在C中为字符数组赋值时，为什么整数数组元素包含错误的值，并且该如何解决？

Question

I wrote the following code:

#include <stdio.h>
#include <string.h>
int main()
{
    char *string = "abcd";
    int i;int j;
    int array[5];
    for(i=0; i<strlen(string); i++)
       array[i] = string[i];  
    for(j=0; j<4; j++)
      printf("array[i] %d\n",array[i]);


     array[0] = string[0]; 
     printf("array[0] %d\n",array[0]);


return 0;
}

I got the following output:

array[i] 4195888
array[i] 4195888
array[i] 4195888
array[i] 4195888
array[0] 97

I thought the output would be:

array[i] 97
array[i] 98
array[i] 99
array[i] 100
array[0] 97

If this assignment:

array[0] = string[0];

assigns 97 to array[0], then why doing similar assignments within the loop, produces a different output, and how can I fix my code so that the output would look as I expected it to be?

Answer 1

The problem is mismatched index values.

for(j=0; j<4; j++)
      printf("array[i] %d\n",array[i]);

should be

for(j=0; j<4; j++)                       // j is the loop control variable
      printf("array[j] %d\n",array[j]);  // so, use j as index here

Otherwise, what happens is you use the last value of i as index, which here, is an attempt to access uninitialized element (ie, array[4] ) which contains indeterminate value. This invokes undefined behavior .

To be on safe side, you better initialize the local variables to your function.

Answer 2

As already pointed out, your printf loop is on j but you are using i as the index into the array, whereas you should index using j. You also have j hardcoded within the printf, and it should be %d and then filled in with j.

#include <stdio.h>
#include <string.h>
int main()
{
    char *string = "abcd";
    int i;int j;
    int array[5];
    for(i=0; i<strlen(string); i++)
       array[i] = string[i];  
    for(j=0; j<4; j++)
      printf("array[%d] %d\n",j,array[j]);


     array[0] = string[0]; 
     printf("array[0] %d\n",array[0]);


return 0;
}

Then your output will be as expected:

array[0] 97
array[1] 98
array[2] 99
array[3] 100
array[0] 97