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

Question

Dominic 2013-3-26

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/68754-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

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Walter Roberson 2013-3-26

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 2013-3-27

cheers for the clarification

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Answer 1

Matt Kindig 2013-3-26

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/68754-beginner-question-if-loop-code-at-beginning-of-post#answer_80041

在 MATLAB Online 中打开

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.