Segmentation fault，求大神指导，看了两天了，就是不道错在哪

#include<stdio.h>
char str[10000];
int main()
{
int n,k,a,j;
float u,i,p,prem,mid;
char ch;
str[0]='q';
scanf("%d",&n);
getchar();
for(k=1;k<=n;k++)
{
u=0,i=0,p=0;j=1;
while((str[j]=getchar())!='\n')
{
j++;
if(str[j-1]=='=')
{
prem=1;
scanf("%f %c",&mid,&ch);
switch(ch)
{
case 'm': prem=0.001;break;
case 'k': prem=1000; break;
case 'M': prem=1000000;break;
}
switch(str[j-2])
{
case 'U': u=mid*prem;break;
case 'I': i=mid*prem;break;
case 'P': p=mid*prem;break;
}
}
}
if(p==0)
printf("Problem #%d\nP=%0.2fW\n\n",k,u*i);
else
if(i==0)
printf("Problem #%d\nI=%0.2fA\n\n",k,p/u);
else
if(u==0)
printf("Problem #%d\nU=%0.2fV\n\n",k,p/i);
}
return 0;
}