My fortran program never stops executing - fortran

Question

I have a fortran program that never stops executing.
program russianmultiplication
implicit none
integer::x,y,ginx,giny,k=0
print*,'give a number'
read*,x
k=k+x
print*,'one more'
read*,y
if (y==1) then
print*,x
end if
do while(y/=1)
ginx=x*2
giny=y/2
if (mod(giny,2)/=0) then
k=k+ginx
end if
end do
print*,'result',k
end program
Why does this program never end?

Answer 1

You never change y inside the loop, and have no exit statement, either. So either y is 1 on input, in which case the condition for the loop is never true and it never executes, or you never leave the loop since the condition is always true.

Answer 2

Here is perhaps a clearer version of the original program to do Russian peasant multiplication:
PROGRAM Russian_peasant_multiplication
IMPLICIT NONE
INTEGER :: total, x, x_save, y, y_save
total = 0!running total
PRINT *, "x ="
READ *, x
x_save = x
total = total + x
PRINT *, "y ="
READ *, y
y_save = y
PRINT *, y, x
!do while(y/=1)
modulo: DO
IF (y == 1) EXIT!replace DO WHILE for better control
!ginx=x*2
x = x * 2!okay to redefine since saved
!giny=y/2
y = y / 2!okay to redefine since saved
!if (mod(giny,2)/=0) then
IF (MOD(y, 2) /= 0) THEN!keep only odd numbers on the left
PRINT *, y, x
!k=k+ginx
total = total + x
END IF
END DO modulo
PRINT *, " "
PRINT *, x_save
PRINT *, "x"
PRINT *, y_save
PRINT *, "="
PRINT *, total
END PROGRAM Russian_peasant_multiplication
I've added useful comments and also pointed out where the logical was flawed. Hope this helps.