Finding three leaders from arrays - arrays

Question

Thank you for visiting this question. I know this looks like a question from a book which it totally is. I couldn't find the solution for this anywhere and I cant get one thing. Supposedly the code compare things like studs[i].score within for loop, but why it can assign the value of studs[i].score to another element of the struct like say first.score? The same goes for studs[i].name = first.name the program wont even compile. Any input matter, have been sitting with this for a week.
Have a great day!
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
struct stud {
char name[50];
float score;
};
//Please do not modify struct stud
//You are only allowed to modify inside printThreeLeaders
void printThreeLeaders(struct stud studs[], int count) { //why is count here? C arrays do not carry any size indicator, we
//must explicitly pass the number of elements in as an argument
//Please do not modify the content of studs array
struct stud first, second, third;
//Your code here
for (int i=0;i<count;i++){
if (studs[i].score>third.score){
if(studs[i].score>second.score){
if (studs[i].score>first.score){
studs[i].score=first.score;
studs[i].name=first.name;
}
}studs[i].score=second.score;
studs[i].name=second.name;
}studs[i].score=third.score;
studs[i].name=third.name;
}
//Please find the top three highest scoring students on the leaderboard
//and print out their names and scores.
//You are allowed to use string functions such as strcmp or strcpy
//Although you might not need them
//Please do not modify the following code
printf("Leader board:\n");
printf("First place: %s, %.2f\n", first.name, first.score);
printf("Second place: %s, %.2f\n", second.name, second.score);
printf("Third place: %s, %.2f\n", third.name, third.score);
}
//Please do not modify main function
int main(void) {
struct stud students[20];
int stud_count = 0;
char temp_name[50];
float grade = 0;
printf("Enter a test score(-1 to quit), or\n");
printf("Enter a grade first, then a student's name\n");
scanf("%f", &grade);
while (grade != -1)
{
scanf("%s", temp_name);
students[stud_count].score = grade;
strcpy(students[stud_count].name, temp_name);
stud_count ++;
printf("Enter a test score(-1 to quit), or\n");
printf("Enter a grade first, then a student's name\n");
scanf("%f", &grade);
}
if(stud_count > 2) {
printThreeLeaders(students, stud_count);
}
return 0;
}

Answer 1

A few issues:
You do modify the studs array with: studs[i].score=second.score;
The three variables first, second, and third are uninitialized so you have UB (undefined behavior)
You don't need to use str* functions to copy the name if you copy the whole struct.
Here is the refactored code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct stud {
char name[50];
float score;
};
// Please do not modify struct stud
// You are only allowed to modify inside printThreeLeaders
void
printThreeLeaders(struct stud studs[], int count)
{
// why is count here? C arrays do not carry any size indicator, we
// must explicitly pass the number of elements in as an argument
// Please do not modify the content of studs array
// NOTE/BUG: first/second/third are _not_ initialized
#if 0
struct stud first, second, third;
#else
struct stud first = { .score = -1 };
struct stud second = { .score = -1 };
struct stud third = { .score = -1 };
#endif
// Your code here
for (int i = 0; i < count; i++) {
const struct stud *st = &studs[i];
float score = st->score;
if (score > first.score) {
third = second;
second = first;
first = *st;
continue;
}
if (score > second.score) {
third = second;
second = *st;
continue;
}
if (score > third.score) {
third = *st;
continue;
}
}
// Please find the top three highest scoring students on the leaderboard
// and print out their names and scores.
// You are allowed to use string functions such as strcmp or strcpy
// Although you might not need them
// Please do not modify the following code
printf("Leader board:\n");
printf("First place: %s, %.2f\n", first.name, first.score);
printf("Second place: %s, %.2f\n", second.name, second.score);
printf("Third place: %s, %.2f\n", third.name, third.score);
}
// Please do not modify main function
int
main(void)
{
struct stud students[20];
int stud_count = 0;
char temp_name[50];
float grade = 0;
printf("Enter a test score(-1 to quit), or\n");
printf("Enter a grade first, then a student's name\n");
scanf("%f", &grade);
while (grade != -1) {
scanf("%s", temp_name);
students[stud_count].score = grade;
strcpy(students[stud_count].name, temp_name);
stud_count++;
printf("Enter a test score(-1 to quit), or\n");
printf("Enter a grade first, then a student's name\n");
scanf("%f", &grade);
}
if (stud_count > 2) {
printThreeLeaders(students, stud_count);
}
return 0;
}
In the above code, I've used cpp conditionals to denote old vs. new code:
#if 0
// old code
#else
// new code
#endif
#if 1
// new code
#endif
Note: this can be cleaned up by running the file through unifdef -k
Here is the test input I used:
1 Fred
2 Bob
3 Alice
4 John
5 Mary
6 Frank
7 Abel
8 Cain
9 Peter
10 Kilroy
11 Smith
12 Jones
-1
Here is the [cleaned up] program output:
Leader board:
First place: Jones, 12.00
Second place: Smith, 11.00
Third place: Kilroy, 10.00
UPDATE:
Amazing, it does work. The problem was in initialization of first, second and third. With this edits it does work. My incorrect reasoning was that initialization happened at 'struct stud first second third'. –
JEDi455
C is all about minimalism and speed.
Initialization [of stack based variables] is not done by default for speed.
Here, for this problem, explicit initialization was needed.
But, in another problem, suppose we had (e.g.):
int x,y,z;
If we explicitly assign them values with:
/* some small blob of code unrelated to x/y/z ... */
x = funcA();
y = funcB(x);
z = funcC(x,y);
Then, we'd be cursing the compiler for wasting time by initializing them to default values, only to overwrite those values with our explicit code.
That is, if the compiler always treated:
int x,y,z;
as:
int x = 0, y = 0, z = 0;
We'd not want the compiler to "help" us in this way. That is, if we wanted the latter, we'd have written that.
C gives the programmers full control [and assumes they know what they're doing]. The compiler will try to help by flagging statements with errors or warnings, but it's often up to the programmer.