C Arrays, Strings and Structures Practice

Practise arrays, 2D arrays, C strings and the string.h functions, structs, files and bit manipulation in C.

Intermediate10 problemsUpdated October 24, 2026

Review first: Arrays, Multidimensional Arrays, Strings, String Functions, Structures, File Handling, Bitwise Operators

1.Reverse an array and find its maximum

Beginner

Read 5 integers into an array, print the largest one and its position (index), then print the array in reverse order.

Show hint

Track the index of the largest element, not just its value. Print in reverse with for (int i = 4; i >= 0; i--).

Show solution

#include <stdio.h>

int main(void) {
    int a[5];
    for (int i = 0; i < 5; i++) {
        if (scanf("%d", &a[i]) != 1) return 1;
    }
    int best = 0;
    for (int i = 1; i < 5; i++) {
        if (a[i] > a[best]) best = i;
    }
    printf("max %d at index %d\n", a[best], best);
    for (int i = 4; i >= 0; i--) printf("%d ", a[i]);
    printf("\n");
    return 0;
}

Output

12
45
7
45
30
max 45 at index 1
30 45 7 45 12

2.Row and column sums of a matrix

Intermediate

For int m[3][3] = {{4, 9, 2}, {3, 5, 7}, {8, 1, 6}}, print each row's sum, each column's sum and the main diagonal's sum. (This is a magic square, so they should all be equal.)

Show hint

Use nested loops. For column sums, put the row loop inside the column loop.

Show solution

#include <stdio.h>

int main(void) {
    int m[3][3] = {{4, 9, 2}, {3, 5, 7}, {8, 1, 6}};
    printf("rows:");
    for (int r = 0; r < 3; r++) {
        int s = 0;
        for (int c = 0; c < 3; c++) s += m[r][c];
        printf(" %d", s);
    }
    printf("\ncols:");
    for (int c = 0; c < 3; c++) {
        int s = 0;
        for (int r = 0; r < 3; r++) s += m[r][c];
        printf(" %d", s);
    }
    int d = 0;
    for (int i = 0; i < 3; i++) d += m[i][i];
    printf("\ndiagonal: %d\n", d);
    return 0;
}

Output

rows: 15 15 15
cols: 15 15 15
diagonal: 15

3.Count vowels in a line

Beginner

Read a whole line with fgets and print the number of vowels (a, e, i, o, u, either case) and the number of other letters.

Show hint

Use tolower and strchr("aeiou", c) to test for a vowel, and isalpha for letters. Cast characters to unsigned char before passing them to <ctype.h> functions.

Show solution

#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main(void) {
    char line[200];
    if (fgets(line, sizeof line, stdin) == NULL) return 1;
    int vowels = 0, consonants = 0;
    for (int i = 0; line[i] != '\0'; i++) {
        unsigned char c = (unsigned char)line[i];
        if (!isalpha(c)) continue;
        if (strchr("aeiou", tolower(c))) vowels++;
        else consonants++;
    }
    printf("vowels %d, other letters %d\n", vowels, consonants);
    return 0;
}

Output

Learning C Is Fun
vowels 5, other letters 9

4.Capitalise every word

Intermediate

Read a line and print it with the first letter of every word in uppercase and the rest in lowercase.

Show hint

Keep a flag that says whether the previous character was a space. Remove the newline fgets keeps with line[strcspn(line, "\n")] = '\0';.

Show solution

#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main(void) {
    char line[200];
    if (fgets(line, sizeof line, stdin) == NULL) return 1;
    line[strcspn(line, "\n")] = '\0';
    int start = 1;
    for (int i = 0; line[i] != '\0'; i++) {
        unsigned char c = (unsigned char)line[i];
        if (isspace(c)) {
            start = 1;
        } else {
            line[i] = (char)(start ? toupper(c) : tolower(c));
            start = 0;
        }
    }
    printf("%s\n", line);
    return 0;
}

Output

hELLO wORLD from c
Hello World From C

5.Parse a CSV line

Intermediate

Split char data[] = "Pune,Chennai,Delhi,Kochi"; at the commas with strtok, print each city with its length, and print the longest city.

Show hint

Call strtok(data, ",") once, then strtok(NULL, ",") until it returns NULL. Store a pointer to the longest token.

Show solution

#include <stdio.h>
#include <string.h>

int main(void) {
    char data[] = "Pune,Chennai,Delhi,Kochi";
    const char *longest = NULL;
    for (char *t = strtok(data, ","); t != NULL; t = strtok(NULL, ",")) {
        printf("%s (%zu)\n", t, strlen(t));
        if (longest == NULL || strlen(t) > strlen(longest)) longest = t;
    }
    printf("longest: %s\n", longest);
    return 0;
}

Output

Pune (4)
Chennai (7)
Delhi (5)
Kochi (5)
longest: Chennai

6.Array of student structs

Intermediate

Define typedef struct { char name[20]; int marks[3]; } Student;. For three students, compute each average, print it, and print the name of the student with the highest average.

Show hint

Write double average(const Student *s) and call it with &list[i]. Inside, use s->marks[j].

Show solution

#include <stdio.h>

typedef struct {
    char name[20];
    int marks[3];
} Student;

double average(const Student *s) {
    return (s->marks[0] + s->marks[1] + s->marks[2]) / 3.0;
}

int main(void) {
    Student list[] = {
        {"Asha", {78, 85, 90}},
        {"Ravi", {88, 92, 79}},
        {"Meena", {95, 70, 81}},
    };
    int best = 0;
    for (int i = 0; i < 3; i++) {
        printf("%-6s %.2f\n", list[i].name, average(&list[i]));
        if (average(&list[i]) > average(&list[best])) best = i;
    }
    printf("top: %s\n", list[best].name);
    return 0;
}

Output

Asha   84.33
Ravi   86.33
Meena  82.00
top: Ravi

7.Sort structs by a field

Advanced

Store four products (name and price) in an array of structs and sort them by price, lowest first, using qsort. Print them in order.

Show hint

The comparison function receives pointers to two structs: convert with const Product *p = a; and compare p->price.

Show solution

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    const char *name;
    double price;
} Product;

int by_price(const void *a, const void *b) {
    const Product *x = a, *y = b;
    return (x->price > y->price) - (x->price < y->price);
}

int main(void) {
    Product items[] = {
        {"Keyboard", 1299.0}, {"Mouse", 499.0}, {"Monitor", 8999.0}, {"Cable", 199.0},
    };
    qsort(items, 4, sizeof items[0], by_price);
    for (int i = 0; i < 4; i++) printf("%-9s %8.2f\n", items[i].name, items[i].price);
    return 0;
}

Output

Cable       199.00
Mouse       499.00
Keyboard   1299.00
Monitor    8999.00

8.Write and read back a file

Intermediate

Write three lines of text to notes.txt, then open it again for reading and print the number of lines and the number of characters (excluding newlines).

Show hint

Check every fopen for NULL. Read with fgetc until EOF, counting '\n' separately.

Show solution

#include <stdio.h>

int main(void) {
    FILE *f = fopen("notes.txt", "w");
    if (f == NULL) return 1;
    fputs("pointers\n", f);
    fputs("structs\n", f);
    fputs("files\n", f);
    fclose(f);

    f = fopen("notes.txt", "r");
    if (f == NULL) return 1;
    int c, lines = 0, chars = 0;
    while ((c = fgetc(f)) != EOF) {
        if (c == '\n') lines++;
        else chars++;
    }
    fclose(f);
    printf("%d lines, %d characters\n", lines, chars);
    return 0;
}

Output

3 lines, 20 characters

9.Permission flags with bits

Intermediate

Using bit flags READ = 1, WRITE = 2 and EXEC = 4, start with no permissions, grant read and write, print the value, then revoke write and grant exec, and print the value and whether write is set.

Show hint

Grant with |=, revoke with &= ~FLAG and test with &.

Show solution

#include <stdio.h>

enum { READ = 1u << 0, WRITE = 1u << 1, EXEC = 1u << 2 };

int main(void) {
    unsigned perms = 0;
    perms |= READ | WRITE;
    printf("perms = %u\n", perms);
    perms &= ~WRITE;
    perms |= EXEC;
    printf("perms = %u, write %s\n", perms, (perms & WRITE) ? "on" : "off");
    return 0;
}

Output

perms = 3
perms = 5, write off

10.Count the 1 bits

Advanced

Write int popcount(unsigned x) that counts the bits set to 1, and print the counts for 0, 7, 255 and 1024.

Show hint

x & (x - 1) clears the lowest set bit. Count how many times you can do that before x is 0.

Show solution

#include <stdio.h>

int popcount(unsigned x) {
    int count = 0;
    while (x != 0) {
        x &= x - 1;
        count++;
    }
    return count;
}

int main(void) {
    unsigned tests[] = {0, 7, 255, 1024};
    for (int i = 0; i < 4; i++) printf("%u -> %d\n", tests[i], popcount(tests[i]));
    return 0;
}

Output

0 -> 0
7 -> 3
255 -> 8
1024 -> 1

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