File31. Дан файл целых чисел, содержащий более 50 элементов. Уменьшить его размер до 50 элементов, удалив из файла необходимое количество начальных элементов.

Решение на Python 3:

# -*- coding: utf-8 -*-
import random
import sys

def GenerateNumbers(fname):
    N = random.randint(50,70)
    print("N = ",N)
    try:
        f = open(fname, "w")
        for i in range(0,N):
            #x = random.randrange(1,10)
            x = i + 1
            line = str(x)+"\n"
            f.write(line)
        f.close()
    except:
        print("Error: ", sys.exc_info()[0])

def CountFile(fname):
#count number of rows in given text-file 
    N = 0
    try:
        with open(fname,'r') as f:
            for line in f:
                N += 1
                #print(N,":",line)
    except IOError:
        print("Open error:",fname)
        return -1
    finally:
        return N

f_input = "file31_in.txt"
f_output = "file31_out.txt"
GenerateNumbers(f_input)
print("Read from:",f_input)
print("Write to:",f_output)

try:
    num_items = CountFile(f_input)
    if num_items <= 50:
        print("File must contain more than 50 items")
        sys.exit()

    num_extra = num_items - 50
    
    with open(f_input, 'r') as f_in, open(f_output, 'w') as f_out:
        for i in range(num_extra):
            line = f_in.readline()
        for i in range(50):
            line = f_in.readline()
            f_out.write(line)

except:
    print("Error: ", sys.exc_info())