Making for loop and if statment and increasing speed using matlab / octave

Several issues but the main one is probably that you don't preallocate - appending like this: amp=[amp;array(row,2)]; is generally slow in MATLAB. You don't need a loop here, though.

Let's start with a simple array, A:

1 500 2 700 3 900 7 1000 9 800 

And our lookup values are [2 6 3 9 7]; We want our output to show these lookup values, sorted, in the first column, and the second column to be either the values from the second column of A (where they exist) or zero.

lookup = sort(lookup); output = zeros(length(lookup),2); output(:,1) = lookup; [c a b ] = intersect(A(:,1),lookup); output(b,2) = A(a,2); 

The output is:

2 700 3 900 6 0 7 1000 9 800 

Purely from an efficiency standpoint, I would rewrite the for loop as follows:

m = 0; % number of omitted values n = 0; % number of found values for ii=1:1:rows(lookupval) [row,col] = find(lookupval(ii) == array(:,1)); if ~isempty(row) %if you find a lookup value in array %disp('found'); n=n+1; amp(n)=array(row,2); freq(n)=;array(row,1); found_array=[freq,amp]; else %add lookup value to another array and make amp value zero m=m+1; notfound_array(2*m-1:2*m)=[lookupval(ii);0]; endif end 

This saves you a find call by using its output directly rather than recomputing it when find returns a position, and grows the arrays in a more efficient way (as shown in this question).

This is a test Divakar suggest I do to see the speed it takes octave 3.8.1 to run this. Results are below along with the code.

1) Using ismember with 2,000,000 is faster but uses more memory
-elapsed time -0.2306sec- or -0.0038mins-
Total is 15000001 elements using 106000008 bytes

2) Using intersect with 2,000,000 is slower but uses less memory.
-elapsed time -0.3057sec- or -0.0051mins-
Total is 11749047 elements using 93992376 bytes

3) Using bskfun with 100,000 produces an error: out of memory or dimensions too large for octave's index type

First test results:

clear all, clc, tic, clf; num_to_test=2000000 %amount of numbers to test a1=(1:1:num_to_test)'; a2=(a1.*num_to_test); array=[a1,a2]; %array where values are stored lookupval=(randperm(num_to_test,num_to_test/2)/4)'; %lookup these random vaules of intergers and floats and get another value lookupval = sort(lookupval); sort_comb_array = [lookupval zeros(size(lookupval))]; [idA1,idB1] = ismember(array(:,1),lookupval); sort_comb_array(idB1(idA1),2) = array(idA1,2); fprintf('\nfinally Done-elapsed time -%4.4fsec- or -%4.4fmins- or -%4.4fhours-\n',toc,toc/60,toc/3600); whos >>>num_to_test = 2000000 >>> finally Done-elapsed time -0.2306sec- or -0.0038mins- or -0.0001hours- >>>Variables in the current scope: Attr Name Size Bytes Class ==== ==== ==== ===== ===== a1 2000000x1 16000000 double a2 2000000x1 16000000 double array 2000000x2 32000000 double idA1 2000000x1 2000000 logical idB1 2000000x1 16000000 double lookupval 1000000x1 8000000 double num_to_test 1x1 8 double sort_comb_array 1000000x2 16000000 double Total is 15000001 elements using 106000008 bytes ======================================================================== 

Second test results:

clear all, clc, tic, clf; num_to_test=2000000 %amount of numbers to test a1=(1:1:num_to_test)'; a2=(a1.*num_to_test); array=[a1,a2]; %array where values are stored lookupval=(randperm(num_to_test,num_to_test/2)/4)'; %lookup these random vaules of intergers and floats and get another value lookupval = sort(lookupval); output = zeros(length(lookupval),2); output(:,1) = lookupval; [c a b ] = intersect(array(:,1),lookupval); output(b,2) =array(a,2); fprintf('\nfinally Done-elapsed time -%4.4fsec- or -%4.4fmins- or -%4.4fhours-\n',toc,toc/60,toc/3600); whos >>>num_to_test = 2000000 >>> finally Done-elapsed time -0.3057sec- or -0.0051mins- or -0.0001hours- >>>Variables in the current scope: Attr Name Size Bytes Class ==== ==== ==== ===== ===== a 250005x1 2000040 double a1 2000000x1 16000000 double a2 2000000x1 16000000 double array 2000000x2 32000000 double b 250005x1 2000040 double c 250005x1 2000040 double lookupval 1000000x1 8000000 double num_to_test 1x1 8 double output 1000000x2 16000000 double Total is 11750016 elements using 94000128 bytes =======================================================================