ch05: new chapter

05-record-like/struct/README

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+To compile `metro_write.c` on MacOS 10.14 with XCode:
+
+$ clang metro_write.c -o metro
+
+Output:
+
+$ xxd metro_areas.bin 
+00000000: e207 0000 546f 6b79 6f00 0000 0000 0000  ....Tokyo.......
+00000010: 0000 0000 0000 0000 4a50 01e7 1158 274c  ........JP...X'L
+00000020: df07 0000 5368 616e 6768 6169 0000 0000  ....Shanghai....
+00000030: 0100 0000 0e00 0000 434e 0000 f8a0 134c  ........CN.....L
+00000040: df07 0000 4a61 6b61 7274 6100 0000 0000  ....Jakarta.....
+00000050: 0000 0000 0000 0000 4944 0000 bcc5 f14b  ........ID.....K
+$ python3 metro_read.py 
+2018	Tokyo, JP	43,868,228
+2015	Shanghai, CN	38,700,000
+2015	Jakarta, ID	31,689,592

05-record-like/struct/metro_read.py

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+from struct import iter_unpack
+
+FORMAT = 'i12s2sf'                             # <1>
+
+def text(field: bytes) -> str:                 # <2>
+    octets = field.split(b'\0', 1)[0]          # <3>
+    return octets.decode('cp437')              # <4>
+
+with open('metro_areas.bin', 'rb') as fp:      # <5>
+    data = fp.read()
+
+for fields in iter_unpack(FORMAT, data):       # <6>
+    year, name, country, pop = fields
+    place = text(name) + ', ' + text(country)  # <7>
+    print(f'{year}\t{place}\t{pop:,.0f}')

05-record-like/struct/metro_write.c

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+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+#define LEN 3
+
+struct MetroArea {
+    int year;
+    char name[12];
+    char country[2];
+    float population;
+};
+
+int main(int argc, char* argv[]) {
+    struct MetroArea metro[LEN];
+    int rank;
+
+    metro[0].year = 2018;
+    strcpy(metro[0].name, "Tokyo");
+    metro[0].country[0] = 'J';
+    metro[0].country[1] = 'P';
+    metro[0].population = 43868229.0;
+
+    metro[1].year = 2015;
+    strcpy(metro[1].name, "Shanghai");
+    metro[1].country[0] = 'C';
+    metro[1].country[1] = 'N';
+    metro[1].population = 38700000.0;
+
+    metro[2].year = 2015;
+    strcpy(metro[2].name, "Jakarta");
+    metro[2].country[0] = 'I';
+    metro[2].country[1] = 'D';
+    metro[2].population = 31689592.0;
+
+    FILE* data;
+    if ( (data = fopen("metro_areas.bin", "wb")) == NULL ) {
+        printf("Error opening file\n");
+        return 1;   
+    }
+
+    fwrite(metro, sizeof(struct MetroArea), LEN, data);
+    fclose(data);
+
+    return 0;
+}