Count the numbers of equal rows in a file - if-statement

Question

Suppose you have a file.dat of the form:
1
1
1
2
2
3
3
3
3
...
I want to count how many equal numbers there are and save them iteratively in a string. For instance:
m = 3 (times 1),
m = 2 (times 2),
m = 4 (times 3).
I put here my code:
program sele
implicit none
integer::j,k,s,n,l,r,m
real*8,allocatable::ID(:)
real*8:: j_r8,i_r8
open(10,file='data.dat')
n=0
DO
READ(10,*,END=100)
n=n+1
END DO
100 continue
rewind(10)
allocate(ID(n))
s=0
do s=1, n
read(10,*) ID(s)
end do
do r=1,n-1
if (ID(r)-ID(r+1) .EQ. 0) then
m = m + 1
print*, m
end if
end do
end program
The last do is the condition I'd like to expand, with something like:
if (condition is true) then
save an index of the number of equal digits
use this to do some operations:
do i = 1, number of equal digits
if (condition is not true) then
restart with the other digits.

Answer 1

If the values you want to read are integer values in a given limited range (for instance from 1 to 100), then the simplest way is the following :
program sele
implicit none
integer, parameter :: vmin=1
integer, parameter :: vmax=100
integer :: list(vmin:vmax)
integer :: value,i
open(10,file='data.dat')
list=0
do
read(10,*,end=10) value
if(value < vmin .OR. value > vmax) then
write(*,*) 'invalid value ',value
stop
endif
list(value)=list(value)+1
enddo
10 continue
do i=vmin,vmax
if(list(i) > 0) then
write(*,*) list(i),' times ',i
endif
enddo
end program
Which gives on your example :
3 times 1
2 times 2
4 times 3
It is possible to improve easily that program to manage variable vmin and vmax (the array list must then be declared allocatable and allocated at the right size).
If the range is too large, then a simple array is not accurate anymore and the right algorithm becomes more complicated : it must avoid to store unused values.