How to check if a variable exists and if yes display the variable.

Question

Mouse 2021-11-2

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1577745-how-to-check-if-a-variable-exists-and-if-yes-display-the-variable

评论： Stephen23 2021-11-2

clear all;
%disp(testresults);     %can't work because variable testresults does not
%exist.
testresults = magic(5)      %create variable
exist testresults;          %returns "1", variable exists
if (exist testresults var)      %why can't I check if this exists? 
    %File: temp.m Line: 6 Column: 11
%Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error. To construct matrices, use brackets instead of parentheses.
    disp(testresults);
end

why is that not possible? What should I do to fix this problem?

The variable is eventually created bevore.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Stephen23 2021-11-2

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1577745-how-to-check-if-a-variable-exists-and-if-yes-display-the-variable#answer_822560

编辑：Stephen23 2021-11-2

在 MATLAB Online 中打开

"Why can't I check if this exists?"

Of course you can, you just need to use function syntax and not command syntax:

https://www.mathworks.com/help/matlab/matlab_prog/command-vs-function-syntax.html

if exist('testresults','var')

Command sytnax is a historical oddity that exists only to confuse beginners who think that certain functions must be called using command syntax, because that is what they see in the MATLAB help. In reality it can be replaced in every situation with function syntax.

"What should I do to fix this problem?"

Avoid command syntax.

2 个评论
显示无隐藏无

Steven Lord 2021-11-2

在 MATLAB Online 中打开

There are situations where command form is more natural IMO.

help plot  % vs help('plot')
doc fft    % doc('fft')
hold on    % hold('on')
quit force % quit('force')
clear all  % clear('all')
close all  % close('all')

Stephen23 2021-11-2

@Steven Lord: yes to HELP, QUIT, and DOC when called from the command window.

No to HOLD: without explicit object handles such a GUI is too fragile for my taste.

请先登录，再进行评论。

Answer 2

Image Analyst 2021-11-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1577745-how-to-check-if-a-variable-exists-and-if-yes-display-the-variable#answer_822535

在 MATLAB Online 中打开

Try this

try
    testresults % Spew results to command window.
catch ME
    message = sprintf('Note: testresults is not a variable in scope.');
    uiwait(warndlg(message))
end