Confusion C begining

Question

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


int main()
{
 char a[250];
 char c1[1],c2[1];
int n,i;


printf("Give text: ");
gets(a);

printf("Give c1: ");
gets(c1);
  printf("Give c2: ");
 gets(c2);

n=strlen(a);

for(i=0;i<n;i++)
{
    if(a[i]==c1)
 {
      a[i]=c2;
   }
  if(a[i]==c2)
 {
    a[i]=c1;
   }
   }
     printf("%s",a);
return 0;
}

In a text I need to switch c1 with c2 and reverse, but when I start the program after I give a, c1, c2 nothing happened. Where am I wrong?

Answer 1

First of all, don't use gets(), it's inherently dangerous, use fgets() instead.

On top of that, when you used gets(c1), c1 being an one-element array, you already overrun the allocated memory which invokes undefined behavior.

That said, you have c1 and c2 as one-element arrays, which are not wrong but neither required. Define them as simple char variables

char c1;
char c2;

and use them like

 scanf(" %c", &c1);  // mind the space and don't forget to to check the return 
 scanf(" %c", &c2);  // value of scanf() to ensure proper scanning.

After that, the check for a[i] == c2 should come as else construct, otherwise, you'll be overwriting the previous operation. Something like

for(i=0;i<n;i++)
{
    if(a[i]==c1)
   {
      a[i]=c2;
   }
  else if(a[i]==c2)
   {
    a[i]=c1;
   }
}

Answer 2

• gets() shouldn't be used because it has unavoidable risk of buffer overrun, is deprecated in C99 and is removed from C11.
• c1 and c2 have insufficient buffer size.
• You should check if readings are successful.
• You should dereference c1 and c2 before compareing to retrieve characters read.
• You should use else if, or the modified character will be modified again.

Try this:

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

/* this is a simple implementation and the buffer for \n is wasted */
char* safer_gets(char* buf, size_t size){
  char* lf;
  if (fgets(buf, size, stdin) == NULL) return NULL;
  if ((lf = strchr(buf, '\n')) != NULL) *lf = '\0';
  return buf;
}

int main()
{
  char a[250];
  char c1[4],c2[4]; /* need at least 3 elements due to the inefficient implementation of safer_gets */
  int n,i;


  printf("Give text: ");
  if(safer_gets(a, sizeof(a)) == NULL)
  {
    fputs("read a error\n", stderr);
    return 1;
  }

  printf("Give c1: ");
  if(safer_gets(c1, sizeof(c1)) == NULL)
  {
    fputs("read c1 error\n", stderr);
    return 1;
  }
  printf("Give c2: ");
  if(safer_gets(c2, sizeof(c2)) == NULL)
  {
    fputs("read c2 error\n", stderr);
    return 1;
  }

  n=strlen(a);

  for(i=0;i<n;i++)
  {
    if(a[i]==*c1)
    {
      a[i]=*c2;
    }
    else if(a[i]==*c2)
    {
      a[i]=*c1;
    }
  }
  printf("%s",a);
  return 0;
}