# Help with for loop and if else statement

1 view (last 30 days)
Rachna Singh on 13 Jan 2022
Commented: Rachna Singh on 13 Jan 2022
I am writing to code for the attached capture 1 image, and it is supposed to obtain the graph as shown in the capture 2 image.
I used for and if-else statement to write the code, however it is not working. If possible can anyone look into it? Thanks in advance.
y=0:1000;
for i=0:length(y)
if y(i) <= 10
Q1=1;
elseif (y(i) > 10) && (y(i)<100)
Q2=0.32*y-2.2;
else y(i) > 100
Q3 =300/sqrt(y);
end
Q=(Q1+Q2+Q3)
end
semilogx(y,Q)

Image Analyst on 13 Jan 2022
Edited: Image Analyst on 13 Jan 2022
You need to index L and Q:
L = linspace(0, 200, 2000);
Q = ones(1, length(L)); % Initialize to 1
for k = 2 : length(L)
if L(k) <= 10
Q(k) = 1;
elseif (L(k) > 10) && (L(k)<= 100)
Q(k)=0.32 * L(k) - 2.2;
elseif L(k) > 100
Q(k) =300/sqrt(L(k));
end
% Q=(Q1+Q2+Q3)
end
loglog(L, Q, 'LineWidth', 3);
grid on;
xlabel('L', 'FontSize', 20)
ylabel('Q', 'FontSize', 20) Of course you don't need a loop - you can do it vectorized if you want but it's your homework so I didn't try to modify it too much.
Rachna Singh on 13 Jan 2022
Thank you

Mathieu NOE on 13 Jan 2022
hello
here you are .
Q andy must be indexed in the loop (Q(k) , y(k))
only one Q variable suffice for all cases - no need to create multiple Q's
also please don't use i or j for loop index as your are shadowing their native role (i = j = sqrt(-1) for complex numbers)
clc
clearvars
y=1:1000;
for k=1:length(y)
if y(k) <= 10
Q(k)=1;
elseif (y(k) > 10) && (y(k)<100)
Q(k)=0.32*y(k)-2.2;
else y(k) > 100
Q(k) =300/sqrt(y(k));
end
end
loglog(y,Q)

Rachna Singh on 13 Jan 2022
Thank you everyone :)

R2021b

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