How to format the %e format specifier in c program

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float num = 10.0111
printf("%E",num);

Output: 1.001110E+001

But I want = 1.00111E+1

What I have tried:

I read many website there were written that no hands on solution is available in C library.

Posted 9-Mar-16 3:23am

Rupam Bhaduri Pro

Updated 9-Mar-16 3:54am

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Jochen Arndt · Accepted Answer · 2016-03-09T03:54:00

Solution 1

The behaviour is defined by the standard (see fprintf[^]):

Quote:
The exponent shall always contain at least two digits. If the value is zero, the exponent shall be zero.

So the only solution would be printing to a buffer using sprintf() and modifying the string afterwards.

Example:

C++

char buffer[64];
sprintf(buffer, "%E", num);
char *expPos = strchr(buffer, 'E');
int exp = atoi(expPos + 1);
sprintf(expPos + 1, "%+d", exp);
printf(buffer);

[EDIT]
A version that removes also trailing zeroes before the exponent:

C++

char buffer[64];
sprintf(buffer, "%E", num);
// Locate the exponent
char *expPos = strchr(buffer, 'E');
// Get the exponent value
int exp = atoi(expPos + 1);
// Skip trailing zeroes using pre-increment to start at the last digit.
// Afterwards expPos points to the last non-zero digit
do expPos--; while (expPos > buffer && *expPos == '0');
// Overwrite trailing zeroes and exponent with short exponent
sprintf(expPos + 1, "E%+d", exp);
printf(buffer);

[/EDIT]

Posted 9-Mar-16 3:54am

Jochen Arndt

Updated 9-Mar-16 7:17am

v4

Comments

Rupam Bhaduri Pro 9-Mar-16 12:30pm

Thank you Jochen, that worked!
But I also want to remove those zeros before 'E'

Jochen Arndt 9-Mar-16 12:52pm

This would be more difficult. The reason is that the E format uses a default precision of 6 digits (after the decimal point) printing all of them. If you know the required precision in advance, you may set it:
sprintf(buffer, "%.5E", num);

Otherwise you have to remove trailing zeroes before the exponent by modifying the string or implement your own print function by getting the exponent with log10(num), rounding that by casting to integer, dividing the num by pow(10, exp), printing the result using the 'f' format and appending the exponent (has to be tested; I'm not sure if I'm right with this).
Example:
int exp = (int)log10(num);
printf("%fE+%d", num / pow(10.0, exp), exp);

But note that the above might result in errors (which should be small enough when the input is float / single precision and the operations are perfomed with double precision).

Rupam Bhaduri Pro 9-Mar-16 12:55pm

Can you tell that how to get the number before 'E' then I think I can do it.

Jochen Arndt 9-Mar-16 13:07pm

See my updated solution.

Rupam Bhaduri Pro 9-Mar-16 13:16pm

Thank you, I just found another solution after lots of trying. Now can print exactly what I want. And your last solution is more easy than mine.

Rupam Bhaduri Pro 9-Mar-16 13:20pm

And please help me by telling me a good book of C, where everything is mentioned in a precise manner like these things.

Jochen Arndt 9-Mar-16 13:39pm

That is difficult for me because I know C since 30 years and have therefore no more books. When starting with C I used the Turbo C Reference Guide. You can download it as PDF from the web but it is outdated (1988).

A good online resource is http://www.cplusplus.com/.
That is mainly for C++ but describes also all C standard library functions.

Some books and online resources can be found at the end of the Wikipedia article:
https://en.wikipedia.org/wiki/C_%28programming_language%29

Finally you can search for "C tutorial recommendations" preferring hits from pages / forums specialised on C.

Rupam Bhaduri Pro 9-Mar-16 15:24pm

Thanks for the info, but C version 11 released in 2011. I find that there are many languages better than C, like C#, Python. I don't like Java. As you have experience of many years, what language you thing is the best for flexibility, speed and security and also provided cross-platform GUI library?

Jochen Arndt 10-Mar-16 3:04am

With those requirements it will be C++.
Java is good for platform independant GUI applications.
C# is coming up.
Plain C is still good for low level and simple command line utilities but not for GUI applications.