command for delaying matlab to execute a certain line

hello im doing an iteration and everytime i execute my code it brings out and error

clc
clear
format longg
gama=0.75;
ppc=667;
tpc=408;
pts=2500;
tts=35+460;
tws=245+460;
l=10000;
theta=0;
x=10;
pws=(pts)+(0.25*(pts/100))*((l*cos(theta))/(100))
for i=1:x
pavg(i)=(pws+pts)/2
tavg=(tws+tts)/2
tpr=tavg/tpc
ppr(i)=pavg(i)/ppc
z(i)=input('enter z: ');
s(i)=(0.0375*gama*l*cos(theta))/(z(i)*tavg)
pws=pts*exp(s(i)/2)

if s(i)==s(i-1);
    display('done')
    break
else 
    display('u need more iterations')
end
end

the error comes out because matlab executes the if statement before another value for "s" has been calculated i want it to wait till a second value of "s" is calculated to execute the if loop i know this code is kinda amateur but bare with me please im new to matlab coding, and if u have any suggestion on a simplest form of this iteration please do help.

Answer 1

[Azzi Abdelmalek]

Matlab execute the lines of your code one after another. The if statement will be executed after the calculation of s(i). What is the error message? have you tried to debug your code?

[mohamed]

this is the error message

Subscript indices must either be real positive integers or logicals.

Error in prob6 (line 23)
if s(i)==s(i-1);

and what is debugging?

[Azzi Abdelmalek]

You need to initialize s, and the for loop should start at i=2, because if i is equal to 1, you can't write s(i)==s(i-1), i-1=0 is not allowed by Matlab

http://www.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html

[mohamed]

how do you suggest i initialize "s" i tried coping the rules under the for loop and calculating them at first then starting the code at i=2:x, it didnt work it only gave me 1 value for pws the new one every time i enter a value for z