{"id":4133,"date":"2022-08-21T17:37:09","date_gmt":"2022-08-21T12:07:09","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-explain-the-output-of-the-following-c-program\/"},"modified":"2022-08-21T17:37:09","modified_gmt":"2022-08-21T12:07:09","slug":"solved-explain-the-output-of-the-following-c-program","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-explain-the-output-of-the-following-c-program\/","title":{"rendered":"[Solved] Explain the output of the following C program?"},"content":{"rendered":"

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\n<\/p>\n

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The code under consideration is, when modernized and given some missing spaces:<\/p>\n

#include <stdio.h>\nint main(void)\n{\n    int a = 1, b = 2, c = 3;\n    printf(\"%d\\n\", a += (a += 3, 5, a));\n    return 0;\n}\n<\/code><\/pre>\n
My initial reaction was “dup of Why are these constructs undefined behaviour”, even though that is primarily about ++<\/code> and --<\/code> operators which are more often seen in such questions.<\/p>\n
A second glance notes that there a comma operators in (a += 3, 5, a)<\/code> and these impose sequence points.  On its own, this much of the expression is fine.  If it were used in b += (a += 3, 5, a)<\/code>, there would be no problem.  The text of the question asks about just (a += 3, 5, a)<\/code>.<\/p>\n
Of course, the , 5<\/code> does nothing useful.  And when that’s eliminated, the only reason to use (a += 3, a)<\/code> is to put a sequence point between the addition and the a<\/code>.  However, there are no circumstances I can think of where you could legitimately need that sequence point.<\/p>\n
The more complex issue is whether the evaluation of the LHS of a += (a += 3, 5, a)<\/code> is properly sequenced with the RHS. It is undefined behaviour because although there are sequence points in the RHS expression, there isn’t a sequence point between the evaluation of the LHS and RHS of the overall expression.<\/p>\n
Remember that the compound assignment operator +=<\/code> behaves more or less as:<\/p>\n
a += x;\na = a + x;\n<\/code><\/pre>\n
The difference is that the expression on the LHS (a<\/code> in this example) is only evaluated once.  In this case, there’s no problem.  If the code were more complex, this detail matters:<\/p>\n
array[index++].member[subindex++] += 23;\n<\/code><\/pre>\n
Now it is crucial that the LHS is only evaluated once!<\/p>\n
Summary<\/h3>\n
The code in the question is tricky code and, because it has undefined behaviour, it should be avoided in production code.<\/p>\n
I wonder why b<\/code> and c<\/code> are in the question at all; they’re unused variables.  So much confusion in so little code!<\/p>\n
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Rule of thumb: Avoid multiple increments, decrements and compound assignment operators in a single expression.<\/p>\n
Advanced rule of thumb (for experts only): don’t apply multiple increments, decrements or compound assignment operators to the same variable in a single expression.<\/p>\n
If you know enough to know when you can safely break the rules (?:<\/code>), then you don’t necessarily need to follow the rules of thumb.  Until then, avoid over-using these operators.<\/p>\n<\/p><\/div>\n
<\/div>\n<\/div>\n
            1<\/span> <\/p><\/div>\n<\/div>\n
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solved Explain the output of the following C program? <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] The code under consideration is, when modernized and given some missing spaces: #include <stdio.h> int main(void) { int a = 1, b = 2, c = 3; printf(“%d\\n”, a += (a += 3, 5, a)); return 0; } My initial reaction was “dup of Why are these constructs undefined behaviour”, even though that is … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324],"class_list":["post-4133","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c"],"yoast_head":"\n[Solved] Explain the output of the following C program? - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-explain-the-output-of-the-following-c-program\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Explain the output of the following C program? - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] The code under consideration is, when modernized and given some missing spaces: #include <stdio.h> int main(void) { int a = 1, b = 2, c = 3; printf("%dn", a += (a += 3, 5, a)); return 0; } My initial reaction was “dup of Why are these constructs undefined behaviour”, even though that is ... 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