Remove duplicate array elements

LAB A07 · 01

Understand the task.

Aim: Remove duplicate array elements.

Review the related theory in Unit 3.

LAB A07 · 02

Know what goes in and what comes out.

First n from 1 to 20, then n integers from -100 to 100. The output preserves first-occurrence order.

8
4 2 4 7 2 9 7 4

Unique values: 4 2 7 9

Source programs read the documented input directly, without prompt text, so the same sample works in the embedded compiler’s STDIN field.

LAB A07 · 03

Plan the steps before writing code.

1. Read the array; set used, the retained-element count, to zero.
2. Visit each original element at index i.
3. Search only the retained prefix a[0] through a[used - 1].
4. If the value is absent, store it at a[used] and increment used.
5. Print the retained prefix.

LAB A07 · 04

One complete program. Ready to run.

#include <stdio.h>
#define MAX 20
int main(void)
{
    int a[MAX], n, i, j, used = 0, found;
    if (scanf("%d", &n) != 1 || n < 1 || n > MAX) {
        puts("Enter a count from 1 to 20.");
        return 1;
    }
    for (i = 0; i < n; i++) {
        if (scanf("%d", &a[i]) != 1 || a[i] < -100 || a[i] > 100) {
            puts("Enter each value from -100 to 100.");
            return 1;
        }
    }
    for (i = 0; i < n; i++) {
        found = 0;
        for (j = 0; j < used; j++) {
            if (a[j] == a[i]) { found = 1; break; }
        }
        if (!found) a[used++] = a[i];
    }
    printf("Unique values:");
    for (i = 0; i < used; i++) printf(" %d", a[i]);
    putchar('\n');
    return 0;
}

gcc A07-remove-duplicates.c -o program

LAB A07 · 05

Connect each statement to its purpose.

• The retained prefix contains unique values in their original first-occurrence order.
• used never exceeds i + 1. A write to a[used] is at or behind the element being processed, so it does not overwrite future unread elements.
• No sorting is performed. Sorting-based removal would change the ordering unless extra work restored it.
• The number of retained values can be smaller than n; the original array capacity remains unchanged.

LAB A07 · 06

Follow the sample calculation.

LAB A07 · 07

Check more than one input.

Time and storage

O(n²) worst-case time from repeated prefix searches; O(1) extra working space because the array is compacted in place.

Start with the sample, then test a boundary and a case that changes the result. A correct sample alone does not establish that every input is handled correctly.

LAB A07 · 08

Use the real C compiler.

Choose an example and load it into the embedded C compiler. Select I/O inside the compiler, check the sample STDIN, and press Run. Change the input or edit the source to explore a different case.

8
4 2 4 7 2 9 7 4

Unique values: 4 2 7 9

#include <stdio.h>
#define MAX 20
int main(void)
{
    int a[MAX], n, i, j, used = 0, found;
    if (scanf("%d", &n) != 1 || n < 1 || n > MAX) {
        puts("Enter a count from 1 to 20.");
        return 1;
    }
    for (i = 0; i < n; i++) {
        if (scanf("%d", &a[i]) != 1 || a[i] < -100 || a[i] > 100) {
            puts("Enter each value from -100 to 100.");
            return 1;
        }
    }
    for (i = 0; i < n; i++) {
        found = 0;
        for (j = 0; j < used; j++) {
            if (a[j] == a[i]) { found = 1; break; }
        }
        if (!found) a[used++] = a[i];
    }
    printf("Unique values:");
    for (i = 0; i < used; i++) printf(" %d", a[i]);
    putchar('\n');
    return 0;
}

LAB A07 · 09

Explain why the program works.

LAB A07 · 10

Build the next skill.

Read the viva answers, try the extra cases, and return to the lab index to mark this exercise as practised.