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

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Dominic
Dominic 2013-3-26
% 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
  2 个评论
Walter Roberson
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".

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采纳的回答

Matt Kindig
Matt Kindig 2013-3-26
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.
  2 个评论
Matt Kindig
Matt Kindig 2013-3-26
编辑：Matt Kindig 2013-3-26
By the way, you don't need the 'run' part of the commands-- just Rectangle_Rule, Trapezium_Rule, etc. will work fine.

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更多回答（1 个）

Dominic
Dominic 2013-3-27
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

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