Arrays — Contiguous Memory

7. Practice Problems

1. Read N numbers into an array and print them in reverse.
   N-টি সংখ্যা input নিয়ে reverse order-এ প্রিন্ট করুন।
   ✨ Show Answer

   rev_arr.c — stdin: 5 / 10 20 30 40 50

   #include <stdio.h>
   int main(void) {
       int n; scanf("%d", &n);
       int a[100];
       for (int i = 0; i < n; i++) scanf("%d", &a[i]);
       for (int i = n - 1; i >= 0; i--) printf("%d ", a[i]);
       putchar('\n');
       return 0;
   }

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2. Find min, max and mean of an array.
   Array-এর min, max ও mean বের করুন।
   ✨ Show Answer

   stats.c

   #include <stdio.h>
   int main(void) {
       int a[] = {42, 7, 19, 85, 3, 21};
       int n = sizeof(a) / sizeof(a[0]);
       int mn = a[0], mx = a[0];
       long sum = 0;
       for (int i = 0; i < n; i++) {
           if (a[i] < mn) mn = a[i];
           if (a[i] > mx) mx = a[i];
           sum += a[i];
       }
       printf("min=%d max=%d mean=%.2f\n", mn, mx, (double)sum / n);
       return 0;
   }

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3. Count occurrences of a target value in an array.
   একটি মানের সংখ্যা Array-তে কতবার আছে গুনুন।
   ✨ Show Answer

   count.c

   #include <stdio.h>
   int main(void) {
       int a[] = {3, 5, 3, 7, 3, 2, 3, 9};
       int n = 8, target = 3, count = 0;
       for (int i = 0; i < n; i++) if (a[i] == target) count++;
       printf("%d appears %d times\n", target, count);
       return 0;
   }

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4. Rotate an array left by k positions.
   Array-কে বামে k অবস্থান rotate করুন।
   ✨ Show Answer

   rotate.c

   #include <stdio.h>
   void reverse(int *a, int l, int r) {
       while (l < r) { int t = a[l]; a[l++] = a[r]; a[r--] = t; }
   }
   int main(void) {
       int a[] = {1, 2, 3, 4, 5, 6, 7};
       int n = 7, k = 3;
       reverse(a, 0, k - 1);
       reverse(a, k, n - 1);
       reverse(a, 0, n - 1);
       for (int i = 0; i < n; i++) printf("%d ", a[i]);
       putchar('\n');
       return 0;
   }

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   Three-reversal কৌশল — O(n) time, O(1) extra space।

5. Check if an array is sorted (non-decreasing).
   Array sorted কি না যাচাই করুন।
   ✨ Show Answer

   is_sorted.c

   #include <stdio.h>
   int is_sorted(const int *a, int n) {
       for (int i = 1; i < n; i++) if (a[i] < a[i-1]) return 0;
       return 1;
   }
   int main(void) {
       int a[] = {1, 3, 5, 7, 9};
       printf("%s\n", is_sorted(a, 5) ? "sorted" : "not sorted");
       return 0;
   }

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6. Merge two sorted arrays into one sorted array.
   দুটি sorted array মিশিয়ে একটি sorted array বানান।
   ✨ Show Answer

   merge.c

   #include <stdio.h>
   int main(void) {
       int a[] = {1, 4, 7, 10};
       int b[] = {2, 3, 8, 12, 15};
       int na = 4, nb = 5;
       int out[20], i = 0, j = 0, k = 0;
       while (i < na && j < nb) out[k++] = a[i] <= b[j] ? a[i++] : b[j++];
       while (i < na) out[k++] = a[i++];
       while (j < nb) out[k++] = b[j++];
       for (int m = 0; m < k; m++) printf("%d ", out[m]);
       putchar('\n');
       return 0;
   }

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7. Remove duplicates from a sorted array in place.
   Sorted array থেকে in-place duplicate remove করুন।
   ✨ Show Answer

   dedup.c

   #include <stdio.h>
   int main(void) {
       int a[] = {1, 1, 2, 3, 3, 4, 5, 5};
       int n = 8, w = 1;
       for (int i = 1; i < n; i++) if (a[i] != a[w - 1]) a[w++] = a[i];
       for (int i = 0; i < w; i++) printf("%d ", a[i]);
       putchar('\n');
       return 0;
   }

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8. Transpose a 3×3 matrix.
   ৩×৩ matrix-এর transpose বের করুন।
   ✨ Show Answer

   transpose.c

   #include <stdio.h>
   int main(void) {
       int m[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
       for (int i = 0; i < 3; i++)
           for (int j = i + 1; j < 3; j++) {
               int t = m[i][j]; m[i][j] = m[j][i]; m[j][i] = t;
           }
       for (int i = 0; i < 3; i++) {
           for (int j = 0; j < 3; j++) printf("%3d ", m[i][j]);
           putchar('\n');
       }
       return 0;
   }

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9. Multiply two 2×2 matrices.
   দুটি ২×২ matrix গুণ করুন।
   ✨ Show Answer

   matmul.c

   #include <stdio.h>
   int main(void) {
       int A[2][2] = {{1,2},{3,4}};
       int B[2][2] = {{5,6},{7,8}};
       int C[2][2] = {{0}};
       for (int i = 0; i < 2; i++)
           for (int j = 0; j < 2; j++)
               for (int k = 0; k < 2; k++) C[i][j] += A[i][k] * B[k][j];
       for (int i = 0; i < 2; i++) {
           for (int j = 0; j < 2; j++) printf("%4d ", C[i][j]);
           putchar('\n');
       }
       return 0;
   }

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10. Find the second largest element without sorting.
    Sort না করেই দ্বিতীয় বৃহত্তম element বের করুন।
    ✨ Show Answer

    second.c

    #include <stdio.h>
    #include <limits.h>
    int main(void) {
        int a[] = {42, 7, 99, 3, 99, 21};
        int n = 6, first = INT_MIN, second = INT_MIN;
        for (int i = 0; i < n; i++) {
            if (a[i] > first) { second = first; first = a[i]; }
            else if (a[i] > second && a[i] < first) second = a[i];
        }
        printf("first=%d, second=%d\n", first, second);
        return 0;
    }

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11. Linear search — find the index of a target or return −1.
    Linear search — target-এর index পেলে return করুন, না পেলে −1।
    ✨ Show Answer

    linear.c

    #include <stdio.h>
    int find(const int *a, int n, int key) {
        for (int i = 0; i < n; i++) if (a[i] == key) return i;
        return -1;
    }
    int main(void) {
        int a[] = {3, 5, 7, 9, 11};
        printf("index of 7 = %d\n", find(a, 5, 7));
        printf("index of 4 = %d\n", find(a, 5, 4));
        return 0;
    }

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12. Iterative binary search on a sorted array.
    Sorted array-তে iterative binary search।
    ✨ Show Answer

    binsearch.c

    #include <stdio.h>
    int bs(const int *a, int n, int key) {
        int lo = 0, hi = n - 1;
        while (lo <= hi) {
            int m = lo + (hi - lo) / 2;
            if (a[m] == key) return m;
            else if (a[m] < key) lo = m + 1;
            else hi = m - 1;
        }
        return -1;
    }
    int main(void) {
        int a[] = {1, 3, 7, 12, 19, 25, 42};
        printf("%d\n", bs(a, 7, 19));
        return 0;
    }

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13. Spiral-print a 3×3 matrix.
    ৩×৩ matrix-কে spiral order-এ প্রিন্ট করুন।
    ✨ Show Answer

    spiral.c

    #include <stdio.h>
    int main(void) {
        int m[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
        int top = 0, bot = 2, lef = 0, rig = 2;
        while (top <= bot && lef <= rig) {
            for (int i = lef; i <= rig; i++) printf("%d ", m[top][i]); top++;
            for (int i = top; i <= bot; i++) printf("%d ", m[i][rig]); rig--;
            if (top <= bot) { for (int i = rig; i >= lef; i--) printf("%d ", m[bot][i]); bot--; }
            if (lef <= rig) { for (int i = bot; i >= top; i--) printf("%d ", m[i][lef]); lef++; }
        }
        putchar('\n');
        return 0;
    }

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14. Find the majority element (appears > n/2 times) — Boyer–Moore.
    Majority element (> n/2 বার) — Boyer–Moore algorithm।
    ✨ Show Answer

    majority.c

    #include <stdio.h>
    int main(void) {
        int a[] = {2, 2, 1, 2, 3, 2, 2};
        int n = 7, cand = 0, cnt = 0;
        for (int i = 0; i < n; i++) {
            if (cnt == 0) cand = a[i];
            cnt += (a[i] == cand) ? 1 : -1;
        }
        printf("majority = %d\n", cand);
        return 0;
    }

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15. Given two arrays, print their intersection (distinct common elements).
    দুটি array-এর intersection প্রিন্ট করুন (ইউনিক মিল)।
    ✨ Show Answer

    intersect.c

    #include <stdio.h>
    int main(void) {
        int a[] = {1, 2, 3, 4, 5};
        int b[] = {3, 4, 5, 6, 7};
        int na = 5, nb = 5;
        for (int i = 0; i < na; i++)
            for (int j = 0; j < nb; j++)
                if (a[i] == b[j]) { printf("%d ", a[i]); break; }
        putchar('\n');
        return 0;
    }

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16. Explain why sizeof(arr) inside a function is different from in main.
    Function-এর ভেতর ও main-এ sizeof(arr) আলাদা কেন?
    ✨ Show Answer

    Array যখন function-এ পাস হয়, তখন এটি একটি pointer-এ decay করে। তাই function-এর ভেতর sizeof(arr) pointer-এর size (সাধারণত 8 byte), পুরো array-এর নয়। main-এ array-র সম্পূর্ণ type জানা, তাই সেখানে সঠিক মোট size পাওয়া যায়। এজন্যই function-এ array পাঠানোর সময় length আলাদাভাবে পাঠাতে হয়।

17. Build a histogram — count how many times each value in 0..9 appears.
    Histogram বানান — 0..9-এর প্রতিটি মান array-তে কতবার আছে।
    ✨ Show Answer

    histogram.c

    #include <stdio.h>
    int main(void) {
        int data[] = {3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 0, 0};
        int n = sizeof(data) / sizeof(data[0]);
        int bins[10] = {0};
        for (int i = 0; i < n; i++) bins[data[i]]++;
        for (int i = 0; i < 10; i++) {
            printf("%d: ", i);
            for (int j = 0; j < bins[i]; j++) putchar('*');
            putchar('\n');
        }
        return 0;
    }

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18. Demonstrate a buffer overrun that -fsanitize=address can catch.
    একটি buffer overrun দেখান যা -fsanitize=address ধরতে পারে।
    ✨ Show Answer

    #include <stdio.h>
    int main(void) {
        int a[5];
        printf("%d\n", a[100]);   // reads out of bounds — UB
        return 0;
    }
    
    // Compile: gcc -fsanitize=address buggy.c
    // Run ./a.out — AddressSanitizer will print a detailed report.

    Online runner-এ AddressSanitizer সবসময় enabled থাকে না; নিজের মেশিনে চালিয়ে দেখুন।