How should I fix these garbage values?

Question

I am an apprentice of C language, trying to learn some stuff here. I had confronted with a problem, and I wish I could have solved it on my own. But, I am a short wrapper and quite prone to procrastination, so I need your good support. As the title explicitly says, I do not know why my topMatches() function is spitting out some random float values, printing out at the beginning and the end of an array of scores[]. Like this,

Output #1
(-0.00000, Lisa Rose),(0.99124, Gene Seymour),(0.92447, Michael Phillips),(0.89341, Claudia Puig),(0.66285, Mick LaSalle),(0.38125, Jack Matthews),(-1.00000, Toby)
Output #2
(107185664961793568883398204719104.00000, Lisa Rose),(0.99124, Gene Seymour),(0.92447, Michael Phillips),(0.89341, Claudia Puig),(0.66285, Mick LaSalle),(0.38125, Jack Matthews),(-1.00000, Toby)
Output #3
(0.99124, Lisa Rose),(0.92447, Gene Seymour),(0.89341, Michael Phillips),(0.66285, Claudia Puig),(0.38125, Mick LaSalle),(-118195603315995709432961818167345152.00000, Jack Matthews),(-1.00000, Toby)
...

The value should be in the range of -1 and 1. I would really appreciate to see your feedback.

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

//data types
struct mInfo
{
    char mName[20];
    float rating;
};
struct cInfo
{
    char name[20];
    struct mInfo movi[7];
};
//prototype fxns
typedef double (*sim_fp)(struct cInfo *, const char *, const char *);
double sim_D(struct cInfo *prefs, const char *person1, const char *person2);
void topMatches(sim_fp fp, struct cInfo *prefs, const char *person1, int num);
int cmpFxn (const void * a, const void * b);
void reverseFxn(float arr[], int num);

int main() 
{
    int num = 7;
    struct cInfo critics[num];
  
    critics[0] = (struct cInfo) {"Lisa Rose", {"Lady in the Water", 2.5, "Snakes on a Plane", 3.5, "Just My Luck", 3, "Superman Returns", 3.5, "The Night Listener", 3, "You, Me and Dupree", 2.5}};
    
    critics[1] = (struct cInfo) {"Gene Seymour",{"Lady in the Water", 3, "Snakes on a Plane", 3.5, "Just My Luck", 1.5, "Superman Returns", 5, "The Night Listener", 3, "You, Me and Dupree", 3.5}};
    
    critics[2] = (struct cInfo) {"Michael Phillips",{"Lady in the Water", 2.5, "Snakes on a Plane", 3, "Superman Returns", 3.5, "The Night Listener", 4}};
    
    critics[3] = (struct cInfo) {"Claudia Puig",{"Snakes on a Plane", 3.5, "Just My Luck", 3, "Superman Returns", 4, "The Night Listener", 4.5, "You, Me and Dupree", 2.5}};
    
    critics[4] = (struct cInfo) {"Mick LaSalle",{"Lady in the Water", 3, "Snakes on a Plane", 4, "Just My Luck", 2, "Superman Returns", 3, "The Night Listener", 3, "You, Me and Dupree", 2}};
    
    critics[5] = (struct cInfo) {"Jack Matthews",{"Lady in the Water", 3, "Snakes on a Plane", 4, "Superman Returns", 5, "You, Me and Dupree", 3.5}};
    
    critics[6] = (struct cInfo) {"Toby",{"Snakes on a Plane", 4.5, "Superman Returns", 4, "You, Me and Dupree",1}};
    
    topMatches(sim_D, critics, "Toby", 7);

    return 0;
}

double sim_D(struct cInfo *prefs, const char *person1, const char *person2)
{
    int i=0;
    int x=0;
    int next=0;
    int p1;
    int p2;
    float X = 0;
    float Y = 0;
    float sumSq1 = 0;
    float sumSq2 = 0;
    float pSum = 0;
    float num = 0;
    float den = 0;
    float Pscore = 0;
    int nElements =0;
    
    
    for (i=0;i<7;i++) {
        if(strcmp(prefs[i].name, person1) ==0)
        {
           p1 = i; 
        }
        else if(strcmp(prefs[i].name, person2) ==0)
        {
            p2 = i;
        }
    }
  
    for (x=0;x<7;x++) {
        for (next=0;next<7;next++) 
        {
            if (!prefs[p1].movi[x].rating && !prefs[p2].movi[next].rating);
            else if (strcmp(prefs[p1].movi[x].mName, prefs[p2].movi[next].mName) == 0)
            {
                  X += prefs[p1].movi[x].rating;
                  Y += prefs[p2].movi[next].rating;
                  sumSq1 += pow(prefs[p1].movi[x].rating,2);
                  sumSq2 += pow(prefs[p2].movi[next].rating,2);
                  pSum += (prefs[p1].movi[x].rating*prefs[p2].movi[next].rating); 
                  nElements++;
            }
        }
        next = 0;
    }
  
    num = pSum-(X*Y/nElements);

    den=sqrt((sumSq1-pow(X,2)/nElements)*(sumSq2-pow(Y,2)/nElements));
    if(den ==0) return -1;

    Pscore = num/den; 

    return Pscore;
}

void topMatches(sim_fp fp, struct cInfo *prefs, const char *person1, int num)
{
    float scores[8];
    char *buf;
    int i=0;

    for (i=0;i<num;i++)
    {
        if(strcmp(person1, prefs[i].name)==0)
        {
          continue;
        }
        scores[i] = fp(prefs, person1, prefs[i].name);
    }
  
  qsort(scores, num, sizeof(float), (*cmpFxn));
  reverseFxn(scores, num);
   printf("\n\n");
        for(i=0;i<num;i++)
        {
          if (i == num-1)
          {
            printf("(%.5f, %s)", scores[num-1], prefs[num-1].name);
          }
          else
          {
            printf("(%.5f, %s),", scores[i], prefs[i].name);
          }
        }
}
void reverseFxn(float arr[], int num)
{
  float scoresTmp[num];
  int j;
        
    for(j=0;j<num;j++)
    {
      scoresTmp[num-1-j] = arr[j];
    }
      for(j=0;j<num;j++)
      {
        arr[j]=scoresTmp[j];
      }
}
int cmpFxn (const void * a, const void * b) 
{
   return ( *(int*)a - *(int*)b );
}

The value of each elements of the scores[] array needs to be within the range of -1 and 1.

Answer 1

1. Unexpected variable output is a hint that uninitialized data is being used. In struct cInfo you have a member struct mInfo movi[7]; but assign between 3 and 6 of these in main() . By switching from assignment to initialization those records that are not explicit set will be zero initialized. This seems to be sufficient to get consistent output.

int main() {
    struct cInfo critics[] = {
        {"Lisa Rose", {
            {"Lady in the Water", 2.5},
            {"Snakes on a Plane", 3.5},
            {"Just My Luck", 3},
            {"Superman Returns", 3.5},
            {"The Night Listener", 3},
            {"You, Me and Dupree", 2.5}
        }},
        {"Gene Seymour", {
            {"Lady in the Water", 3},
            {"Snakes on a Plane", 3.5},
            {"Just My Luck", 1.5},
            {"Superman Returns", 5},
            {"The Night Listener", 3},
            {"You, Me and Dupree", 3.5}
        }},
        {"Michael Phillips", {
            {"Lady in the Water", 2.5},
            {"Snakes on a Plane", 3},
            {"Superman Returns", 3.5},
            {"The Night Listener", 4}
        }},
        {"Claudia Puig", {
            {"Snakes on a Plane", 3.5},
            {"Just My Luck", 3},
            {"Superman Returns", 4},
            {"The Night Listener", 4.5},
            {"You, Me and Dupree", 2.5}
        }},
        {"Mick LaSalle", {
            {"Lady in the Water", 3},
            {"Snakes on a Plane", 4},
            {"Just My Luck", 2},
            {"Superman Returns", 3},
            {"The Night Listener", 3},
            {"You, Me and Dupree", 2}
        }},
        {"Jack Matthews", {
            {"Lady in the Water", 3},
            {"Snakes on a Plane", 4},
            {"Superman Returns", 5},
            {"You, Me and Dupree", 3.5}
        }},
        {"Toby", {
            {"Snakes on a Plane", 4.5},
            {"Superman Returns", 4},
            {"You, Me and Dupree",1}
        }}
    };
    topMatches(sim_D, critics, "Toby", sizeof critics / sizeof *critics);
}

and here is example output:

(0.99124, Lisa Rose),(0.92447, Gene Seymour),(0.89341, Michael Phillips),(0.66285, Claudia Puig),(0.38125, Mick LaSalle),(0.00000, Jack Matthews),(-1.00000, Toby)

2. (not fixed) Consider rewriting the unusual if expression with an empty body:

if (!prefs[p1].movi[x].rating && !prefs[p2].movi[next].rating);
else if (strcmp(prefs[p1].movi[x].mName, prefs[p2].movi[next].mName) == 0) {

to:

if (
   (prefs[p1].movi[x].rating || prefs[p2].movi[next].rating) && 
   !strcmp(prefs[p1].movi[x].mName, prefs[p2].movi[next].mName)
) {

If you introduce a couple of vanity variables it could be written a more compactly.

3. Replace float with double and fix the sort function:

int cmpFxn (const void * a, const void * b) {
    double a2 = *(double *) a;
    double b2 = *(double *) b;
    if(a2 < b2) return -1;
    if(a2 > b2) return 1;
    return 0;
}

4. valgrind reports use of uninitialized data in topMatches specially double scores[8]; . You only iterate over num which is 7 , and you don't set scores[i] for the entry that corresponds to person1 so I suggest you do:

    double scores[num];
    memset(scores, 0, sizeof(scores));

5. (not fixed) Audit your code for magic values, like 7 and 20. Use constants, pass in an argument or sizeof a / sizeof *a if you need the size of an array (be careful arrays degrade to pointers when passed as arguments).

Answer 2

At least these issues:

Wrong compare function

qsort() is called with an array of float and cmpFxn , yet cmpFxn() compares int . ^*1

Need to cast to float . ^*2

// return (*(int*)a - *(int*)b )
return (*(float*)a > *(float*)b ) - (*(float*)a < *(float*)b );

No not use return (*(float*)a - *(float*)b ) as the float difference may overflow int range or truncate to 0.

Code risks sqrt(some_negative)

Watch out for slight floating point artifacts that result in a negative that mathematically should never be less than 0.

// den=sqrt((sumSq1-pow(X,2)/nElements)*(sumSq2-pow(Y,2)/nElements));
t = (sumSq1-pow(X,2)/nElements)*(sumSq2-pow(Y,2)/nElements);
den = t >= 0.0 ? sqrt(t) : 0;

float vs. double

Code is casual about mixing float and double objects and functions. Suggest double only.

For debug purposes, easy enough to use #define float double to temporarily do this.

---

^*1
cmpFxn() is also a weak compare function for int too as it risks int overflow.

// return ( *(int*)a - *(int*)b );
return (*(int*)a > *(int*)b ) - (*(int*)a < *(int*)b );

^*2
If not-a-numbers are involved, the compare is more complicated. Let us assume, for now, that is not the case.