What does "I get a number from an API: 8.1784e+17" mean? Is this a char vector, string or a UINT64 or double scalar? In the last case, remember that doubles contain about 16 digits only. So there is no chance to store a value with 18 digitis accurately.
x = uint64(817835078876192769)
% uint64 817835078876192769
double(x)
% 8.17835078876193e+17
uint64(double(x))
% uint64 817835078876192768
num2str(x)
% '817835078876192769'
sprintf('%d', x)
% '817835078876192769'
sprintf('%.0f', x)
% '817835078876192768'
So the problem might be, that the value is provided as double value. You need to store the value as UINT64 or as char array or string.
