In Matlab, how to specify number of digits in the exponent using formatSpec formatting operator?

Question

I'm writing data to an output text file using the fprintf command in Matlab. How to write a number to the output file, for instance, 1.12345678e-001, with three digits in the exponent?

formatSpec = '%1.8e\n';

gives 1.12345678e-01, not the desired result!

There's a similar question here https://se.mathworks.com/matlabcentral/answers/100772-how-do-i-use-fprintf-to-write-numbers-in-exponential-notation-of-variable-exponent-digits-on-a-windo

But following the instructions given there didn't solve the problem!

Answer 1

You can use this non-regex method:

num = 0.112345678
pow = floor(log10(abs(num)));
sprintf('%.8fe%+.3d', num/10^pow, pow)

ans =
1.12345678e-001

For multiple inputs use this:

num= [.123 .456 .789];
pow = floor(log10(abs(num)));
sprintf('%.8fe%+.3d ', [num./10.^pow; pow])

Answer 2

Not sure if this falls under the category of solution or work-around, but here it goes:

x = .123e25; % example number
formatSpec = '%1.8e\n'; % format specification
s = sprintf(formatSpec, x); % "normal" sprintf
pat = '(?<=e)[+-]\d+'; % regex pattern to detect exponent
s = regexprep(s, pat, sprintf('%+04i', str2double(regexp(s, pat ,'match')))); % zero-pad

It uses regular expressions to identify the exponent substring and replace it with the exponent zero-padded to three digits. Positive exponents include a plus sign, as with fprintf.

Answer 3

This isn't the cleanest answer but you could do something like this. Basic steps are write as is, get the exponent with a regexp, re-write that portion, and replace.

formatSpec = '%1.8e'

tempStr = sprintf(formatSpec,1.12345678e-1);
oldExp  = regexp(tempStr,'e[+-]([0-9]+)$','tokens');
newExp  = num2str(str2double(oldExp{1}{1}),'%03d');
fixedStr = regexprep(tempStr,[oldExp{1}{1} '$'],newExp)

This outputs:

fixedStr =    
1.12345678e-001