Function returns a wrong number? - c++

Question

Can anybody help me with this? It gives me a wrong number. matrix[i][j].spath is filled with the correct values but when i return the shortest path between any two nodes it gives me a wrong number. The compiler gives me this
warning: control reaches end of non-void function
But the if-statement where i check whether the end is reached will always perform so the return statement, because i set up the end coordinates in main(). But i noticed when i add return 1 or return anything at the end of the function it gives the correct result. Is this a kind of a rule or what? I have written a functions like this where i had an if-statement and the only return statement in it and it worked without problems. Thanks :)
#include <iostream>
#include <queue>
using namespace std;
struct node
{
int x,y,spath,val;
}v,c;
node mat[100][100];
int dy[] = {-1,1,0,0}, dx[] = {0,0,-1,1}, n, m;
void input()
{
cin >> n >> m;
for (int i=0; i<n; i++) {
for (int j=0; j<m; j++) {
cin >> mat[i][j].val;
mat[i][j].spath = 0;
}
}
}
int shortest_path(node start, node end)
{
queue<node> q;
q.push(start);
mat[start.y][start.x].val = 1;
while (!q.empty())
{
v = q.front();
q.pop();
for (int i=0; i<4; i++) {
c.y = v.y + dy[i];
c.x = v.x + dx[i];
if (c.y == end.y && c.x == end.x) {
return mat[v.y][v.x].spath + 1;
}
else if (c.y >=0 && c.y < n && c.x >=0 && c.x < m && mat[c.y][c.x].val == 0)
{
mat[c.y][c.x].val = 1;
mat[c.y][c.x].spath = mat[v.y][v.x].spath + 1;
q.push(c);
}
}
}
}
int main()
{
node start,end;
start.x = start.y = 0;
end.y = end.x = 4;
input();
cout << shortest_path(start,end) << endl;
return 0;
}

Answer 1

As you noticed, the problem is that it misses a return statement. You may know that it will always go through the return in the if statement, but the compiler doesn't, hence the warning.
You supposed the input was correct, but you should "never trust user input". Never Ever.
There should always be a return statement for all routes the execution might take in your non-void function.

Answer 2

It seems that you are writing BFS.Here's my code:
#include"stdio.h"
#include"string.h"
#include"queue"
using namespace std;
#define N 200
int n,m;
int move[][2]={{0,1},{1,0},{0,-1},{-1,0}};
struct node
{
node(int _x=0,int _y=0)
{
x=_x,y=_y;
}
int x,y; //mark the Coord of the node
};
int data[N][N];//store data.
bool map[N][N]; //mark whether the node is visited.
int input()//input & initialize the array
{
memset(map,false,sizeof(map));
scanf("%d%d",&n,&m);
for(int i=0;i<n;i++)
for(int j=0;j<m;j++)
{
int t;
scanf("%d",&t);
data[i][j]=t;
map[i][j]=false;
}
return 0;
}
bool judge(node x)
{
if(x.x<n&&x.x>=0&&x.y<m&&x.y>=0) return true;
return false;
}
int shortest_path(node s,node e)
{
queue<int>dist;//means 'spath' in your code.
int dst=0;
queue<node>q;
q.push(s);
map[s.x][s.y]=true;
dist.push(0);
node v,c;
while(!q.empty())
{
v=q.front();
q.pop();
dst=dist.front();
dist.pop();
for(int i=0;i<4;i++)
{
c.x=v.x+move[i][0];
c.y=v.y+move[i][1];
if(judge(c)&&!map[c.x][c.y])
{
dist.push(dst+1);
q.push(c);
map[c.x][c.y]=true;
if(c.x==e.x&&c.y==e.y)
return dst+1;
}
}
}
return -1;//if the path not found return -1;
};
int main()
{
input();
node s(0,0),e(4,4);
printf("%d\n",shortest_path(s,e));
return 0;
}
the input should be:
n>=5 m>=5 because end point is(4,4).
and an n*m matrix.

Answer 3

About the warning :
Your code is not protected against bad input : if end is outside your n x m grid, or is on a "wall", or if there is no path from start to end your function will exit without executing a return statement.
The compiler has no way of predicting what input will be fed to the function.