scanf not passing value to variable - c

Question

I wanted to ask a little bit about scanf in C using Xcode IDE. If I not initially set value for variable choice, anytime I open my program and enter any choice(either 1/2) it will go to else case every time. So I check the value after select any choice then I got a strange number. Could you please take a look at my code. Thank you in advance.
Here's my actual code:
/* Bubble Sort using MPI */
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <time.h>
#define N 1000
double startT,stopT;
double startTime;
void showElapsed(int id, char *m)
{
printf("%d: %s %f secs\n",id,m,(clock()-startTime)/CLOCKS_PER_SEC);
}
void showVector(int *v, int n, int id)
{
int i;
printf("%d: ",id);
for(i=0;i<n;i++)
printf("%d ",v[i]);
putchar('\n');
}
int * merge(int *v1, int n1, int *v2, int n2)
{
int i,j,k;
int * result;
result = (int *)malloc((n1+n2)*sizeof(int));
/*
i : pointer of v1
j : pointer of v2
k : pointer of k
*/
i=0; j=0; k=0;
while(i<n1 && j<n2)
if(v1[i]<v2[j])
{
result[k] = v1[i];
i++; k++;
}
else
{
result[k] = v2[j];
j++; k++;
}
if(i==n1)
while(j<n2)
{
result[k] = v2[j];
j++; k++;
}
else
while(i<n1)
{
result[k] = v1[i];
i++; k++;
}
return result;
}
void swap(int *v, int i, int j)
{
int t;
t = v[i];
v[i] = v[j];
v[j] = t;
}
void sort(int *v, int n)
{
int i,j;
for(i=n-2;i>=0;i--)
for(j=0;j<=i;j++)
if(v[j]>v[j+1])
swap(v,j,j+1);
}
int main(int argc, char **argv)
{
int * data;
int * chunk;
int * other;
int m,n=N;
int id,p;
int s;
int i;
int step;
int choice = 0;
//start asking user to select option between sequential or parallel version of BubbleSort
printf(":: Welcome to BubbleSort Project for CSS333 ::\n");
printf("Please select option that you prefer\n");
printf("Type \"1\" for sequential mode or \"2\" for parallel mode\n");
printf("");
fflush(stdout);
scanf("Enter here: %d", &choice);
printf("Test value of choice(should be either 1 or 2): %d\n", choice);
//end asking
if(choice == 1){
// do seq
printf("You have selected option 1 which is running BubbleSort in Sequential mode\n");
printf("Please wait...");
}
else if(choice == 2){
// do parallel
printf("You have selected option 2 which is running BubbleSort in parallel mode\n");
printf("Please wait...");
MPI_Status status;
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&id);
MPI_Comm_size(MPI_COMM_WORLD,&p);
if(id==0)
{
int r;
srandom(clock());
s = n/p;
r = n%p;
data = (int *)malloc((n+p-r)*sizeof(int));
for(i=0;i<n;i++)
data[i] = random();
if(r!=0)
{
for(i=n;i<n+p-r;i++)
data[i]=0;
s=s+1;
}
startT = clock();
MPI_Bcast(&s,1,MPI_INT,0,MPI_COMM_WORLD);
chunk = (int *)malloc(s*sizeof(int));
MPI_Scatter(data,s,MPI_INT,chunk,s,MPI_INT,0,MPI_COMM_WORLD);
sort(chunk,s);
}
else
{
MPI_Bcast(&s,1,MPI_INT,0,MPI_COMM_WORLD);
chunk = (int *)malloc(s*sizeof(int));
MPI_Scatter(&data,s,MPI_INT,chunk,s,MPI_INT,0,MPI_COMM_WORLD);
sort(chunk,s);
}
step = 1;
while(step<p)
{
if(id%(2*step)==0)
{
if(id+step<p)
{
MPI_Recv(&m,1,MPI_INT,id+step,0,MPI_COMM_WORLD,&status);
other = (int *)malloc(m*sizeof(int));
MPI_Recv(other,m,MPI_INT,id+step,0,MPI_COMM_WORLD,&status);
chunk = merge(chunk,s,other,m);
s = s+m;
}
}
else
{
int near = id-step;
MPI_Send(&s,1,MPI_INT,near,0,MPI_COMM_WORLD);
MPI_Send(chunk,s,MPI_INT,near,0,MPI_COMM_WORLD);
break;
}
step = step*2;
}
if(id==0)
{
FILE * fout;
stopT = clock();
printf("%d; %d processors; %f secs\n",N,p,(stopT-startT)/CLOCKS_PER_SEC);
fout = fopen("result","w");
for(i=0;i<s;i++)
if (chunk[i] != 0)
fprintf(fout,"%d\n",chunk[i]);
fclose(fout);
}
MPI_Finalize();
}
else{
printf("Invalid value\n");
printf("Program exiting...\n");
exit(0);
}
}

Answer 1

This is your problem:
scanf("Enter here: %d", &choice);
You might be expecting this to display "Enter here: " then accept a number as input and store it in the variable choice. But that's not what it does.
What this does is that it goes through the formatting string ("Enter here: %d"), one character by one. For each character that is not '%', it reads a character from stdin and compares them together. If they don't match, it pushes the character back to the buffer of stdin and stops scanning.
So unless the user types in something starting with Enter here: followed immediately by a number, it fails at reading that number.
What you probably wanted to do is to:
printf("Enter here: ");
scanf("%d", &choice);
(and then read the documentation for scanf().