A= RandomReal[ {0, 2}, {40,50}];
(* Making a vector of 40 1's *)
x=Table[2, {40}];
A[[23,33]]=10;
(* Setting Row 5 to zero *)
 A[[5]]= 0*A[[5]]; 
(* Setting Column 7 to zero *)
A[[All,7]] =x;
MatrixPlot[A]

A= RandomReal[ {0, 2}, {40,50}];
(* Messing with a single row *)
iVal = 3;
For[j=1,j≤50,j=j+1,
A[[iVal,j]]=0
]
MatrixPlot[A]

{mDim,nDim}={40,50}
A= RandomReal[ {0, 2},{mDim,nDim}];
(* Messing with a single row *)
iVal = 3;
Do[
A[[iVal,j]]=0,
{j, 3, nDim}
]
MatrixPlot[A]

{mDim,nDim}={40,23}
A= RandomReal[ {0, 2},{mDim,nDim}];
(* Messing with a Diagonal *)
Do[
A[[j,j]]=0,
{j, 1, Min[mDim,nDim]}
]
MatrixPlot[A]

{mDim,nDim}={40,23}
A= RandomReal[ {0, 2},{mDim,nDim}];
(* Messing with a Diagonal *)
Do[
If[i==j,A[[i,j]]=0],
{i,1, Min[mDim,nDim],2},
{j, 1, Min[mDim,nDim],2}
]
MatrixPlot[A]

{mDim,nDim}={40,23}
A= RandomReal[ {0, 2},{mDim,nDim}];
(* Messing with a Diagonal *)
Do[
If[
And[ i==j, EvenQ[i]],
A[[i,j]]=0
],
{i,1, Min[mDim,nDim]},
{j, 1, Min[mDim,nDim]}
]
MatrixPlot[A]

?Do

ListPlot[x]

?Module

Clear[x,f]
f[x_]:= Module[
{t},
t=(x^2);
{Sin[t], Cos[t]}
]

Plot[f[t],{t, 0, 4}]

f[0.1]

?t