C Handout

Every C program starts in main. #include <stdio.h> pulls in a header 头文件 so you can use printf to print. A function 函数 is a named block you can call; \n starts a new line. main returns 0 to mean "success".

#include <stdio.h>

void greet(void) {
    printf("Hello, world!\n");
}

int main(void) {
    greet();
    printf("I am learning C.\n");
    return 0;
}

C needs a type for every variable: int (integer 整数), double (floating-point 浮点数), char (one character). printf uses a format specifier 格式说明符 — %d, %f, %c — for each value. / between two ints is integer division; % is the remainder 余数.

#include <stdio.h>

int main(void) {
    int n = 17;
    double pi = 3.14;
    char grade = 'A';
    printf("%d %.2f %c\n", n, pi, grade);   // 17 3.14 A
    printf("%d %d\n", 7 / 2, 7 % 2);        // 3 1
    return 0;
}

The format string drives both printing and reading:

scanf("%d", &n) reads keyboard input with the same specifiers (note the &). Its sibling sscanf parses values out of a string, so it runs anywhere:

#include <stdio.h>

int main(void) {
    char line[] = "Mei 88";
    char name[20];
    int score;
    sscanf(line, "%19s %d", name, &score);   // parse text -> values
    printf("%s scored %d\n", name, score);   // Mei scored 88
    return 0;
}

A comment 注释 is a note for humans; the compiler ignores it. Use // for one line and /* ... */ for a block. Good indentation 缩进 and clear names make code easy to read.

#include <stdio.h>

int main(void) {
    // a single-line comment
    /* a block
       comment */
    int total = 3 + 4;       // clear names help
    printf("%d\n", total);   // 7
    return 0;
}

Common mistakes

• Every statement ends with ;, and main should return int (return 0;).
• printf needs a format string: %d for int, %f for double, and \n for a new line.
• A wrong format like %d for a double prints garbage — match the type.

An if runs a block when a condition 条件 is true. C has no real boolean 布尔值 type by default: 0 is false and any non-zero value is true. Chain choices with else if and else. Compare with ==, !=, <, >, <=, >=.

#include <stdio.h>

int main(void) {
    int score = 72;
    if (score >= 90) {
        printf("A\n");
    } else if (score >= 60) {
        printf("Pass\n");
    } else {
        printf("Fail\n");
    }
    return 0;
}

A switch picks one case by an integer value. Each case needs a break to jump out 跳出 — without it, C "falls through" into the next case. default runs when nothing matches.

#include <stdio.h>

int main(void) {
    int day = 3;
    switch (day) {
        case 1: printf("Mon\n"); break;
        case 2: printf("Tue\n"); break;
        case 3: printf("Wed\n"); break;
        default: printf("Other\n");
    }
    return 0;
}

Common mistakes

• if (x = 5) assigns and is always true; use == to compare.
• Each switch case needs a break;, or it falls through to the next.
• 0 is false and any non-zero value is true in a condition.

A loop 循环 repeats a block. A while loop runs as long as a condition is true. A for loop packs the start, the test, and the increment 自增 (i++) into one line — best when you know how many times to repeat.

#include <stdio.h>

int main(void) {
    int i = 1;
    while (i <= 3) {
        printf("%d ", i);
        i++;
    }
    printf("\n");                    // 1 2 3
    for (int j = 0; j < 5; j++) {
        printf("%d ", j);
    }
    printf("\n");                    // 0 1 2 3 4
    return 0;
}

A common pattern: start an accumulator 累加器 at 0, then add to it inside a loop. The same idea counts items or finds a running total.

#include <stdio.h>

int main(void) {
    int total = 0;
    for (int i = 1; i <= 5; i++) {
        total += i;            // 1 + 2 + 3 + 4 + 5
    }
    printf("%d\n", total);     // 15
    return 0;
}

A loop inside another loop is a nested 嵌套 loop. The inner loop finishes fully for each step of the outer loop — perfect for grids and patterns.

#include <stdio.h>

int main(void) {
    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 3; col++) {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}

Common mistakes

• for (i = 0; i < n; i++) runs n times; a semicolon right after for (...) makes an empty loop.
• A while whose condition never becomes false loops forever.
• Declare the counter (int i) before or inside the for header.

A function takes parameters 参数 (inputs) and gives back a return value 返回值. The return type comes first (int, double, …); void means it returns nothing.

#include <stdio.h>

int square(int x) {        // x is a parameter
    return x * x;          // hand back a value
}

int main(void) {
    int r = square(5);
    printf("%d\n", r);     // 25
    return 0;
}

C reads top to bottom, so a function must be known before it is called. A prototype 函数原型 — the header line plus ; — declares it early so you can keep main first. A variable's scope 作用域 is the block it lives in: it is local 局部 and disappears when the block ends.

#include <stdio.h>

int add(int a, int b);     // prototype: declared before use

int main(void) {
    printf("%d\n", add(3, 4));   // 7
    return 0;
}

int add(int a, int b) {    // definition comes later
    int sum = a + b;       // sum is local to add
    return sum;
}

Common mistakes

• A function used before it is defined needs a prototype above main.
• Arguments are passed BY VALUE — changes inside a function do not affect the caller unless you pass a pointer.
• A variable declared inside a function is local to it.

An array 数组 holds several values of one type. Index 索引 from 0. C does not store an array's length, so a common trick computes the element 元素 count: sizeof(a) / sizeof(a[0]).

#include <stdio.h>

int main(void) {
    int scores[3] = {88, 71, 95};
    printf("%d\n", scores[0]);     // 88
    scores[1] = 100;
    printf("%d\n", scores[1]);     // 100
    int n = sizeof(scores) / sizeof(scores[0]);
    printf("%d\n", n);             // 3
    return 0;
}

Traverse 遍历 an array with a for loop to find a max, a total, or a count. Always loop from 0 up to n - 1.

#include <stdio.h>

int main(void) {
    int a[] = {3, 9, 2, 7};
    int n = sizeof(a) / sizeof(a[0]);
    int max = a[0], total = 0;
    for (int i = 0; i < n; i++) {
        if (a[i] > max) max = a[i];
        total += a[i];
    }
    printf("%d %d\n", max, total);   // 9 21
    return 0;
}

A 2-D array is a grid of rows 行 and columns 列: grid[row][col]. Use two nested loops to visit every cell.

#include <stdio.h>

int main(void) {
    int grid[2][3] = {{1, 2, 3}, {4, 5, 6}};
    printf("%d\n", grid[1][2]);    // 6
    for (int r = 0; r < 2; r++) {
        for (int c = 0; c < 3; c++) {
            printf("%d ", grid[r][c]);
        }
    }
    printf("\n");                  // 1 2 3 4 5 6
    return 0;
}

Common mistakes

• C does NOT check array bounds: writing a[n] in a size-n array is undefined behaviour.
• Valid indexes run from 0 to n - 1.
• The array name is a pointer to its first element; sizeof only gives the size in the array's own scope.

A pointer 指针 stores the address 地址 of a variable. &x gives the address of x; *p dereferences 解引用 the pointer — it reads or writes the value stored there. Passing a pointer lets a function change the caller's variable (pass-by-pointer).

#include <stdio.h>

void addOne(int *p) {     // p holds an address
    *p = *p + 1;          // change the value at that address
}

int main(void) {
    int x = 10;
    int *ptr = &x;        // & takes the address of x
    printf("%d\n", *ptr); // * reads the value: 10
    addOne(&x);
    printf("%d\n", x);    // 11 — changed through the pointer
    return 0;
}

The classic use is a swap function. Passing values only copies them — the swap is lost. Passing pointers lets the function reach the caller's variables:

#include <stdio.h>

void swap_values(int a, int b) {      // copies: the caller sees nothing
    int t = a; a = b; b = t;
}

void swap_pointers(int *a, int *b) {  // addresses: the swap is real
    int t = *a; *a = *b; *b = t;
}

int main(void) {
    int x = 1, y = 2;
    swap_values(x, y);
    printf("%d %d\n", x, y);   // 1 2  (unchanged!)
    swap_pointers(&x, &y);
    printf("%d %d\n", x, y);   // 2 1  (swapped)
    return 0;
}

An array name acts as a pointer to its first element, and pointer arithmetic 指针运算 steps by whole elements:

#include <stdio.h>

int main(void) {
    int a[] = {10, 20, 30};
    int *p = a;                 // same as &a[0]
    printf("%d\n", *p);         // 10
    printf("%d\n", *(p + 2));   // 30 — same as a[2]
    return 0;
}

• NULL is the pointer that points at nothing — check for it before you dereference.

Common mistakes

• &x is the address of x; *p is the value stored at p.
• Never use *p on an uninitialised or NULL pointer — it crashes or corrupts memory.
• To change a caller's variable, pass its address and write through the pointer.

A C string 字符串 is an array of char. Every string ends with a hidden null terminator 空终止符 '\0', which marks where it stops. strlen (from <string.h>) counts characters up to that '\0'. Print a whole string with %s and one character 字符 with %c.

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

int main(void) {
    char name[] = "Mei";            // 'M', 'e', 'i', '\0'
    printf("%s\n", name);           // Mei
    printf("%zu\n", strlen(name));  // 3 (stops at '\0')
    printf("%c\n", name[0]);        // M
    return 0;
}

You can traverse a string by looping until you reach '\0'.

#include <stdio.h>

int main(void) {
    char word[] = "banana";
    int count = 0;
    for (int i = 0; word[i] != '\0'; i++) {
        if (word[i] == 'a') count++;
    }
    printf("%d\n", count);   // 3
    return 0;
}

#include <string.h> gives you the everyday string tools:

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

int main(void) {
    char full[40];
    strcpy(full, "Mei");           // full is now "Mei"
    strcat(full, " Chen");         // append -> "Mei Chen"
    printf("%s (%zu)\n", full, strlen(full));       // Mei Chen (8)
    printf("%d\n", strcmp("apple", "apple") == 0);  // 1 (equal)
    return 0;
}

• strcmp returns 0 for equal, so the test is strcmp(a, b) == 0 — never a == b.
• The destination array must have room for the result plus the terminator.

Common mistakes

• A C string needs one extra byte for the \0 terminator: a 5-letter word needs char[6].
• Copy and compare strings with strcpy / strcmp, not = / ==.
• Forgetting the \0 makes printf("%s", ...) run past the end of the text.

A struct 结构体 groups related values into one type. Each value is a member 成员. typedef gives the struct a short name so you can write Dog instead of struct Dog. Use . on a struct value, but -> on a pointer to a struct.

#include <stdio.h>

typedef struct {
    char name[20];
    int age;
} Dog;

int main(void) {
    Dog d = {"Rex", 3};
    printf("%s is %d\n", d.name, d.age);   // Rex is 3   (. on a value)

    Dog *p = &d;
    p->age = 4;                            // -> on a pointer
    printf("%s is %d\n", d.name, d.age);   // Rex is 4
    return 0;
}

Common mistakes

• Use . on a struct value and -> on a struct pointer.
• typedef lets you drop the word struct when declaring; without it you write struct Point p;.
• Assigning one struct to another copies all its fields.

malloc allocates 分配 memory on the heap 堆 at run time and returns a pointer to it. realloc resizes that block; free gives it back. Every malloc needs a matching free, or you get a memory leak 内存泄漏. These live in <stdlib.h>.

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

int main(void) {
    int n = 3;
    int *a = malloc(n * sizeof(int));    // room for 3 ints
    for (int i = 0; i < n; i++) a[i] = i * 10;

    a = realloc(a, 4 * sizeof(int));     // grow to 4 ints
    a[3] = 30;

    for (int i = 0; i < 4; i++) printf("%d ", a[i]);
    printf("\n");                        // 0 10 20 30

    free(a);                             // hand the memory back
    return 0;
}

• calloc(n, size) allocates an array like malloc AND fills it with zeros.

Common mistakes

• Every malloc needs a matching free; forgetting to free leaks memory.
• Using memory after free (a "use-after-free") is a serious bug.
• Check that malloc did not return NULL before you use the memory.

A linked list 链表 is a chain of nodes 节点. Each node holds a value and a pointer to the next node; the last one points to NULL. Unlike an array it grows one node at a time with malloc. Always free every node when done.

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

typedef struct Node {
    int value;
    struct Node *next;
} Node;

int main(void) {
    Node *head = NULL;
    for (int v = 10; v <= 30; v += 10) {   // prepend 10, 20, 30
        Node *n = malloc(sizeof(Node));
        n->value = v;
        n->next = head;
        head = n;
    }
    for (Node *p = head; p != NULL; p = p->next) {
        printf("%d ", p->value);           // 30 20 10
    }
    printf("\n");

    while (head != NULL) {                 // free the whole list
        Node *t = head;
        head = head->next;
        free(t);
    }
    return 0;
}

A stack 栈 is LIFO 后进先出 (last in, first out): push and pop at the same end. A queue 队列 is FIFO 先进先出 (first in, first out): add at the back, remove from the front. Both are easy to build on an array with index variables.

#include <stdio.h>

int main(void) {
    int stack[10];
    int top = 0;                       // next free slot
    stack[top++] = 1;                  // push
    stack[top++] = 2;
    stack[top++] = 3;
    while (top > 0) {
        printf("%d ", stack[--top]);   // pop: 3 2 1
    }
    printf("\n");
    return 0;
}

#include <stdio.h>

int main(void) {
    int queue[10];
    int front = 0, back = 0;
    queue[back++] = 1;                       // enqueue
    queue[back++] = 2;
    queue[back++] = 3;
    while (front < back) {
        printf("%d ", queue[front++]);       // dequeue: 1 2 3
    }
    printf("\n");
    return 0;
}

Common mistakes

• In a linked list, never lose the head pointer, or the whole list is unreachable.
• Free every node when you are done, walking the list before you unlink.
• Check for an empty list (a NULL head) before you pop.

Linear search 线性查找 checks each element in turn — it works on any array. Binary search 二分查找 is far faster but needs a sorted 已排序 array: it checks the middle and discards half each step. Both return the index, or -1 if missing.

#include <stdio.h>

int linear(int a[], int n, int target) {
    for (int i = 0; i < n; i++)
        if (a[i] == target) return i;
    return -1;
}

int binary(int a[], int n, int target) {
    int lo = 0, hi = n - 1;
    while (lo <= hi) {
        int mid = (lo + hi) / 2;
        if (a[mid] == target) return mid;
        else if (a[mid] < target) lo = mid + 1;
        else hi = mid - 1;
    }
    return -1;
}

int main(void) {
    int a[] = {2, 5, 8, 12, 16, 23};
    int n = sizeof(a) / sizeof(a[0]);
    printf("%d %d %d\n", linear(a, n, 12), binary(a, n, 12), binary(a, n, 9));
    return 0;   // 3 3 -1
}

Bubble sort 冒泡排序 repeatedly compares neighbours and swaps 交换 any pair that is out of order. After each pass the largest value "bubbles" to the end.

#include <stdio.h>

int main(void) {
    int a[] = {5, 2, 9, 1, 7};
    int n = sizeof(a) / sizeof(a[0]);
    for (int i = 0; i < n - 1; i++) {
        for (int j = 0; j < n - 1 - i; j++) {
            if (a[j] > a[j + 1]) {
                int t = a[j]; a[j] = a[j + 1]; a[j + 1] = t;
            }
        }
    }
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    printf("\n");   // 1 2 5 7 9
    return 0;
}

Recursion 递归 is a function that calls itself. It needs a base case 基准情形 to stop, and a recursive call that steps toward it. Without a base case it never ends.

#include <stdio.h>

int factorial(int n) {
    if (n <= 1) return 1;          // base case
    return n * factorial(n - 1);   // recursive call
}

int main(void) {
    printf("%d\n", factorial(5));  // 120
    return 0;
}

Common mistakes

• Binary search needs a sorted array; recursion needs a base case.
• Each recursive call must move closer to the base case, or the stack overflows.
• Bubble and insertion sort are O(n²) — fine to learn, slow at scale.

fopen returns a file pointer 文件指针; the mode 模式 string says what to do — "w" write, "r" read, "a" append. Write with fprintf, read with fscanf, and always fclose when done.

#include <stdio.h>

int main(void) {
    FILE *out = fopen("scores.txt", "w");   // open for writing
    fprintf(out, "Alice 80\nBob 95\n");
    fclose(out);

    FILE *in = fopen("scores.txt", "r");    // open for reading
    char name[20];
    int score;
    while (fscanf(in, "%s %d", name, &score) == 2) {
        printf("%s -> %d\n", name, score);
    }
    fclose(in);
    return 0;
}

C has no exceptions. Instead a function signals failure with a return code 返回码 — by convention 0 means success and non-zero means an error. The caller checks the code before trusting the result. (fopen follows the same idea: it returns NULL when it fails.)

#include <stdio.h>

// returns 0 on success, -1 if the divisor is 0
int safe_divide(int a, int b, int *result) {
    if (b == 0) return -1;          // error code
    *result = a / b;
    return 0;                       // success
}

int main(void) {
    int r;
    if (safe_divide(10, 2, &r) == 0)
        printf("10 / 2 = %d\n", r);          // 10 / 2 = 5
    if (safe_divide(10, 0, &r) != 0)
        printf("cannot divide by zero\n");   // error reported
    return 0;
}

Common mistakes

• Check that fopen did not return NULL before reading or writing.
• Always fclose a file when you are done.
• Return a non-zero code from main to signal an error to the caller.

A bit 位 is a single 0 or 1; numbers are stored in binary 二进制. Bitwise 按位 operators work on the bits directly: & AND, | OR, ^ XOR, << shift left (×2 each step), >> shift right (÷2).

#include <stdio.h>

int main(void) {
    int a = 12;              // 1100
    int b = 10;              // 1010
    printf("%d\n", a & b);   // 8   AND  -> 1000
    printf("%d\n", a | b);   // 14  OR   -> 1110
    printf("%d\n", a ^ b);   // 6   XOR  -> 0110
    printf("%d\n", a << 1);  // 24  left shift
    printf("%d\n", a >> 1);  // 6   right shift
    return 0;
}

Hexadecimal 十六进制 (base 16) writes binary compactly: one hex digit is exactly four bits. Write hex literals with 0x; print with %x:

#include <stdio.h>

int main(void) {
    printf("%d\n", 0xFF);   // 255  (0x marks a hex literal)
    printf("%x\n", 255);    // ff
    printf("%x\n", 12);     // c
    return 0;
}

Run-length encoding (RLE) is a simple compression 压缩 method: replace each run 游程 of repeated characters with a count and the character. It is lossless 无损 — the original is fully recoverable.

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

int main(void) {
    char *s = "aaabbc";
    int n = strlen(s);
    for (int i = 0; i < n; ) {
        char c = s[i];
        int run = 0;
        while (i < n && s[i] == c) { run++; i++; }
        printf("%d%c", run, c);     // 3a2b1c
    }
    printf("\n");
    return 0;
}

Common mistakes

• & is bitwise AND and && is logical AND — do not confuse them.
• A left shift << 1 doubles a value; a right shift >> 1 halves it.
• Bit n has value 1 << n (C has no ** power operator); check a bit with (x >> n) & 1.

The preprocessor 预处理器 runs before compiling. #define makes a macro 宏 — plain text replaced everywhere. const makes a constant 常量 that cannot change. static inside a function keeps a variable's value between calls. An enum (enumeration 枚举) names a set of integers starting at 0.

#include <stdio.h>

#define MAX 100                     // macro: text replaced before compiling

enum Color { RED, GREEN, BLUE };    // named integers 0, 1, 2

int next_id(void) {
    static int count = 0;           // keeps its value between calls
    count++;
    return count;
}

int main(void) {
    const double PI = 3.14;         // cannot be changed
    printf("%d %.2f\n", MAX, PI);            // 100 3.14
    printf("%d %d %d\n", RED, GREEN, BLUE);  // 0 1 2
    int a = next_id();
    int b = next_id();
    printf("%d %d\n", a, b);                 // 1 2
    return 0;
}

Bigger programs split across files: declarations go in a header 头文件 file (mine.h), included with #include "mine.h" — quotes mean your own file, < > means the standard library. An include guard (#ifndef MINE_H / #define MINE_H / #endif) stops a header being included twice.

Common mistakes

• #define does plain text replacement with no type checking; wrap macro bodies and arguments in parentheses.
• A const value cannot be changed after it is set.
• A static local variable keeps its value between calls to the function.