Converting string to a double
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Hello everyone.
i'm programming a software that gets different values from a txt file, and plotting a graph based on these values.
In order to create the graph I had to convert the strings in the txt file to a double, but the str2double function rounds the values so the graph isn't accurate.
for example, the string to convert is: 1.7356922617E-3, and the doublue value after applying the method is 1.73569E-3.
is there any method that convert without rounding?
thank you.
1 个评论
" but the str2double function rounds the values so the graph isn't accurate."
I very much doubt that str2double "rounds the values", what is much more likely is that you are confusing how numeric data are displayed with how numeric data are stored, which are two completely different things.
You can easily change how the data are displayed:
format short
str = '1.7356922617E-3';
val = str2double(str)
format long
val
So str2double has not "rounded" the value at all, all of the digits are converted and stored as expected.
回答（1 个）
Fangjun Jiang
2021-3-24
Are you sure?
>> format long
>> a=str2double('1.7356922617E-3')
a =
0.001735692261700
7 个评论
Jan
2021-3-24
Exactly. The values are not rounded, but just the display in the command window.
Fangjun Jiang
2021-3-24
Note, there is a limit on the precision. I believe it is 15 digits of precision. Need to find it somewhere in the doc.
>> b=str2double('100.12345678901234567')
b =
1.001234567890124e+02
"I believe it is 15 digits of precision"
The "precision" is more complex than that, it really depends what you are doing with the values:
Fangjun Jiang
2021-3-24
In MATLAB, it seems to be 15 digits after the decimal point.
format long
c=str2double('10.12345678901234567890')
c=str2double('1.12345678901234567890')
c=str2double('0.12345678901234567890')
c =
10.123456789012346
c =
1.123456789012346
c =
0.123456789012346
Stephen23
2021-3-24
"In MATLAB, it seems to be 15 digits after the decimal point."
Your examples only demonstrate the display format you selected (which has 15 digits after the decimal point)
Your examples do not demonstrate any "limit" on the conversion precision.
c=str2double('10.12345678901234567890')
fprintf('%.999g\n', c)
This has apparently remembered 16 total digits correctly, 10.12345678901234. Is that really the case?
fprintf('%.999g\n', 10.12345678901234567890) %do we get the same?
fprintf('%.999g\n', 10.123456789012340)
fprintf('%.999g\n', 10.123456789012344)
fprintf('%.999g\n', 10.123456789012345)
fprintf('%.999g\n', 10.123456789012346)
fprintf('%.999g\n', 10.123456789012350)
fprintf('%.999g\n', 10.12345678901234567890*(1-eps))
fprintf('%.999g\n', 10.12345678901234567890*(1+eps))
So after 10.12345678901234 it is taking into account that what follows is something on the high side of 5x in order to decide whether to round down to 10.123456789012344 or up to 10.123456789012346
Fangjun Jiang
2021-3-24
good points. Now I understand better. It certainly can read in more than 15 digits after decimal point.
>> p1=sprintf('%40.30f',pi)
p1 =
' 3.141592653589793115997963468544'
>> d=str2double(p1)
d =
3.141592653589793
>> p2=sprintf('%40.30f',d)
p2 =
' 3.141592653589793115997963468544'
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