small problem In Pascal , Could you please help? - pascal

Question

I write this program with pascal
which ask the user to enter two arrays and constant value which is K
the program muli the K with arrays .
and then save the answer in new array
and do some operation in new array
addition << work well
Subtraction << also work
BUT the problem in Multi << I am trying to ask the user to enter a new array and do Muti but still there is a problem.
ALSO
I want these operation repeated until the user press exit <<< I could not do this options because i am not perfect with pascal .
I would be grateful if you could help me
This is My Code
program BST6;
const maxN=100;maxM=100;
type mat=array[1..maxN,1..maxM]of integer;
var A,B,c:mat;
n,m,l,s,i,j,k:integer;
ch : char;
procedure readMat(var A:mat;var m,n:integer);
begin
for i:=1 to m do
for j:=1 to n do
begin
write('mat[',i,',',j,']=');
readln(A[i,j]);
end;
end;
procedure writeMat(A:mat;m,n:integer);
begin
for i:=1 to m do
begin
for j:=1 to n do
write(a[i,j]:4);
writeln;
end;
end;
function multK(A:mat;k:integer):mat;
begin
for i:=1 to n do
for j:=1 to m do
begin
B[i,j]:= K*A[i,j];
end;
multK:=B;
end;
function minus(A,B:mat):mat;
begin
for i:=1 to m do
for j:=1 to n do
C[i,j]:=A[i,j]-B[i,j];
minus:=C;
end;
function plus(A,B:mat):mat;
begin
for i:=1 to m do
for j:=1 to n do
C[i,j]:=A[i,j]+B[i,j];
plus:=C;
end;
function mult(A,B:mat;m,l,n:integer):mat;
begin
for i:=1 to m do
for j:=1 to n do
for k:=1 to l do
c[i,j]:=c[i,j]+A[i,k]*B[k,j];
mult:=C;
end;
begin
write('input m<=',maxM,'.. m=' );readln(m);
write('input n<=',maxN,'.. n=');readln(n);
readMat(A,m,n);
writeln('input the const K');readln(k);
B:=multK(A,K);
writeln('The matrix A : ');
writeMat(A,m,n);
writeln('The matrix B=K*A : ');
writeMat(B,m,n);
writeln('choose the operation + , - or * ');
readln(ch);
case ch of
'+' : c:=plus(A,B);
'-' : c:=minus(A,B);
'*' : begin
writeln('input m<=',maxM,'input l<=',maxN);readln(m,l);readMat(A,m,l);
writeln('input l<=',maxN);readln(n);readMat(B,l,n);
c:=mult(A,B,m,l,n);
end;
end;
writeMat(c,m,n);
readln;
end.

Answer 1

First of all having global one letter variables which collide with function parameters with the same name is insane.
Why does multK modify the global variable B as a sideeffect?
Why does minus modify the global variable C as a sideeffect?
Why global integers as for index variables?
And mult is even worse: It doesn't only modify C as a sideeffect, but it assumes C contains meaningful values beforehand. I think it needs to initialize C to all zeros beforehand.
My guess is some of your side effects interfere in strange ways. But I don't want to think it through. Refactor your code first. In particular learn how and when to use local variables.