printf("how long do you want the string?"); scanf("%d", &i);
buffer = (char*) malloc(i+1); if (buffer==NULL) exit(1);
for (n=0; n < i; n++) buffer[n] = rand()%26 + 'a'; buffer[i] = '\0';
printf("Random string: %s\n", buffer); free(buffer); } ------------------ ./a.out ------------------------------- how long do you want the string?5 Random string: nwlrb =========================================================== ------------------------------------------------------------------------------------------ # 4, 未声明大小的数组 [cppd - array_size](https://github.com/timtingwei/cppd/blob/master/tmp/array_size.cpp)
> * 问题 =================== source code =========================== int size = 5; int iarr[size] = {}; ------------------- ./a.out ---------------------------------- error: variable-sized object ‘iarr1’ may not be initialized =========================================================== > * const =========== source code ========== const unsigned int size = 5; int iarr1[size] = {}; ================================== > * define ========== source code ============== #define ARRAY_SIZE 5; // #define ARRAY_SIZE = 5; // ERROR int iarr[ARRAY_SIZE] = {} ====================================== ------------------------------------------------------------------------------------------------- # 5, continue 和 break 区别 [cppd - continue_break](https://github.com/timtingwei/cppd/blob/master/tmp/continue_break.cpp) > * 概念 continue: 跳过for, range-for, while or do-while, 循环体剩余部分 break: 闭合的for, range-for, while, do-while 循环或 switch选择终止 > * continue实例: loop: =================== source code ============================ // 跳过loop剩余部分 for (int i = 0; i < 5; i++) { for (int j = 2; j <7; j++) { if (j - i == 2) { std::cout << "j-i = 2" << std::endl; continue;} // ... std::cout << "after continue " << std::endl; } } ============================================================
> * break实例: 1, loop: ==================== source code =========================== // 终止循环 for (int i = 0; i < 5; i++) { for (int j = 2; j <7; j++) { if (j - i == 2) {break;} // 终止内存循环 std::cout << "j - i = " << j - i << std::endl; } } ============================================================ 2, switch =================== source code =========================== int i = 2; switch (i) { case 1: std::cout << "1\n"; case 2: std::cout << "2\n"; // 选择到这里 case 3: std::cout << "3\n"; case 4: case 5: std::cout << "45 \n"; break; // 终止switch case 6: std::cout << "6\n"; } ------------------- ./a.out ------------------------------ 2 3 45 ============================================================ ----------------------------------------------------------------------------------------------------- # if...else 和 if...else if的区别 [cppd - if_else_elseif](https://github.com/timtingwei/dsacpp/blob/master/src/02/vector_ordered.cpp) >* 问题 ==================== source code ========================== while (lo < hi) { // 区间存在 Rank mi = (lo + hi) >> 1; // 取中点 if (e < elem[mi]) hi = mi; // 深入前半段查找 else if (elem[mi] < e) lo = mi + 1; // 深入后半段查找 else return mi; // 在mi命中 } =========================================================== if, else...if 和 else是什么关系？ 从结果上看, 二分查找, 在前半段找到至少比较1次, 后半段2次。 >* if...else ==== int i = 6; if (i == 5) {std::cout << "i = 5" << std::endl; } else { if (i > 5) {std::cout << "i > 5" << std::endl; } else { std::cout << "i < 5" << std::endl;} } --------------------- ./a.out --------------------------- i > 5 === >* if...else if...else if === int i = 4; if (i == 5) std::cout << "i = 5" << std::endl; else if (i < 5) std::cout << "i < 5" << std::endl; else if (i > 5) std::cout << "i > 5" << std::endl; ----- i < 5 === >* if === int i = 6; if (i == 6) std::cout << "i = 6" << std::endl; if (i != 7) std::cout << "i != 7" << std::endl; ----------------- ./a.out -------------------------- i = 6 i != 7 ==================================================== >* 各自的作用域辨析 ==================== source code ========================== bool foo = false, bar = true, baz = true; if (foo) { // <- this block is only executed if 'foo' is true std::cout << "if(foo)\n"; } else if (bar) { // <- 'bar' is only checked if 'foo' is false // <- this block only executed if 'foo' is false and 'bar' is true std::cout << "else if(bar)\n"; } else { // <- this block only executed if 'foo' and 'bar' are both false std::cout << "else {}\n"; } if (baz) { // <- no 'else', so previous 'ifs' don't matter // <- this block only executed if 'baz' is true. foo/bar don't matter std::cout << "if(baz)\n"; } ---------------------- ./a.out ---------------------------------- // else if(bar) // if(baz) ============================================================== 我用的是Google cpplint的cpp书写方式, else要在两个反花括号之间。 这样更容易看到 if, else..if, else 是一个语句块, if又是单独的一个。 >* 总结 if(i < n), if...else(n < i), 这两个语句条件除了n = i互补。 如果有一个true, 则不会执行接下来的else; 如果都为false, else执行, 条件相当于(n==i)为true. 因此，对于二分查找来说, 前半段找到只执行一次if, 后半段找到执行 if 和 if else两次比较, 还节省一次判等的操作。 1, if...else if...else 可作三个并列关系的选择, 但运行时有先后顺序 2, if...else 多用 (cond)? expre1 : expre2; 代替 ===== source code ====== int i = 4, count = 0; (i == 4) ? count++ : i = 4; ======================== 缺点是执行的语句只能为一个表达式且单行, 也不能return 3, if, if 语句多为两个condtions没有并列关系。