为什么这个 for 循环无限运行？

Question

I was trying a problem, where I had to write "Accept" for a string ending with b. I'm having trouble with this part of code:

char s[100];
int length,i=0;
gets(s);
length=strlen(s);
for( i=0; i<length; i+2)
{
  if(s[i]=='b' && s[i+1]=='\0')
      printf("Accept");
  else printf("Not accept");
}

But this loop runs infinitely. The same problem is solved when I use this:

while(s[i]!='\0')
{ 
   if(s[i]=='b' && s[i+1]=='\0')
      printf("Accept");
   else printf("Not accept");
   i+2;
}

I understand there is a problem in the logic of the for loop, & have tried a lot to figure it out, but failed. I'm new to programming.Can you please help me with this.

Answer 1

for( i=0; i<length; i+2)

i is not incremented. So it should be:

for( i=0; i<length; i+=2)

Answer 2

You're not incrementing i in your for loop:

for( i=0; i<length; i+2)

All this does is add 2 to i and throws away the result. You were probably intending to increment by 2, but that won't work either. If your string contains an even number of characters, you'll never find the last character.

So what you need to do in increment by 1 on each iteration:

for( i=0; i<length; i++)

EDIT:

Better yet, just check the last character and get rid of the loop altogether:

if (s[length-1] == 'b') {
    printf("Accept\n");
} else {
    printf("Not Accept\n");
}

Answer 3

Where do you initialize your string? And also are you missing a parentheses? Also you probably want to print "Not accept" once. Check below for solution.

But really it will be much easier if you directly check the last character of array for 'b' (using as index length-1 ), and you don't need the loop anymore (since you know the length).

#include <stdio.h>
#include <string.h>
int main()
{
  char s[100];
  strcpy(s,"test");
  int length = 0, i = 0, j = 0;
  length=strlen(s);
  for( i=0; i<length; i++)
  {
    if(s[i]=='b' && s[i+1]=='\0')
    {
       printf("Accept");
       j = 1;
    }

  }
  if(j==0) printf("Not accept");
  return 0;
}

Answer 4

在您的for循环中，您从未增加i变量：

for( i=0; i<length; i+2) // should probably use i++ instead of i+2

Answer 5

The problem that you are trying to solve (if I am reading the attempts correctly) is that you want to establish if the last character is the letter 'b'.

You have found out the length of the string by using:

length=strlen(s);

So using this fact you can do the following

if (length > 0 && s[length-1] == 'b') {
   printf("Accept");
}
else
{
   printf("Not accept");
}