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C: char into an int question/problem


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I just got back into learning C and finished a tutorial on Arrays. I'd been thinking about a program like this one, where you can add a bunch of numbers without having to declare a whole bunch of different variables. I knew an array would be how I could do this but I wasn't sure exactly how so when I finished the array tutorial I immediately started visualizing this:

#include <stdio.h>

int main()
{
  int array[50], numstoadd;
  int total = 0;
  printf("How many numbers would you like to add?\n");
  while(1){
    scanf("%d", &numstoadd);
    if(numstoadd < 2){
      printf("Please use at least two numbers.\n");
      continue;
    }
    else if(numstoadd > 50){
      printf("Fifty number limit, just cuz I said. :)\n");
      continue;
    }
    else{
      int i = 0;
      printf("Input one number per line.\n");
    for( i = 0; i < numstoadd; i++){
      scanf("%d", &array[i]);
      total = total + array[i];
    }
    for( i = 0; i < numstoadd - 1; i++){
      printf("%d + ", array[i]);
    }
    printf("%d = %d", array[numstoadd - 1], total);
    fflush(stdin);
    getchar();
    return 0;
    }
  }
}

The thing is, when a char is put into "numstoadd" int the 50 max nums line keeps going until I end it. Also, if a char is put into one of the array elements that is to be added, it jumps straight to the total with a bunch of random numbers.

How can I stop something like this from occurring by maybe verifying that the input is an integer or something similar? And, is this a buffer overflow? If not, what is going on in memory (other than "You can't put a char into an int variable" because I already know that)?

Thanks in advance.

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I have had a quick look, not exactly how I would tackle the job but then as you are learning it is a reasonable way to tackle the task. I have made a few changes, mainly removing the scanf's and replacing them with fgets's followed by atoi to convert the string into and integer. I also changed the structure of your code slightly so that you don't keep running pointless checks (that sort of thing doesn't make a noticable difference at this sort of thing but once you start coding apps that need to process massive amounts of data it will be very noticable).

#include <stdio.h>
#include <stdlib.h>
#define BUFFER_SIZE 50

int main(int argc, char ** argv)
{
  char input[BUFFER_SIZE];
  int array[50];
  int numstoadd=0;
  int total = 0;
  int i;
  printf("How many numbers would you like to add?\n");
  while(!numstoadd)
  {
    fgets(input, BUFFER_SIZE, stdin);
    numstoadd=atoi(input);
    if(numstoadd < 2)
    {
      printf("Please use at least two numbers.\n");
    }
    else if(numstoadd > 50)
    {
      printf("Fifty number limit, just cuz I said. :)\n");
    }
  }

  for( i = 0; i < numstoadd; i++)
  {
    printf("\nEnter number => ");
    fgets(input, BUFFER_SIZE, stdin);
    array[i]=atoi(input);
    total = total + array[i];
  }
  for( i = 0; i < numstoadd - 1; i++)
  {
    printf("%d + ", array[i]);
  }
  printf("%d = %d", array[numstoadd - 1], total);
  fflush(stdin);
  getchar();
  return EXIT_SUCCESS;
}

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  • 3 weeks later...
Looks very good, one thing to remember is > is not inclusive so you gotta do >= or <=, but thats me just nit picking.

Arrays in C use an offset for indexing (i.e. the first element is 0). So, for example, when you want to loop through a 50 elements you want your loop to run from 0 to 49 so you usually find that in the for loop you want to compare against one less than the number the user entered.

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