求大神！！if为真却不执行怎么回事

clc
close all
clear all
i=-0.4:0.025:0.4;
j=0.2:0.025:0.8;
m=length(i);
n=length(j);
k=1;
for a=-0.4:0.025:0.4
for b=0.2:0.025:0.8
if (a==-0.1 && b==0.5)
A(k)=1;
elseif (a==0 && b==0.5)
A(k)=0.6;
elseif (a==0.1 && b==0.5)
A(k)=1;
elseif (a==0 && b==0.4)
A(k)=1;
elseif (a==0 && b==0.6)
A(k)=1;
else
A(k)=0;
end
k=k+1;
end
end
B=reshape(A,n,m);
imagesc(i,j,B);
axis('xy');
colorbar
A只赋值了两个，其他三个没赋值到，，这是咋回事？？？