Error in strcmp() when I use a structure as a parameter

Question

My program needs these functionalities:

NOTE: I did not include the codes for the numbers 1,2 & 4 since I already finished them. The 3rd one is my problem.

1. The program should continuously allow input from the user as long the user still wants to. (Dynamically)
2. Get the final grade of a student (average of frst_grade, scnd_grade, fnl_grade)
3. Get the number of students per college.
4. Get student name by inputting s_id.

My problem is how to compare the search input to the user input in s_college to get the number of students. The only way I know is by using strcmp() but it gives me this error: invalid conversion from 'char' to 'const char*' [-fpermissive]

So how do I compare these two to get the number of students per college?

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

int i,n,sum,search,num=0,ctr=0;

char answer,choice,choice2,search2;

struct record{

int s_id;
char s_name[100];
char s_course;
char s_college[5];
int s_scoress;
}id[100],name[100],course,college[100],scores;

struct s_scores{
int frst_grade;
int scnd_grade;
int fnl_grade;
}first,second,final;



void ADD();
void COLLEGE();
void ID();


void COLLEGE(){
printf("Enter college (abbreviation only)");
scanf("%s",&search2);
for(i=0;i<num;i++){
if(strcmp(college[i].s_college,search2)==0);
ctr++;    
}  
printf("The number of students in %s is %d",search2,ctr);

Answer 1

Lets take a look at these (partial) lines:

char ..., search2;
...
scanf("%s",&search2);
...
...strcmp(college[i].s_college,search2)...

The variable search2 is a single character. Trying to put a string into it will write at least two character: The string you read plus the string terminator. That means you will write out of bounds.

You then use the character variable as an argument to strcmp which converts the contents of search2 into a pointer and uses that pointer as a pointer to a string.

Both of these problems will lead to undefined behavior.

Is search2 supposed to be a string? Then declare it as an array, like

char ..., search2[100];

If search2 is supposed to be a single character then first you need to read a single character

scanf("%c", &search2);  // Note the changed format to read a single character

And then you need to change your comparison to not use strcmp.

Answer 2

You cannot use strcmp with what is not a null-terminated string. You can write

if(college[i].s_college[0] == search2 && college[i].s_college[1] == '\0')

Don't forget to remove the junk semicolon to have the if statement work.

Answer 3

Your search2 is just a character. You need a string

Perhaps declare search2 as follows:

char search2[50];

Also read up about scanf to prevent buffer overruns:

scanf("%49s", search2); // Do not go past the end of search2[50]

Answer 4

Well compiler tells you error: Variable search2 is char while s_college[5]; is array of chars. Function strcmp requires two arrays/pointers in order to work.

If search2 is only one byte then you can create: char Search2[2]; that would hold that one char and terminal null. But this would work only if search2 is one byte. If you however have to compare two array of chars where search2 is more then one byte, then you should probably think about dynamic allocation or create a static array char search2[some_length];.

Answer 5

this is not a complete 'answer', however, it does fix some of the major problems in the code:

Define your struct's like this;

struct s_scores
{
    int frst_grade;
    int scnd_grade;
    int fnl_grade;
};


struct record
{
    int  s_id;
    char s_name[100];
    char s_course;
    char s_college[5];
    struct s_scores  s_scoress;
};

struct record records[100];

Then access the individual fields similar to:

if( 1 != scanf( "%4s", records[i].s_college ) )
{
    perror( "scanf for college abbreviation failed" );
    exit( EXIT_FAILURE )
}

// implied else, scanf successful

// validate the college abbreviation

for( size_t j=0; j< (sizeof(collegesAbbrevationsTable)/(sizeof( *collegeAbbreviationsTable ); i++ )
{
    if( strncmp( collegeAbbreviationsTable[j], records[i].s_college, 4)
    { // then found matching abbreviation
        break; // exit 'validation' loop
    }
}

Note: perror() found in stdio.h. exit() and EXIT_FAILURE found in stdlib.h.

Note: In C, when referencing an array, the result is a pointer to the first byte of that array, so in the call to scanf() must not use & when referencing the array s_college[].

`

Answer 6

declae search2 as char search2[10]; or char * search2;

Reason : string2 is a character variable and college is a null terminating char array.

Signature of stncmp id int strcmp(const char* s1, const char*s2);

So the function to properly you need to passs char* or char array(which is again a char*).