绝知此事要躬行

Author: vonzhou

chapter02/README.md

@@ -92,7 +92,16 @@ show_bytes((byte_pointer)s, strlen(s));
 ### 证明符号扩展的正确性, 利用数学归纳法，证明一位扩展正确即可
 ![](sign_extension_prove.jpg)
 
+### C语言标准：把short转换成unsigned时，先改变大小再完成从有符号到无符号的转换。 short -> int -> unsigned
+[type_cast_mystery.c](type_cast_mystery.c)
 
+### 习题2.25 问题在于无符号运算, 0U - 1 = UMax, 然后访问非法地址，coredump
+[sum_elements.c](sum_elements.c)
+
+### 习题2.26 问题仍然是无符号运算的微妙，size_t定义为unsigned int. 在做差和比较时会采用无符号运算，如果a<b, a-b是负数，会成为一个很大的无符号数，所以结果不正确
+[stronger.c](stronger.c)
+
+### 
 
 
 

chapter02/sign_extension_prove.jpg

@@ -0,0 +1,14 @@
+#include <stdio.h>
+#include <string.h>
+
+int stronger(char *s, char *t){
+    return strlen(s) - strlen(t) > 0;
+    // return strlen(s) > strlen(t);
+}
+
+int main(){
+    char *s = "abc";
+    char *t = "abcdef";
+    printf("stringer = %d\n", stronger(s,t));
+    return 0;
+}

chapter02/stronger

@@ -0,0 +1,18 @@
+#include <stdio.h>
+
+
+int sum_elements(float a[], unsigned length){
+    int i;
+    float result = 0;
+    for(i=0; i<=length-1;i++){
+        result += a[i];
+    }
+    return result;
+}
+
+int main(){
+    int i = 0;
+    unsigned len = 0U;
+    printf("%u\n", len-1);
+    return 0;
+}

chapter02/stronger.c

@@ -0,0 +1,20 @@
+#include <stdio.h>
+
+
+typedef unsigned char *byte_pointer;
+
+void show_bytes(byte_pointer start, int len){
+    int i;
+    for (i = 0; i < len; i++)
+        printf(" %.2x", start[i]);
+    printf("\n");
+}
+
+int main(){
+    short sx = -12345;
+    unsigned uy = sx;
+
+    printf("uy = %u:\t", uy);
+    show_bytes((byte_pointer)&uy, sizeof(unsigned));
+    return 0;
+}