???index exceeds matrix dimensions

in my program i have used for loop and if loop at the end i am getting  ???index exceeds matrix dimensions this comment after calculating 1 value, where i have to calculate 42 values i used 
example
for x=1:42
if x==1
   a=1;
   b=2;
end
if x==2
   a=2;
   b=3;
end
:
:
:
:
:
:
if x==42
   a=42;
   b=43;
end
c=a+b
end

[Walter Roberson]

There is no such thing as an if loop.

Answer 1

[Andrei Bobrov]

x=1:42
c = 2*x+1

more :)

c = zeros(1,42);
for x = 1:42
    a = x;
    b = a + 1;
    c(x) = a + b;
end

more 2

x=1:42
c = zeros(size(x));
for i1 = 1:numel(x)
    if x(i1) == i1
        a = x(i1);
        b = a + 1;
    end
    c(i1) = a + b;
end