A simple C program with unexpected output

Question

I am trying to solve leetcode 506. (Relative Ranks problem). This is the c code not complete yet,and there's a puzzle that output is unexpected.

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int i=0,j,nums[]={5,4,3,2,1},tmp;
    char str[21];
    char **ret=(char **)malloc(sizeof(*ret)*5);
    for(i=0;i<5;i++)     //bubble sort
    {
        for(j=5-1;j>i;j--)
        {
            if(nums[i]>nums[j])
            {
                tmp=nums[i];
                nums[i]=nums[j];
                nums[j]=tmp;
            }
        }
    }
    for(i=0;i<5;i++)
    {
        if(i<3)
        {
            if(i==0)
            *(ret+i)="Gold Medal";
            else if(i==1)
            *(ret+i)="Silver Medal";
            else
            *(ret+i)="Bronze Medal";
        }
        else
        {

            sprintf(str,"%d",nums[i]);      
            *(ret+i)=str;   
            //printf("%s ",*(ret+i));
        }
    }
    for(i=0;i<5;i++)
    printf("%s ",*(ret+i));

}

I think the output should be:

Gold Medal
Silver Medal
Bronze Medal
4
5

but the actual output is:

Gold Medal
Silver Medal
Bronze Medal
5
5

ans:

else
        {
            char *str=malloc(sizeof(char)*10);
            sprintf(str,"%d",nums[i]);      
            *(ret+i)=str;   
            //printf("%s ",*(ret+i));
        }

score 3 · Accepted Answer · answered Sep 30 '18 at 02:00

3

You are assigning str twice in your first loop. ret[3] and ret[4] both point to the same memory address. When you output your array, in your second loop, you will therefore get 5 twice, because its value is overwritten in the previous loop.

The statement *(ret+i) = str; does not copy the value of the variable str, but simply points ret+1 to the address of str. The address of str does not change when using sprintf to assign values to its bytes.

answered Sep 30 '18 at 02:00

knittl

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thanks for your reply! I got it. Is there any alternative way to solve the problem? – Howard Sep 30 '18 at 02:20
Allocate a memory region to `sprintf` to, then point your array item to the new memory address. – knittl Sep 30 '18 at 02:26
I have tried to allocate **str to store data, but it Segmentation Fault. like this sprintf(str[i],"%d",nums[i]); *(ret+i)=str[i]; – Howard Sep 30 '18 at 02:39
No, use `malloc` inside the loop to allocate memory. Then `sprintf` to that memory. Have your array elment point to the newly allocated memory address/area. – knittl Sep 30 '18 at 02:43
oh! I got ! thank you very much: ) – Howard Sep 30 '18 at 02:45
@Howard and then of course do not forget to `free` your allocated memory, as soon as you do not need it anymore. – knittl Sep 30 '18 at 02:47
ok, I know thx :) – Howard Sep 30 '18 at 02:51

A simple C program with unexpected output

1 Answers1