How to improve the code in C (make "prettier")?

Question

C++ / C#

Task:

Главный вождь племени Абба не умеет считать. В обмен на одну из его земель вождь другого племени предложил ему выбрать одну из трех куч с золотыми монетами. Но вождю племени Абба хочется получить наибольшее количество золотых монет. Помогите вождю сделать правильный выбор! 
INPUT.TXT
В первой строке входного файла INPUT.TXT записаны три натуральных числа через пробел. Каждое из чисел не превышает 10100. Числа записаны без ведущих нулей. 
OUTPUT.TXT
В выходной файл OUTPUT.TXT нужно вывести одно целое число — максимальное количество монет, которые может взять вождь.

#include <stdio.h>

#define N 103
char* maxNum(char *n1, char *n2);
int length(char s[]);
int main()
{
  FILE *in;
  FILE *out;
  int i,j;
  char c;
  char num1[N];
  char num2[N];
  char num3[N];
  char *nums[3] = {num1,num2,num3}; 
  in = fopen("input.txt","r");
  for (i=0;i<3; ++i){
    j = 0;
    while(c=fgetc(in)){
      if ( c == '\n' || c == ' ' ){
        (*(nums+i))[j] = '\0';
        break;
      }			
      (*(nums+i))[j] = c;
      j++;
      
    }
  }
  fclose(in);
  out = fopen("output.txt","w");
  fprintf(out, "%s",maxNum(*nums, maxNum(*(nums+1),*(nums+2))));
  fclose(out);
  //printf("%s", maxNum(*nums, maxNum(*(nums+1),*(nums+2))));
  return 0;
}

int length(char s[]){
  int i;
  for(i=0; i<N,s[i]!='\0';i++)
    ;
  return i;
}

char* maxNum(char *n1, char *n2)
{
  int i;
  if (length(n1)>length(n2))
    return n1;
  if (length(n2)>length(n1))
    return n2;
  for (i=0; i<N;i++){
    if (n1[i]>n2[i])
      return n1;
    if (n2[i]>n1[i])
      return n2;
  }
}

Now the question is: how to make the code smaller, "prettier". I can somewhere not those data types used. Tell me please.

Answer 1

char num1[N];
  char num2[N];
  char num3[N];
  char *nums[3] = {num1,num2,num3};

Why not nums[3][N]?
When do you think the while condition will stop executing?
I would add a check at the end of the file:if ( c == '\n' || c == ' ' || c == EOF)
Why not nums[i][j]?
There is such a ready-made function strnlenfrom string.h
maxNum does not handle the case when the arguments are equal.

Answer 2

You can improve in the following ways:

• knowledge and use of standard functions
  
• checking results and handling errors
  

#include <stdio.h>
#include <string.h>

#define MAX_NUM_LENGTH 100

const char* chose(const char* left, const char* right)
{
  const size_t len_left = strlen(left);
  const size_t len_right = strlen(right);
  if (len_left > len_right)
    return left;
  if (len_right > len_left)
    return right;
  if (strcmp(left, right) < 0)
    return right;
  return left;
}

const char* get_max(const char* n1, const char* n2, const char* n3)
{
  return
      chose(
          chose(n1, n2),
          chose(n2, n3));
}

int main()
{
  FILE* in = NULL;
  FILE* out = NULL;
  char n1[MAX_NUM_LENGTH + 1];
  char n2[MAX_NUM_LENGTH + 1];
  char n3[MAX_NUM_LENGTH + 1];
  in = fopen("input.txt", "r");
  if (!in) {
    perror("Cannot open file 'input.txt'");
    return 1;
  }
  if (3 != fscanf(in, "%100[0-9] %100[0-9] %100[0-9]", n1, n2, n3)) {
    perror("Wrong input format");
    fclose(in);
    return 1;
  }
  const char* max_num = get_max(n1, n2, n3);
  out = fopen("output.txt", "w");
  if (!out) {
    perror("Cannot open file 'output.txt'");
    return 1;
  }
  fprintf(out, "%s", max_num);
  fclose(in);
  fclose(out);
  return 0;
}

Answer 3

You need to find your style of code design, and try to stick to
it, I recommend for the future S. McConnell "Perfect Code"
----------------------- -------------------------------------------------- --------------
indents after mathematical signs ( +, -, *, /, =), after a comma, after checks ( ==, !=, <=, <, >, > =), logical indents - after each task, variables must be declared as close as possible to the place of use

#include <stdio.h>
#include <stdlib.h>     /* atoi */

int main()
{
  char nums[3][N];

  // читаем
  FILE *in = fopen("input.txt", "r");
  for (int i = 0; i < 3; ++i)
  {
    int j = 0;

    char c;
    while(c = fgetc(in))
    {
      nums[i][j++] = c;

      if (c == '\n' || c == ' ')
      {
        nums[i][j] = '\0';
        break;
      }
    }
  }
  fclose(in);
  
  // находим
  int maxNums = 0;
  for (int i = 0; i != sizeof(nums); ++i)
  {
    const int curNum = atoi(nums[i]);
  if (curNum <= maxNums)
    continue;
    
  maxNums = curNum;
  }

  // выводим
  FILE *out = fopen("output.txt", "w");
  fprintf(out, "%s", maxNums);
  fclose(out);

  // printf("%s", maxNum(*nums, maxNum(*(nums+1),*(nums+2))));

  return 0;
}