how to compare string?

i want to take input fuel=CH4 or CO or Co2, %its num + string input if fuel==CH4 % if fuel is CH4 i need to fix some value for variable %if fuel is Co or Co2 i need to fix some value can you please help me how to do it?

Answer 1

[Birdman]

Try this:

fuel=input('select the gaseous component CH4,ethane,propane,butane,H2,CO,CO2,N2\n'); 
if prod(ismember(fuel,'CH4'))
    density=0.716; %kg/m^ 
    LCV=35.83; %MJ/m^3 
    cc=1; hh=4; oo=0; nn=0; 
    TMmass=cc+hh+oo+hh; 
    disp('total mass of CH4'); 
    disp(TMmass); 
end
if prod(ismember(fuel,'ethane'))
    density=1.342; %kg/m^3
    disp(density);
    LCV=63.76; %MJ/m^3
    disp(LCV);
    cc=2;
    disp(cc);
    hh=6;
    disp(hh);
    oo=0;
    disp(oo);
    nn=0;
    disp(nn);
    TMmass=cc+hh+oo+hh;
    disp('total mass of CH4');
    disp(TMmass);
end

Do not forget to enter your input in char form. Like

>>'CH4'

or

>>'ethane'

[SRIGUHAN MURUGASUNDARAM]

thank you, i will try..

[Guillaume]

prod(ismember(...))  ?

If you want to test that all values are members, then it would make sense to use all not prod.

all(ismember(...))

In addition, the test will say that CH4, C, H, CH4CH4C are all the same chemical. Why not use strcmp?

[Birdman]

Thanks.

[Stephen23]

This is a buggy way to compare strings. It will work incorrectly even if the order of the characters is different, or if any characters are repeated. Avoid using this code.

Strings should be compared simply using strcmp and strcmpi. See the other answers for simpler and much more robust code.

[Birdman]

Yes, thank you for attention.

Answer 2

[Steven Lord]

The approach Birdman suggested won't work if the user enters more characters than you expected.

fuel = 'CH4CC4'
prod(ismember(fuel, 'CH4')) % returns 1

It also won't work if the user enters lower-case characters when you expected upper-case.

prod(ismember('ch4', 'CH4')) % returns 0

Use isequal, strcmp, strcmpi, or a switch / case statement. To handle case matching, you might also want to use upper or lower to convert the input into a consistent case.

isequal(fuel, 'CH4') % false
strcmp(fuel, 'CH4') % false

switch fuel
  case 'CH4'
    result = true
  otherwise
    result = false
end

[SRIGUHAN MURUGASUNDARAM]

ok i will try this... thank you

[SRIGUHAN MURUGASUNDARAM]

Hi Mr.Steven Lord, your help was really usefull. i have some doubts in my mathwork. can you please help me ? if you dont mind... may i know how to contact you? email or any chat..

[Steven Lord]

Please use the Ask button at the top of the page to ask a new question. That way I can read it and so can others who may respond before I can.

Answer 3

[Guillaume]

string comparisons are done with strcmp, or strcmpi for case insensitive. Never with ==