IF statement wrongly checked

I have wrote a simple matlab script that calculate a function given 2 variables and tell me when it's smaller than a given value and save the two initial variables that have triggered the IF statements in a matrix; code below

clear all
close all

f = 17.5*10^9;
a = 3*10^-2;
b = 2*10^-2;
c = 3*10^8;
j = 1;
k = 0;
modi = zeros(2,25);
for m = 1:1:5
    for n = 1:1:5
        f_mn = (c/2)*(sqrt(((m/a)^2)+((n/b)^2)));
        if f_mn < f
            modi(1,j+k)=n;
            modi(2,j+k)=m;
            j+1;
        end
    end
    k+1;
end

I don't know if I'm dumb or I'm missing something but, the first iteration of the code (m=1 and n=1) should produced a f_mn=9.01e+09 which is definetly smaller than f=1.75e+10 but the code doesn't enter the IF statement...

Answer 1

[Walter Roberson]

            j+1;

That line retrieves the current value of j and invokes the plus() method between j and 1 (whatever that happens to be for the datatype of j), assigns the result to the ans variable, and then discards the result otherwise, since the semi-colon says not to display the result.

That line does not increase j by 1, as it does not assign to j.

[Nicola Caldognetto]

ok, that was the error but I don't get why, at least on the first iteration, it doesn't enter the IF statement

[Walter Roberson]

It did enter the if body.

clear all
close all

f = 17.5*10^9;
a = 3*10^-2;
b = 2*10^-2;
c = 3*10^8;
j = 1;
k = 0;
modi = zeros(2,25);
for m = 1:1:5
    for n = 1:1:5
        f_mn = (c/2)*(sqrt(((m/a)^2)+((n/b)^2)));
        if f_mn < f
            fprintf('if entered m = %d, n = %d\n', m, n);
            modi(1,j+k)=n;
            modi(2,j+k)=m;
            j+1;
        end
    end
    k+1;
end

if entered m = 1, n = 1
if entered m = 1, n = 2
if entered m = 2, n = 1
if entered m = 3, n = 1

Answer 2

[Khalid Mahmood]

clear all
%close all, %no need to close all, as nothing was opened by this code.. no figure, port etc 
f = 17.5*10^9; %same as f=1.75e+10
a = 3*10^-2;   %same as a=3e-2
b = 2*10^-2;   %same as b=2e-2;
c = 3*10^8;    %same as c=3e8
j = 1;
k = 0;
i=1;  %for saving values of successive f_mn
modi = zeros(2,25);
for m = 1:1:5
    for n = 1:1:5
        f_mn(i) = (c/2)*(sqrt(((m/a)^2)+((n/b)^2)));  %store in array, to view all values in history of f_mn calculations
        if f_mn(i) < f
            modi(1,j+k)=n;
            modi(2,j+k)=m;
            j=j+1;   % Must update j by assigning new value to j
        end
    i=i+1;    
    end
    k=k+1 % Must update k by assigning new value to k
end
f=get(0,'Format') %store old format of display
format compact   %display in compact format
format short e   %additionally, short exponential format
f_mn
modi
format(f)  %restore format

[Walter Roberson]

f==format  %stotre old format

That does not store the format into f: that compares the format to f. It relies on an R2021a change to permit format to return the current format.

format f  %restore format

That does not restore anything, it sets the current format to 'f' (which is not a valid format.) You would need

format(f)

[Khalid Mahmood]

True, that == was mistyped. change it to single =

[Walter Roberson]

... and change to

format(f)

[Khalid Mahmood]

I checked format in R2020a. f=format; and format f; commands both dont work.

instead use:

f = get(0,'Format')
format short e
format compact

.. and at last

format(f)