Finding the number of lines of a data file using the command line

There is a common way to read each line one at a time and check iostat

for a nonzero or negative value each time it is read. However, I would like to name the procedure system(command)

and use the command wc -l

to count the number and then want to allocate the size of the array where I want to put the data. For example, I am printing the number of lines in both directions:

Program Test_reading_lines
    integer:: count,ios, whatever
    character(LEN=100):: command

    Print*,'Reading number of lines in a standard way'

    open (10, file='DATA_FILE')
           read (10,*,iostat=ios) whatever
           if (ios/=0) exit     
      End Do

    Print*,'Number of lines =', count

    Print*,'Reading number of lines using shell command'

    command='cat DATA_FILE | wc -l'
    call system(command)

    Print*,'Number of lines =','< ? >' 

    End Program Test_reading_lines


Unfortunately, in the latter case I can assign a variable type count

, as in the standard case? That is, I want to print the variable instead '< ? >'

in the last print command. In the meantime, there is no need to know about it. ”


source to share

2 answers

It is impossible directly. You can redirect the command output to a file then open it and read it

Or use some more complex Unix function features and call it C API (see first answer in this thread).

EXECUTE_COMMAND_LINE () also has no function to read the command output directly.



If you want to use a Unix command $ wc -l

, you can call a subroutine Fortran execute_command_line

, which is common to many Fortran compilers gfortran


Here is a working example that calculates the number of lines in a nlines

file named style.gnuplot

and then uses it nlines

to add some lines in style.gnuplot

by replacing the last one.

PROGRAM numLines

    integer, parameter :: n = 100
    integer :: i, nLines
    real, parameter :: x0 = -3.14, xEnd = 3.14
    real :: dx
    real, dimension (:), allocatable :: x, fun

    allocate(x(0:n)) ! Allocate the x array
    allocate(fun(0:n)) ! Allocate the fun array

    dx = abs(xEnd-x0)/n
    x(0:n) = [(x0+i*dx, i = 0,n)] ! Create the x array
    fun(0:n) = [(sin(x0+i*dx), i = 0,n)] ! Create the fun array

        DO i=0,size(x)-1
            write(1,*) x(i), ' ', fun(i) ! Save the function to a file to plot
        END DO

    deallocate(x) ! Deallocate the x array
    deallocate(fun) ! Deallocate the fun array

    open(unit=7, file="style.gnuplot")
        write(7,*) "set title 'y = sin(x)' font 'times, 24'"
        write(7,*) "set tics font 'times, 20'"
        write(7,*) "set key font 'times,20'"
        write(7,*) "set grid"
        write(7,*) "set key spacing 1.5"
        write(7,*) "plot '<cat' u 1:2 w l lw 2  linecolor rgb 'orange' notitle "

    CALL execute_command_line("wc -l style.gnuplot | cut -f1 -d' ' > nlines.file") ! COunt the lines

        read(1,*) nlines ! Here the number of lines is saved to a variable

    CALL execute_command_line("rm nlines.file") ! Remove nlines.file

    CALL execute_command_line("cat plotFunction.dat | gnuplot -p style.gnuplot") ! Show the plot within the executable

        DO i = 1,nLines-1 
            read(7,*) ! Read the file untile the penultimate row,
        END DO        ! then append the other rows
        write(7,*) "set object rectangle at -3.14,0 size char 1, char 1", & 
                                                                    " fillcolor rgb 'blue' fillstyle solid border lt 2 lw 1.5"
        write(7,*) "set object rectangle at 0,0 size char 1, char 1", & 
                                                                    " fillcolor rgb 'blue' fillstyle solid border lt 2 lw 1.5"
        write(7,*) "set object rectangle at 3.14,0 size char 1, char 1", & 
                                                                    " fillcolor rgb 'blue' fillstyle solid border lt 2 lw 1.5"
        write(7,*) "plot 'plotFunction.dat' u 1:2 w l lw 2  linecolor rgb 'orange' notitle"

    CALL execute_command_line("gnuplot -p 'style.gnuplot'") ! Load again style.gnulot with the appended lines



My code may not be fancy, but it works!



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