Scientific notation in c

Question

How can I print the output as 123.4e+04 by increasing the number of digits before the comma instead of 1.234e+06 when using scientific notation in c

I tried the %*.*e representations, as shown below, but it didn't work. It shows any result as a single digit before the comma.

#include <stdio.h>
int main() {
    double num =12340000;
    printf("%1.1e\n", num);
    printf("%2.1e\n", num);
    printf("%2.2e\n", num);
    printf("%3.1e\n", num);
    printf("%4.1e\n", num);
    printf("%5.1e\n", num);
    printf("%5.2e\n", num);
    printf("%6.3e\n", num);
    printf("%8.4e\n", num);
    printf("%3.0e\n", num);
}

1.2e+07
1.2e+07
1.23e+07
1.2e+07
1.2e+07
1.2e+07
1.23e+07
1.234e+07
1.2340e+07
1e+07

`I tried the %*.*e` what numbers did you give to `*`? Please show the whole `printf` call. — KamilCuk, Mar 23 '23 at 14:28
As far as I am aware this is not foreseen in C (and derivates) as this representation is rather unusual. You would have to re-format it on your own. — Aconcagua, Mar 23 '23 at 14:30
@Aconcagua I'm new and need to do it, can you share a resource on how to do it with me? — tugtagfatih, Mar 23 '23 at 14:41
@ikegami Is there any other way other than converting it to string and printing it with arrays? — tugtagfatih, Mar 23 '23 at 14:44
There are no solution provided by the standard. You'll need to look elsewhere, or write your own. If you write your own, using `sprintf` and manipulating the resulting string would be simplest (though not simple) — ikegami, Mar 23 '23 at 14:45
If there is more than 1 digit before the `.` then is it really scientific notation? — Lundin, Mar 23 '23 at 14:46
@Lundin, According to Wikipedia, yes. For example, engineering notation is a form of a scientific notation where the exponents are factors of 3. It's just not *normalized* scientific notation. /// Anyway, the name itself is not relevant to the question. — ikegami, Mar 23 '23 at 14:49
@ikegami I'd call engineering notation... engineering notation :) And since I work in electronics I use that all the time. But anyway, printf is too crude a tool for such things. — Lundin, Mar 23 '23 at 14:52
@Lundin, That doesn't mean it's not a form of scientific notation. Apples are fruits. — ikegami, Mar 23 '23 at 14:54
@ikegami *where the exponents are factors of 3*. That means the exponents are either 1 or 3, which are the only two factors of 3. Perhaps you meant *multiples* of 3? — Tom Karzes, Mar 23 '23 at 14:56
A super-ugly work-around might be something along the lines of this: https://godbolt.org/z/6dK5K5Grn. — Lundin, Mar 23 '23 at 15:12

score 1 · Accepted Answer · answered Mar 23 '23 at 15:04

How can I print the output as 123.4e+04 by increasing the number of digits before the comma instead of 1.234e+06 when using scientific notation in c

Only by performing the formatting yourself, or finding some third-party library that will do it for you, or relying on some extension that your C implementation happens to provide. The printf-family functions, as described in the language specification, do not provide such an option.

Standard printf and its siblings offer two main alternatives for formatting floating-point values:

via a %e conversion specifier, which is what you are now using. The documentation for this specifies that it provides exactly one digit before the decimal point.
via a %f conversion specifier, which uses plain decimal notation, not scientific notation.

For cases where the range of values to be formatted is very large, there is also %g, which chooses between the other two based on the magnitude of the value being formatted.

I'm unsure why it's important to you to vary the number of digits before the decimal point in a scientific-notation-like format, but here are a couple of alternatives that might suit:

if you are simply scaling outputs, such that you expect the exponent part to have some specific value, then scale the value manually, format it using %f and, optionally, add an exponent part manually. Example:
```
double x = 12345.67;
printf("%8.2fe+03", x / 1000);
// prints "   12.35e+03"
```
That doesn't seem to be exactly what you asked, but similar could be applied to that.
For a general-purpose implementation, you could consider formatting your number as a string, using sprintf with %e, then adjusting the position of the decimal point and the value of the exponent in the resulting formatted string before printing (left as an exercise).

Scientific notation in c

1 Answers1