so the code is pretty simple. it just loops and if statements. first i have a main. i took the headers and prototypes out to make it smaller.
In main all iam doing is asking user to enter command and call one funtion.
// main.c

int main(int argc, char *argv[])
{
  char command[10];

  printf("Enter (a) to slove Linear equation(1 variable):\n");fflush(stdout);
  printf("Enter (b) to slove Linear equation(2 variable):\n");fflush(stdout);
  printf("Enter (slope) to find  Slope\n");fflush(stdout);
  printf("Enter (exit) to  Exit:\n");fflush(stdout);
  scanf("%s", command);
  while(strcmp(command, "exit") != 0)
    {
      if(strcmp(command, "a") == 0)
    {
      linear_1(&buff1, &buff2);
    }
      printf("Enter (a) to slove Linear equation(1 variable):\n");fflush(stdout);
      printf("Enter (b) to slove Linear equation(2 variable):\n");fflush(stdout);
      printf("Enter (slope) to find  Slope\n");fflush(stdout);
      printf("Enter (exit) to  Exit:\n");fflush(stdout);
      scanf("%s", command);
    }//end of while loop
  return 0;
}

In this linear_1 function the while loop doesnt work.. for some reason it doesnt go inside while loop.
any one know how can i fix this.

 while((ch = getchar()) != '\n')
        {
          two_side(ch, left_side, right_side, buff1);
        }

already so here is linear_1 funtion below.

void linear_1(struct two_side *buff1, struct slove_eq *buff2)
{
  char ch;

  char left_side[SIZE];
  char left_num[SIZE];
  char left_var[SIZE];
  char left_var_num[SIZE];
  int l_total_num = 0;
  int l_total_var = 0;

  char right_side[SIZE];
  char right_num[SIZE];
  char right_var[SIZE];
  char right_var_num[SIZE];
  int r_total_num = 0;
  int r_total_var = 0;

  int ans_num = 0;
  int ans_var = 0;



  buff1->l = 0;
  buff1->r = 0;
  buff1->flag1 = 0;

  set_null(left_side);
  set_null(left_num);
  set_null(left_var);
  set_null(left_var_num);

  set_null(right_side);
  set_null(right_num);
  set_null(right_var);
  set_null(right_var_num);



  printf("Enter a Linear equation(1 variable): ");fflush(stdout);
  while((ch = getchar()) != '\n')
    {
      two_side(ch, left_side, right_side, buff1);
    }

  /**** Left_side: Sparate numbers from variables ****/
  separate(left_side, left_num, left_var);

  /**** Left_side: calculator numbers ***/
  l_total_num = cal_number(left_num);

  /********Left_side: variables to number ***/
  variables_to_num(left_var, left_var_num);

  /**** Left_side: calculator variables ***/
  l_total_var = cal_number(left_var_num);

  /**** Right_side: Sparate numbers from variables ****/
  separate(right_side, right_num, right_var);

  /**** Right_side: calculator numbers ***/
  r_total_num = cal_number(right_num);

  /******* Right_side: variables to number ***/
  variables_to_num(right_var, right_var_num);

  /**** Right_side: calculator variables ***/
  r_total_var = cal_number(right_var_num);


  /*** Slove equation ***/
  slove_eq(l_total_var, l_total_num, r_total_var, r_total_num, buff2);


  if(buff2->ans_var == 0)
    {
      printf("- Error -\n"); fflush(stdout);
    }
  else
    {
      printf("x = %d/%d\n",buff2->ans_num, buff2->ans_var); fflush(stdout);
    }
}

ahhh i found the problem. its when i enter a in main. ex, a\n
for some reason \n is going inside the linear_1 function. thats why its skiping the while loop.

but i still dont know how to fix this

See this. Check out the rest of the series, too.

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