c语言中的for循环

Question

#include<stdio.h>
int n;
struct process
{
    char name[2];
    int at;
    int bt;
    int priority;
    int st;
    int wt;
    int tat;
    int completed;
};

int main()
{
    int i;
    struct process p[n];
    
    printf("Enter no of processes:");
    scanf("%d",&n);
    
    i=0;
    while(i<n)
    {
        printf("\nenter name of process:");
        scanf("%s",p[i].name);
        
        printf("enter arrival time:");
        scanf("%d",&p[i].at);
        
        printf("enter burst time:");
        scanf("%d",&p[i].bt);
        
        printf("enter priority:");
        scanf("%d",&p[i].priority);
        
        p[i].completed=0;
        
        i++;
    }
    
}

In this code i have used a for loop for a program it has to work n times but it works for n-1 only.For example when n is 3 it is **printing 2 times only .could any one can explain reason?

Answer 1

There is no for loop in your code.

But anyway there are two distinct problems in your code:

int main()
{
    int i;
    struct process p[n];   // here the content of n is zero
                           // so you allocate space for zero processes
    
    printf("Enter no of processes:");
    scanf("%d",&n);        // here you ask the user to enter a value for n
                           // but it's too late, the memory for p has already
                           // been allocated based on the value of n which was 0

    

You want this:

int main()
{
    int i;
    
    printf("Enter no of processes:");
    scanf("%d",&n);
    struct process p[n];
    ...

The second problem is here:

struct process
{
    char name[2];   // name can only contain strin gs with a maximal length 
                    // of one (one char + the null char terminator
    ...

You probably want this (assuming your process names can be longer than 1 char):

    char name[50]; room for 49 character + the null terminator

Answer 2

Don't do this:

int n;

int main() {
    struct process p[n];
    scanf("%d",&n);
}

When you do this, the value of n is uninitialized and defaults to 0. So struct process[n] declares an empty array of struct process .

Then when you do: scanf("%s",p[i].name) you are just writing to arbitrary memory on your stack, and will eventually hit a segfault.

Instead do this:

int main() {
    int n, idx;
    struct process *p;
    scanf("%d", &n);
    p = (struct process *)malloc(sizeof(struct process) * n);

    for (idx = 0; idx < n; idx++) {
       printf("\nenter name of process:");
       scanf("%1s", p[idx].name);
       ...
    }
}

Using scanf is problematic too, but if you do make sure the string is no longer than the buffer that contains it plus space for the terminating '\\0' character.