problem in if loop

Dear friends, I want to program following equation. when x1>a1 & x2>a2 F1 = -P21*Apb+Pa*Apt - Fpi - Ff1;

F2 = -P21*Apvt + P11*Apvb - Fpvi - Ff2;

F3 = P13*Ast - P22*Asb - Fsi - Ff3;

F4 = -P12*Asvb+P22*Asvt-Fsvi-Ff4;

when x1>a1 & x2<=a2

F1 = -P21*Apb+Pa*Apt - Fpi - Ff1;

F2 = -P21*Apvt + P11*Apvb - Fpvi - Ff2;

F3 = P13*Ast - P22*Asb - Fsi - Ff3;

F4 = -P12*Asvb+P22*Asvt-Fsvi; % when x1<=a1 x2<=0

F1 = -P21*Apb+Pa*Apt - Fpi - Ff1;

F2 = -P21*Apvt + P11*Apvb - Fpvi;

F3 = P13*Ast - P22*Asb - Fsi;

F4 = -P12*Asvb+P22*Asvt-Fsvi;

 Below given is my matlab script. in this i am not able to figure out why the out put of this program is contact when t is varying from 0 to 10.

t = 0:0.01:10; x1 = 13*sin(t); x2 = 13*sin(t);

for i = 1:length(t);

if x1<=a1 & x2<=0

F1(i) = -P21*Apb+Pa*Apt - Fpi - Ff1;

F2(i) = -P21*Apvt + P11*Apvb - Fpvi;

F3(i) = P13*Ast - P22*Asb - Fsi;

F4(i) = -P12*Asvb+P22*Asvt-Fsvi;

elseif x1>a1 & x2<=a2

F1(i) = -P21*Apb+Pa*Apt - Fpi - Ff1;

F2(i) = -P21*Apvt + P11*Apvb - Fpvi - Ff2;

F3(i) = P13*Ast - P22*Asb - Fsi - Ff3;

F4(i) = -P12*Asvb+P22*Asvt-Fsvi; elseif x1>a1 & x2>a2

F1(i) = -P21*Apb+Pa*Apt - Fpi - Ff1;

F2(i) = -P21*Apvt + P11*Apvb - Fpvi - Ff2;

F3(i) = P13*Ast - P22*Asb - Fsi - Ff3;

F4(i) = -P12*Asvb+P22*Asvt-Fsvi-Ff4; end end

[Walter Roberson]

There is no such thing as an "if loop". "if" is a sequential control statement, and the body of an "if" is executed only once, not in a loop.