Basics of MATLAB and Beyond

Further increases in speed can be achieved by pre-allocating the outputmatrix y. If we have an m-file called factorialpre.m:y = zeros (1,500);for number = 1:500y(number) = prod(1:number);endthe execution time is about 10% faster: 4>> clear>> tic;factorialpre;tocelapsed_time =0.3752More on vectorising code is given in Part II (see page 175).8.4 Comparing StringsThe tests in flow control statements often involve strings (arrays of characters).For example you may want to ask the user of an m-file a questionwhich has a “yes” or “no” response, and adjust the flow accordingly.Although matlab has sophisticated menu utilities, the following is oftensufficient to get a user input:input(’Do you want to continue (y or n) ? ’,’s’);The ’s’ at the end tells matlab to expect a string response, ratherthan a numerical response. The following matlab code tests for a ‘y’response:if strcmp(lower(ans(1)),’y’)go_aheadelsereturnendThe strcmp function compares strings, lower converts to lower-casecharacters and ans(1) selects the first letter of the response. Typehelp strcmp for more information. The return command returns tothe invoking function or to the matlab prompt.9 Data FilesMany techniques are available to read data into matlab and to save datafrom matlab. The load and save functions can load or save matlabformat binary or plain ASCII files, and low-level input-output routinescan be used for other formats.4 See matlab’s gamma function if you are interested in computing factorials.c○ 2000 by CRC Press LLC