My if statement is not doing what I need it to do - if-statement

Question

so for my Numerical methods module we are requested solving a real world problem using Numerical methods and write a script for it through octave I am the last part of my project but it just doesn't want to do as I intend it to do? can someone perhaps help me here?
The section of code is as follows:
Code in question
F=1;
M=3;
input('has this met your criterion? (F/M): ')
if (F)
printf('\n'); %Spacer
printf('__________________________________________________ \n');%Divider
printf('\n'); %Spacer
printf('summation of results are as follows:');
printf('\n'); %Spacer
printf('___________________________________________\n');%Divider
printf('\n'); %Spacer
printf(__________________________________________\n');%Divider
printf('\n'); %Spacer
printf('flows (m3/s)');
QinPipe1=Q1x2=Di(1,1);
QinPipe2=Q2x2=Di(2,1);
QinPipe3=Q3x2=Di(3,1);
printf('\n'); %Spacer
printf('\n'); %Spacer
QinPipe1=Q1x2
QinPipe2=Q2x2
QinPipe3=Q3x2
printf("______________________________________\n");
printf("criterion check(percentage):\n");
printf('\n'); %Spacer
printf('\n'); %Spacer
RelError1=Q1e1=abs(((abs(Q1x)-abs(Q1x2))/abs((Q1x2))))*100;
RelError2=Q2e2=abs(((abs(Q2x)-abs(Q2x2))/abs((Q2x2))))*100;
RelError3=Q3e3=abs(((abs(Q3x)-abs(Q3x2))/abs((Q3x2))))*100;
RelError1=Q1e1
RelError2=Q2e2
RelError3=Q3e3
else if (M)
cramerwdntest
end
endif
What I need it to do is that after it has run the previous commands up until this point once the user answers with either F or M it should either print out the summation (F) of the final results from the script run earlier and if not it should run the following m file (M) again until the user eventually answers with an (F) instead no matter what letter you enter (M or F) it runs the first part of the if statement regardless
I attempted to use the "yes_or_no" function but that seemed to not work either
Could someone please help me on this.

Answer 1

What do F and M represent, and why do you set them to 1 and 3?
There's a lot to improve here, but to answer your question:
F=1;
M=3;
in = input('has this met your criterion? (F/M): ','s')
if in == 'F'
...
's' is used to specify that you input a string (character). You must store the input in a variable. I've called it in but you should choose a variable name that matches its purpose in your script.
You must compare the input to the character F, or input a number instead of a string.
New answer:
in = input('Is this OK? (Y / N)', 's');
while in ~= 'Y' && in ~= 'N'
in = input('Please answer with "Y" or "N". Is this OK?', 's');
end
if in == 'Y'
...