IF statement wrongly checked

1 次查看（过去 30 天）
Nicola Caldognetto
编辑： Khalid Mahmood 2021-4-7
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...

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采纳的回答

Walter Roberson
Walter Roberson 2021-4-7
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.
  2 个评论
Nicola Caldognetto
Nicola Caldognetto 2021-4-7
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
Walter Roberson 2021-4-7
Ran in:
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

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更多回答（1 个）

Khalid Mahmood
Khalid Mahmood 2021-4-7
编辑：Khalid Mahmood 2021-4-7
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
  4 个评论
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Khalid Mahmood
Khalid Mahmood 2021-4-7
编辑：Khalid Mahmood 2021-4-7
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)

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