Beginner Question -- IF loop (code at beginning of post)

% 1. Greetings
fprintf(['\n Welcome! This program integrates and plots a 2D function of x. You may now choose your integration method.' ... 
    ' \n Enter ''1'' for Rectangle Rule; ''2'' for Trapezium Rule; and ''3'' for Simpson''s Rule. \n\n'])
C = input ('Enter your choice = ');  % Variable 'C' is chosen as 'C' for Choice
if C == 1;
    run Rectangle_Rule
elseif C == 2;
    run Trapezium_Rule
elseif C == 3;
    run Simpsons_Rule
else
    fprintf(['\n ------------------------------------------------------------------------------------------------------------' ... 
        '\n OPTION INVALID. PLEASE TRY AGAIN.'... 
        '\n ------------------------------------------------------------------------------------------------------------ \n'])
    run Numeric_Integration
end

I want the fprintf and re-run of numeric integration to work at all cases. The code above works fine when what is input is a number or a an alphabet with apostrophes, e.g., 'code', 'A'; but if it is just a letter with no apostrophes, it doesn't *. For example, I try to enter *test This message appears:

Error using input
Undefined function or variable 'test'.

Error in Numeric_Integration (line 7)
C = input ('Enter your choice = ');  % Variable 'C' is chosen as 'C' for Choice

How do I go around this problem?

Also, is there anyway that when it loops back to give the user another try to not show the welcome message again - this is what happens if I use run Numeric_Integration after else. Is there a code to make it just loop back to C = input ('Enter your choice = '); ?

Cheers

[Walter Roberson]

A note on your title: there is no such thing as an "if loop". There are "for loop" and "while loop", but "if" is an "if statement".

[Dominic]

cheers for the clarification

Answer 1

[Matt Kindig]

For the input issue, use the option 's' to convert the input to string at the prompt, and then use str2double() to convert it to numeric if necessary. For example:

C = input('Enter your choice = ', 's');
C = str2double(C);   %this will be NaN for *test and other strings, and thus will fall through to the "else" statement.

As for the looping question, wrap all of your statements after the welcome message with a while() loop, with some appropriate break condition. At the prompt, type

doc while

to bring up some useful help.

[Matt Kindig]

By the way, you don't need the 'run' part of the commands-- just Rectangle_Rule, Trapezium_Rule, etc. will work fine.

[Dominic]

Cheers

Answer 2

[Dominic]

It works now!

C = 0; 
while C ~= 1 || 2 || 3;
    C = input ('Enter your choice = ', 's');
    C = str2double(C);
    if C == 1;
        Rectangle_Rule
    elseif C == 2;
        Trapezium_Rule
    elseif C == 3;
        Simpsons_Rule
    else
        fprintf(['\n ------------------------------------------------------------------------------------------------------------' ...
        '\n OPTION INVALID. PLEASE TRY AGAIN.'... 
        '\n ------------------------------------------------------------------------------------------------------------ \n\n'])
    end
end