Another map question

I'm working on a simple data structure and some functions that take various shapes and perform calculations on them. The shapes will be constructed out of real physical materials, and I need a way to count each segment length. The count and lengths are arbitrary, as are the number of lengths.

I know this code isn't complete, I'm just showing an outline of what I'm trying to accomplish.

Code:

---

#include <map>
#include <vector>

using std::map;
using std::vector;

struct SEG_PROPERTIES
        {
        int material;
        unsigned int thickness;
        bool is_coated;
        };

struct SEG_COUNTS
        {
        struct SEG_PROPERTIES segProperties;

        // For total length of all segments with same properties
        unsigned int totalLength;

        // first = length of segment
        // second = count of segments with length
        map <unsigned int, unsigned int> segment;
        };

vector <SEG_COUNTS> segCounts;

void someFunction(void)
        {
        // Misc code...

        // Line in question
        segCounts[1].segment.first = 5;
        }

---

When I try to refer to the .first or .second elements of segCounts[].segment I get this compilation error:

Code:

---

error C2039: 'first' : is not a member of std::map<_Kty,_Ty>

---

So I thought that maybe for some reason I need to specify using a pair in the declaration of segment...

Code:

---

#include <utility>
using std::pair;

...

struct SEG_COUNTS
        {
...

        map <pair<unsigned int, unsigned int> > segment;
        };

---

But that gives me different errors, and I really don't think that's the way I'm supposed to declare a map. It's obvious I'm doing something wrong, maybe the syntax is eluding me. Anyone care to enlighten me?

segCounts[1].segment is a std::map, and a std::map does not have a public member named first (or second). What exactly were you trying to do on that line?

first is part of std::pair which map::find() returns. You are using the map incorrectly.

What I'm really looking to do is retrieve and set the two values in segment. I thought that this was done with the .first and .second members of the map, but I guess I'm mistaken.

Perhaps you are looking to use a pair instead of a map?

I need a map, I think. For each segCounts there will be an assortment of length/count pairs that need to be stored.

hmm... okay, in your original code, you need to understand that segCounts[1].segment is a map. You might write segCounts[1].segment[5], which would then be an unsigned int presumably representing the count of segments with length 5.

Okay, I think I've got it. I just needed to get an iterator to the segment in question...

Code:

---

void someFunction(void)
        {
        map <unsigned int, unsigned int>::iterator iterCurrent;
        unsigned int length = 4;
        unsigned int count = 1;

        iterCurrent = segCounts[1].segment.find(length);
        iterCurrent->second = count;

        }

---

...which is what I said in my post.

Yes, thank you Bubba. When I first read your post I didn't quite understand, I'd never used the ::find member function before. You definitely got me on track there.