I want to preface my question by mentioning that I am a CS student that is very new to coding.

I am attempting to write a script that will add a set of array of characters to two arrays that are nested within a loop. The array of characters are prefixes for two courses, and I am using a function called "matcher" (that accepts an array that will hold a prefix, and an integer that holds the course number that we want to match to the corresponding prefix) within the loop.

The issue that I am facing is that when I run this, the first array(.prefix3) gets concatenated with the prefix for the second array(.prefix4), which seems to happen after the loop runs for a second time. I am unsure as to why this is occurring, specially since each array can only hold 7 characters. Here is the code, thank you all in advance:

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

typedef struct {
  char name[20];
  int ID;
  int number_of_courses;
  int course_numbers[4];
  char prefix1[7];
  char prefix2[7];
  char prefix3[7];
  char prefix4[7];
  int taken;


} student_info;

void matcher(char *,int );

int main()
{

student_info students[100];
int j=2;
int course_placeholder;
    
    for(int i=0;i<2;i  )
    {

        printf("Enter course number %d:",i 1);//Type "9696" or "1232"
    scanf("%d",&course_placeholder);
        
    if(j==1)
                        {
                            matcher(students[0].prefix2,course_placeholder);
                        }
  
            else if(j==2)
                        {   
                            
                            matcher(students[0].prefix3,course_placeholder);//this is concatenating the string for some reason. Trying                                                                                                                       to determine why.                          
                        }
        else if(j==3)
                        {
                            matcher(students[0].prefix4,course_placeholder);

                        }
    
        j  ;
        
        }
     
printf("%s\n%s\n",students[0].prefix3,students[0].prefix4);//students[0].prefix3 is concatenating for an unknown reason. 
               
    return 0;
}
  
void matcher(char *prefix_placeholder,int course_placeholder) 
{                                                 
        if(course_placeholder==9696)
        {
           strcpy(prefix_placeholder,"MAT 236");
        }
        else if(course_placeholder==1232)
        {

           strcpy(prefix_placeholder,"COP 220");
        }
}

CodePudding user response：

The arrays

  char prefix1[7];
  char prefix2[7];
  char prefix3[7];
  char prefix4[7];

are too short to store 7-character strings like "MAT 236" and "COP 220" because they require at least 8-character arrays including terminating null-characters.

Allocate enough elements to avoid troubles.

CodePudding user response：

Although you cannot see it, string literals with 7 visible characters, such as this one:

"COP 220" 
        ^ - NULL terminator is implied here  

actually contain 8 characters, not 7. The last character is \0 ( AKA NULL )

So, when allocating memory in an array which is designed to contains C strings ( defined as a NULL terminated char array ) always include space enough for the NULL

char prefix1[8] = {0};  //1 extra element for NULL, all elements initialized to NULL   

To test this yourself, do this:

size_t size = sizeof "MAT 236" ;//expect size == 8