Compute alternating series: 1 − 2 + 3 − 4 + ... ± n Dry Run in C

Compute alternating series: 1 − 2 + 3 − 4 + ... ± n is an interactive C dry run visualizer from the For programs section. Study the source code, then use the execution controls to follow each step, variable update, highlighted line, and console output.

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Compute alternating series: 1 − 2 + 3 − 4 + ... ± n Program Code

#include <stdio.h>
int main() {
    int n, sum = 0, i;
    printf("Enter n: ");
    scanf("%d", &n);
    
    for(i = 1; i <= n; i++) {
        if(i % 2 == 0) {
            sum -= i;   // even: subtract
        } else {
            sum += i;   // odd: add
        }
    }
    
    printf("Alternating sum = %d\n", sum);
    
    // Formula: if n even: -n/2, if n odd: (n+1)/2
    if(n % 2 == 0)
        printf("Formula verification: -%d/2 = %d\n", n, -n/2);
    else
        printf("Formula verification: (%d+1)/2 = %d\n", n, (n+1)/2);
    
    return 0;
}

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