ADVANCED DATA STRUCTURES PROGRAM • LEVEL 21 — PLACEMENT-ORIENTED STRUCTURES
Answer Range Sums with a Fenwick Tree
Learn how to answer range sums with a fenwick tree using a clear C program.
PROBLEM UNDERSTANDING
Input and expected output
Sample input
No input required
Sample output
Sum[1,4] = 14
COMPLETE C PROGRAM
Complete C implementation
#include <stdio.h>
void update(int tree[],int count,int index,int delta){for(;index<=count;index+=index&-index)tree[index]+=delta;}int query(const int tree[],int index){int sum=0;for(;index>0;index-=index&-index)sum+=tree[index];return sum;}
int main(void)
{
int values[]={1,2,3,4,5,6},tree[7]={0};for(int i=0;i<6;i++)update(tree,6,i+1,values[i]);printf("Sum[1,4] = %d\n",query(tree,5)-query(tree,1));return 0;
}CURRENT STEP
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SELECTED LINE
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EXPECTED OUTPUT FOR THE SAMPLE
Sum[1,4] = 14
Step 0 of 0
PROGRAM EXPLANATION
Algorithm and explanation
- Read the required input values.
- Store partial sums indexed by the least significant set bit and subtract two prefixes.
- Display the computed result.
Store partial sums indexed by the least significant set bit and subtract two prefixes.
EFFICIENCY
Time and space complexity
Time complexity
O(log n) update/query
Auxiliary space
O(n)
DEBUGGING CHECKLIST
Common mistakes
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Use the correct format specifier for every variable.
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Initialize variables before using their values.
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Check braces, semicolons and input order carefully.
Try it yourself
Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.