats with ostringstream

4 Answers4

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