Question

c语言程序设计基础题目求助

程序填空，不要改变与输入输出有关的语句。
输入一个正整数repeat (0<repeat<10)，做repeat次下列运算：
读入一个正整数 n，计算1－1/4＋1/7－1/10＋……的前 n 项之和，输出时保留3位小数。
输入输出示例：括号内是说明
输入
2 (repeat=2)
3
10
输出
sum = 0.893
sum = 0.819

#include <stdio.h>
int main(void)
{
int flag, i, n, t;
int repeat, ri;
double item, sum;

scanf("%d", &repeat);
for(ri = 1; ri <= repeat; ri++){
scanf("%d", &n);
/*---------*/
printf("sum = %.3f\n", sum);
}
}
我的程序;

#include <stdio.h>
int main(void)
{
int flag, i, n, t;
int repeat, ri;
double item, sum;

scanf("%d", &repeat);
for(ri = 1; ri <= repeat; ri++){
flag=1;
scanf("%d", &n);
item=1.0;
sum=0;
t=1;
for(i=1;i<=n;i++){
flag=-flag;
t=t+3;
item=flag*1.0/t;
sum=sum+item;}
printf("sum = %.3f\n", sum);
}
}
为什么总是错误的啊

Answer 1

scanf("%d", &repeat);
for(ri = 1; ri <= repeat; ri++){
flag=1;
scanf("%d", &n);
item=1.0;
sum=0;
t=1;
for(i=0;i<n;i++){
t=3*i+1;
item=flag*1.0/t;
sum=sum+item;
flag=-flag;}

Answer 2

#include <stdio.h>
int main(void)
{
int flag, i, n, t;
int repeat, ri;
double item, sum;

scanf("%d", &repeat);
for(ri = 1; ri <= repeat; ri++){

scanf("%d", &n);
flag=1;
item=1.0;
sum=0;
t=1;
for(i=1;i<=n;i++){
flag=-flag;
t=t+3;
item=flag*1.0/t;
sum=sum+item;}
printf("sum = %.3f\n", sum);
}
}
d