Control Flow, Arrays, and Strings in C

🎯 Learning Objectives

• Use if/else, switch, loops (for, while, do-while)
• Declare and access 1D, 2D, and multidimensional arrays
• Manipulate strings using standard library functions
• Understand the relationship between arrays and pointers

1. Control Flow

1.1 Conditional Statements

c
// if-else ladder
int score = 85;
if (score >= 90) {
    printf("A grade\n");
} else if (score >= 80) {
    printf("B grade\n");  // This executes
} else {
    printf("C grade\n");
}
// switch statement
switch (score / 10) {
    case 10:
    case 9: printf("A\n"); break;
    case 8: printf("B\n"); break;
    default: printf("Below B\n");
}

1.2 Loops

c
// for loop
for (int i = 0; i < 10; i++) {
    printf("%d ", i);
}
// while loop
int i = 0;
while (i < 10) {
    printf("%d ", i++);
}
// do-while (executes at least once)
int j = 0;
do {
    printf("%d ", j++);
} while (j < 10);

1.3 break, continue, goto

c
for (int i = 0; i < 10; i++) {
    if (i == 3) continue;    // Skip iteration (i=3 not printed)
    if (i == 7) break;       // Exit loop (prints 0,1,2,4,5,6)
    printf("%d ", i);
}

2. Arrays

2.1 1D Arrays

c
int arr[5] = {10, 20, 30, 40, 50};
// Access and modify
arr[0] = 100;          // First element
arr[4] = 500;          // Last element
int size = sizeof(arr) / sizeof(arr[0]);  // 5
// Iterate
for (int i = 0; i < size; i++) {
    printf("%d ", arr[i]);
}

2.2 2D Arrays

c
int matrix[3][4] = {
    {1, 2, 3, 4},
    {5, 6, 7, 8},
    {9, 10, 11, 12}
};
// Access: matrix[row][col]
printf("%d\n", matrix[1][2]);  // 7
// Iterate (row-major order)
for (int r = 0; r < 3; r++) {
    for (int c = 0; c < 4; c++) {
        printf("%3d ", matrix[r][c]);
    }
    printf("\n");
}

3. Strings

3.1 String Representation

c
char str1[] = "hello";     // {'h','e','l','l','o','\0'} — 6 bytes
char str2[6] = "hello";    // Same as above
char str3[] = {'h','e','l','l','o','\0'};  // Explicit
printf("Length: %zu\n", strlen(str1));  // 5 (not counting null)
printf("Size: %zu\n", sizeof(str1));    // 6 (including null)

3.2 String Functions

c
#include <string.h>
char src[] = "Hello ";
char dest[50];
strcpy(dest, src);          // Copy src to dest
strcat(dest, "World!");     // Concatenate: "Hello World!"
int cmp = strcmp("abc", "abd");  // Negative (-1): "abc" < "abd"
int len = strlen(dest);     // 12
// Safe versions (with buffer size)
strncpy(dest, src, 49);
strncat(dest, "World!", 49);
strncmp("abc", "abd", 2);   // Compares only first 2 chars (equal → 0)

3.3 String Input

c
char buffer[100];
// UNSAFE: no bounds check
gets(buffer);
// SAFE: reads at most 99 chars
fgets(buffer, sizeof(buffer), stdin);
// Parse input
int num;
char word[50];
sscanf("42 hello", "%d %s", &num, word);  // num=42, word="hello"

4. 📝 Practice Questions

Q1: Write a function that reverses a string in place.

c
void reverse(char *s) {
    int len = strlen(s);
    for (int i = 0; i < len / 2; i++) {
        char temp = s[i];
        s[i] = s[len - 1 - i];
        s[len - 1 - i] = temp;
    }
}

Q2: Why is it dangerous to use gets()?

Answer: gets() has NO bounds checking. If the input exceeds the buffer size, it causes a buffer overflow, overwriting adjacent memory. This is a common security vulnerability. Always use fgets() instead. Q3: What is the output of printf("%d", sizeof("abc\0def"))?

Answer: 8. The string "abc\0def" has characters a,b,c,\0,d,e,f = 7 characters plus the terminating null = 8. The embedded \0 is just another byte in the array. Q4: Given int arr[5] = {10,20,30,40,50};, what is arr[5]?

Answer: Undefined behavior. arr[5] is one past the end of the array. It accesses whatever memory follows the array (could be another variable, could cause a crash). Q5: Write a program that counts the frequency of each character in a string.

c
void count_chars(const char *str) {
    int freq[256] = {0};
    for (int i = 0; str[i]; i++) {
        freq[(unsigned char)str[i]]++;
    }
    for (int i = 0; i < 256; i++) {
        if (freq[i] > 0) {
            printf("'%c': %d\n", i, freq[i]);
        }
    }
}

5. 🔗 Cross-References

• Week 3 - Pointers: Array-pointer equivalence
• Week 4 - Functions: Passing arrays to functions
• BSCS3005 Week 7 - File I/O: Reading/writing strings to files Join Discord PreviousMemory LayoutNextPointers Deep Dive