help with input check

**negevy** · 09-02-2005

I need to check if the user is ,for example, typing a letter insted of a number or that he is typing too many pieces ,for example
if i asked him to Enter an array of size 3 and he by mistake puts 4.
how do I check this?

this is part of the main function only:

Code:

void main() {
	
	int width[max_pieces], // the array of the pieces' heights
		height[max_pieces], // the array of the pieces' widths
		i, // index
		numH,numW,numP,//heights widths and number of pieces.
		puzzle[361];//the array of the puzzle.
	printf("Enter puzzle height( < 20 ) width( < 20 ) number of pieces( <= 10 )\n");
	scanf("%d%d%d",&numH,&numW,&numP);

	while(numH>=20||numW>=20||numP>=10){
		printf("Incorrect input ,try again\n");
		printf("\nEnter puzzle height( < 20 ) width( < 20 ) number of pieces( <= 10 )\n");
	scanf("%d%d%d",&numH,&numW,&numP);
	}//while
	
	printf("Enter array of size %d of piece heights:\n",numP);
	for (i=0; i<numP; i++)
		scanf("%d",&height[i]);
	
	printf("Enter array of size %d of piece widths:\n",numP);
	for (i=0; i<numP; i++)
		scanf("%d",&width[i]);

**Narf** · 09-02-2005

All that falls under input validation, and it's a broad topic. You can test for whether the user enters a number or not by reading everything as a string and then checking the string for validity:

Code:

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

int is_number(char *s) {
  char *e;

  strtol(s, &e, 0);

  return *e == '\0';
}

int main() {
  char *n, s[128];

  if (fgets(s, sizeof s, stdin)) {
    n = strchr(s, '\n');
    if (n) *n = '\0';
    if (is_number(s))
      puts("It's a number");
    else
      puts("Not a number");
  }

  return 0;
}

Or you can use the return value of scanf to see if the conversion worked. If it didn't work then you can assume an invalid format:

Code:

#include <stdio.h>

int main() {
  int i;

  if (scanf("%d", &i) == 1)
    puts("It's a number");
  else
    puts("Not a number");

  return 0;
}

But that's not as complete because it only tests up to the first invalid character, not the whole line like the first example. If the user is typing too many pieces, you can just ignore the extra ones or remove them when you want to ask for more input. That's an easy fix and it usually works great.

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