How to change back to showing numbers based on significant digits after applying the "fixed" parameter to setprecision()

Question

The setprecision() function can display numbers based on significant digits or number of decimals.

But I have found a problem using this. In particular, when I start using the "fixed" parameter to display based on decimal numbers accordingly, subsequent uses of setprecision() continue to display numbers based on decimal numbers, regardless of whether I re-specify the absence/presence of the "fixed" parameter later on.

The problem code is as follows:

#include <iostream> #include <iomanip> using namespace std; int main () { float fValue = 123.456789; cout << "123.456789 float type at 1 significant digit:" << endl; cout << setprecision(1) << fValue << endl << endl; double dValue = 123.456789; cout << "123.456789 double type at 1 decimal numbers:" << endl; cout << fixed << setprecision(1) << dValue << endl << endl; long double lValue = 123.456789876543210; cout << "123.456789876543210 long double type at 1 significant digit:" << endl; cout << setprecision(1) << lValue << endl << endl; }

Output is shown unexpectedly as:

123.456789 float type at 1 significant digit: 1e+002 123.456789 double type at 1 decimal numbers: 123.5 123.456789876543210 long double type at 1 significant digit: 123.5

Output is expected to be shown as:

123.456789 float type at 1 significant digit: 1e+002 123.456789 double type at 1 decimal numbers: 123.5 123.456789876543210 long double type at 1 significant digit: 1e+002

It's working exactly as it should. What did you expect it to do differently? — Ken White
– Ken White, Commented Mar 19 at 2:21
Please post a minimal reproducible example. The shown code can't be outside a function. — 3CEZVQ
– 3CEZVQ, Commented Mar 19 at 2:21
I think you have it backwards. geeksforgeeks.org/… says "std::fixed – Fixed Floating-point notation : It write floating-point values in fixed-point notation. The value is represented with exactly as many digits in the decimal part as specified by the precision field (precision) and with no exponent part." So, setting std::fixed will make std::setprecision(1) print exactly 1 decimal place. — Jerry Jeremiah
– Jerry Jeremiah, Commented Mar 19 at 3:48
What did you expect? 100? With scientific, the precision is the number of significant digits. With fixed , the precision is the number of decimals. (You probably want to look for better tutorials.) — molbdnilo
– molbdnilo, Commented Mar 19 at 8:25
we could point you to the flaw in the tutorial or in your misunderstanding, but we dont know what tutorials you read nor what you expected. You should tell us what output you expect and why. — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Mar 19 at 10:27

SK90 · Accepted Answer · 2025-03-31 18:45:03Z

Based on the small comments I have found the answer (and some misinterpretation of the function on my part, in my original unvetted/unedited question). As you know from above, the problem transformed into a problem of not being able to change back from using the "fixed" parameter, to which one prophetic comment pointed to resetting some other parameter to solve it.

The solution code is as follows:

#include <iostream> #include <iomanip> using namespace std; int main () { const auto default_precision{std::cout.precision()}; float fValue = 123.456789; cout << "123.456789 float type at 1 significant digit:" << endl; cout << defaultfloat << setprecision(default_precision); cout << setprecision(1) << fValue << endl; double dValue = 123.456789; cout << "123.456789 double type at 1 decimal number:" << endl; cout << defaultfloat << setprecision(default_precision); cout << fixed << setprecision(1) << dValue << endl; long double lValue = 123.456789876543210; cout << "123.456789876543210 long double type at 1 significant digit:" << endl; cout << defaultfloat << setprecision(default_precision); cout << setprecision(1) << lValue << endl; }

Also, fixed and setprecision are interchangeable, such that:

cout << fixed << setprecision(1) << dValue << endl;

is equal to:

cout << setprecision(1) << fixed << dValue << endl;

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