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The Department of Computer Science & Engineering
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STUART C. SHAPIRO: CSE
305
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w
- x / y - z or w / x - y / z. Operators with
higher precedence are done first. Parentheses are used to override
precedence. Languages tend to be similar in their operator precedence
order.
x - y - z or x / y / z.
Most operators in most languages are left associative, except that
exponentiation (when included) is usually right associative.
Parentheses are, again, used to override associativity.
For example, Python's subtraction operator is left-associative, and its exponentiation is right-associative:
>>> 100-50-25 25 >>> (100-50)-25 25 >>> 100-(50-25) 75 >>> 2**2**2**2 65536 >>> 2**(2**(2**2)) 65536 >>> ((2**2)**2)**2 256
A mathematical operator, •, is associative when ((x • y) • z) = (x • (y • z)), so that the order of evaluation does not matter. Addition and multiplication are mathematically associative, but may not be associative computationally. For example,
So when adding a series of floating point numbers, it is better to add them starting with the smallest terms than starting with the largest terms.bsh % print(3e17+4.+4.+4.+4.+4.+4.+4.+4.+4.+4.+4.); 3.0E17 bsh % print(3e17+(4.+4.+4.+4.+4.+4.+4.+4.+4.+4.+4.)); 3.0000000000000006E17
in an expression such asIt will not matter unless one of the<expression>1 • <expression>2,
is<expression>1or<expression>2evaluated first?
expressions has a
side effect.
A side effect occurs when the evaluation of an expression causes a change to some variable, rather than only producing a value. Here's an example in Java:
public class OperandOrder {
public static int x;
public static int f(int y) {
x++;
return y;
}
public static void main (String[] args) {
int result;
x = 3;
result = x + f(5);
System.out.println("First evaluation = " + result);
x = 3;
result = f(5) + x;
System.out.println("Second evaluation = " + result);
} // end of main ()
}// OperandOrder
------------------------------------------------
<timberlake:Tests:1:102> javac OperandOrder.java
<timberlake:Tests:1:103> java OperandOrder
First evaluation = 8
Second evaluation = 9
And, here's C:
#include <stdio.h>
int x;
int f(int y) {
x++;
return y;
}
int g(int y) {
return y;
}
int main() {
int result;
x = 3;
result = x + f(5);
printf("First evaluation = %3d\n", result);
x = 3;
result = f(5) + x;
printf("Second evaluation = %3d\n", result);
x = 3;
result = g(x) + f(5);
printf("Third evaluation = %3d\n", result);
x = 3;
result = f(5) + g(x);
printf("Fourth evaluation = %3d\n", result);
return 0;
}
------------------------------------------------
<timberlake:Programs:2:180> gcc -Wall -o operandOrder operandOrder.c
<timberlake:Programs:2:180> ./operandOrder
First evaluation = 9
Second evaluation = 9
Third evaluation = 8
Fourth evaluation = 9
"Except as specified later (for the function-call (), &&, ||, ?:, and comma operators), the order of evaluation of subexpressions and the order in which side effects take place are both unspecified." [ISO C Standard, Sect. 6.5[#3]]
Optimizing compilers may change the order of operand evaluation. They sometimes allow the programmer to specify that a certain section of code should not be optimized.
+
compiling into one prodecure for ints, another for
floats, and another for Strings.
C++ and some other languages allow the programmer to supply additional overloadings to most or all of its operators.
int to double.
A narrowing conversion converts a value from type A to type B where
multiple values of type A convert to the same value of type B (even
within reasonable bounds of range and precision). An example is
conversion from double to int.
However, here's an example where even widening doesn't work because of loss of precision:bsh % int i; bsh % double x; bsh % i = 3; bsh % x = i; bsh % print(x); 3.0 bsh % x = 3.5; bsh % i = (int)x; bsh % print(i); 3
Usually, if the operands of one operator are of different types, the value of one is automatically converted (coerced) to the type of the other as long as that is a widening conversion, but a narrowing conversion must be explicitly specified by a cast.bsh % float y; bsh % y = 123456792; bsh % print(y); 1.23456792E8 bsh % y = 123456793; bsh % print(y); 1.23456792E8
For example, in Python:
This example also shows an especially obvious example of loss of precision and representation errors when dealing with floating point numbers: Python converts the literal 9.7 to 9.6999999999999993!>>> 9.7 9.6999999999999993 >>> type(9.7) <type 'float'> >>> type(2) <type 'int'> >>> 9.7 + 2 11.699999999999999 >>> type(9.7+2) <type 'float'> >>> int(9.7) 9 >>> type(int(9.7) + 2) <type 'int'>
x == y. Note, however, that two
floating-point values, computed differently, are seldom equal.
Boolean expressions apply a Boolean operator to Boolean values. An
example is i < max && a[i] == x. Usually, Boolean
operators have lower precedence than relational and arithmetic
operators. However, using parentheses is often a good idea, for
example, (i < max) && (a[i] == x).
A standard warning given to novice programmers is that i <
j < k is not correct;
it must be written (i <
j) && (j < k). Since C uses 0 and
1 for Boolean values, one can get:
#include <stdio.h>
int main() {
int i=5, j=7, k=3;
if (i < j < k) {
printf("It's true that %d < %d < %d\n", i, j, k);
}
return 0;
}
-----------------------------------------------------
<timberlake:Test:1:43> gcc -Wall -o boolTest boolTest.c
boolTest.c: In function รข:
boolTest.c:6: warning: comparisons like X<=Y<=Z do not have their mathematical meaning
./boolTest
It's true that 5 < 7 < 3
#include <stdio.h>
int main() {
int i=5, j=7, k=3;
if (i < j < k) {
printf("%d < %d : %d\n", i, j, i < j);
printf("%d < %d : %d\n", j,k, j < k);
}
return 0;
}
-----------------------------------------------------
bash-3.2$ gcc -Wall -o boolTest2 boolTest2.c
boolTest2.c: In function 'main':
boolTest2.c:6: warning: comparisons like X<=Y<=Z do not have their mathematical meaning
bash-3.2$ ./boolTest2
5 < 7 : 1
7 < 3 : 0
Python, however, allows these expressions:
Python 2.6.4 (r264:75706, Dec 21 2009, 12:48:32) [GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> i = 5 >>> j = 7 >>> k = 3 >>> i<j<k False >>> k<i<j True >>>
Trust the Boolean type! I have often seen code like
if (<test>) {return true;}
else {return false;}
return <test>;
return x enclosed in a conditional statement:
#include <stdio.h>
int absval(int x)
{
if (x >= 0) {
return x;
}
else {
return -x;
}
}
int main() {
printf("The absolute value of %3d is %3d\n", 5, absval(5));
printf("The absolute value of %3d is %3d\n", -5, absval(-5));
return 0;
}
-------------------------------------------------------------------
<timberlake:Test:1:34> gcc -Wall -o absval absval.c
<timberlake:Test:1:35> ./absval
The absolute value of 5 is 5
The absolute value of -5 is 5
Declarative style, rather than declarative programming language, because some "imperative languages" include the conditional expression also. Again, in C:<timberlake:Test:1:37> ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> let absval x = if x>=0 then x else -x Prelude> absval 5 5 Prelude> absval (-5) 5
The conditional expression uses the trinary operator#includeint absval(int x) { return x >= 0 ? x : -x; } int main() { printf("The absolute value of %3d is %3d\n", 5, absval(5)); printf("The absolute value of %3d is %3d\n", -5, absval(-5)); return 0; } ----------------------------------------------------------------- <timberlake:Test:1:38> gcc -Wall -o absval2 absval2.c <timberlake:Test:1:39> ./absval2 The absolute value of 5 is 5 The absolute value of -5 is 5 <timberlake:Test:1:40>
This operator exists in all the C-based languages.<condition> ? <then-expression> : <else-expression>
Haskell uses lazy evaluation. Under lazy evaluation, expressions are evaluated only when their values are needed. Evaluation of an expression is delayed until its value is required, e.g. in the course of evaluating another expression. In Haskell, when an expression is passed as the actual argument to a function, it is not evaluated unless and when its value is required somewhere in the body of the function.
Prelude> let x = [1..] Prelude> head(x) 1 Prelude> head(tail(x)) 2 Prelude> head(tail(tail(x))) 3 Prelude> head(tail(tail(tail(x)))) 4 Prelude> tail(x) [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,746,747,748,749,750,751,752,753,754,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838,839,840,841,842,843,844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927,928,929,930,931,932,933,934,935,936,937,938,939,940,941,942,943,944,945,946,947,948,949,950,951,952,953,954,955,956,957,958,959,960,961,962,963,964,965,966,967,968,969,970,971,972,973,974,975,976,977,978,979,980,981,982,983,984,985,986,987,988,989,990,991,992,993,994,995,996,997,998,999,1000,1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1011,1012,1013,1014,1015,1016,1017,1018,1019,1020,1021,1022,1023,1024,1025,1026,1027,1028,1029,1030,1031,1032,1033,1034,1035,1036,1037,1038,1039,1040,1041,1042,1043,1044,1045,1046,1047,1048,1049,1050,1051,1052,1053,1054,1055,1056,1057,1058,1059,1060,1061,1062,1063,1064,1065,1066,1067,1068,1069,1070,1071,1072,1073,1074,1075,1076,1077,1078,1079,1080,1081,1082,1083,1084,1085,1086,1087,1088,1089,1090,1091,1092,1093,1094,1095,1096,1097,1098,1099,1100,1101,1102,1103,1104,1105,1106,1107,1108,1109,1110,1111,1112,1113,1114,1115,1116,1117,1118,1119,1120,1121,1122,1123,1124,1125,1126,1127,1128,1129,1130,1131,1132,1133,1134,1135,1136,1137,1138,1139,1140,1141,1142,1143,1144,1145,1146,1147,1148,1149,1150,1151,1152,1153,1154,1155,1156,1157,1158,1159,1160,1161,1162,1163,1164,1165,1166,1167,1168,1169,1170,1171,1172,1173,1174,1175,1176,1177,1178,1179,1180,1181,1182,1183,1184,1185,1186,1187,1188,1189,1190,1191,1192,1193,1194,1195,1196,1197,1198,1199,1200,1201,1202,1203,1204,1205,1206,1207,1208,1209,1210,1211,1212,1213,1214,1215,1216,1217,1218,1219,1220,1221,122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2,6223,6224,6225,6226,6227,6228,6229,6230,6231,6232,6233,6234,6235,6236,6237,6238,6239,6240,6241,6242,6243,6244,6245,6246,6247,6248,6249,6250,6251,6252,6253,6254,6255,6256,6257,6258,6259,6260,6261,6262,6263,6264,6265,6266,6267,6268,6269,6270,6271,6272,6273,6274,6275,6276,6277,6278,6279,6280,6281,6282,6283,6284,6285,6286,6287,6288,6289,6290,6291,6292,6293,6294,6295,6296,6297,6298,6299,6300,6301,6302,6303,6304,6305,6306,6307,6308,6309,6310,6311,6312,6313,6314,6315,6316,6317,6318,6319,6320,6321,6322,6323,6324,6325,6326,6327,6328,6329,6330,6331,6332,6333,6334,6335,6336,6337,6338,6339,6340,6341,6342,6343,6344,6345,6346,6347,6348,6349,6350,6351,6352,6353,6354,6355,6356,6357,6358,6359,6360,6361,6362,6363,6364,6365,6366,6367,6368,6369,6370,6371,6372,6373,6374,6375,6376,6377,6378,6379,6380,6381,6382,6383,6384,6385,6386,6387,6388,6389,6390,6391,6392,6393,6394,6395,6396,6397,6398,6399,6400,6401,6402,6403,6404,6405,6406,6407,6408,6409,6410,6411,6412,6413,6414,6415,6416,6417,6418,6419,6420,6421,6422,6423,6424,6425,6426,6427,6428,6429,6430,6431,6432,6433,6434,6435,6436,6437,6438,6439,6440,6441,6442,6443,6444,6445,6446,6447,6448,6449,6450,6451,6452,6453,6454,6455,6456,6457,6458,6459,6460,6461,6462,6463,6464,6465,6466,6467,6468,6469,6470,6471,6472,6473,6474,6475,6476,6477,6478,6479,6480,6481,6482,6483,6484,6485,6486,6487,6488,6489,6490,6491,6492,6493,6494,6495,6496,6497,6498,6499,6500,6501,6502,6503,6504,6505,6506,6507,6508,6509,6510,6511,6512,6513,6514,6515,6516,6517,6518,6519,6520,6521,6522,6523,6524,6525,6526,6527,6528,6529,6530,6531,6532,6533,6534,6535,6536,6537,6538,6539,6540,6541,6542,6543,6544,6545,6546,6547,6548,6549,6550,6551,6552,6553,6554,6555,6556,6557,6558,6559,6560,6561,6562,6563,6564,6565,6566,6567,6568,6569,6570,6571,6572,6573,6574,6575,6576,6577,6578,6579,6580,6581,6582,6583,6584,6585,6586,6587,6588,6589,6590,6591,6592,6593,6594,6595,6596,6597,6598,6599,6600,6601,6602,6603,6604,6605,6606,6607,6608,6609,6610,6611,6612,6613,6614,6615,6616,6617,6618,6619,6620,6621,6622,6623,6624,6625,6626,6627,6628,6629,6630,6631,6632,6633,6634,6635,6636,6637,6638,6639,6640,6641,6642,6643,6644,6645,6646,6647,6648,6649,6650,6651,6652,6653,6654,6655,6656,6657,6658,6659,6660,6661,6662,6663,6664,6665,6666,6667,6668,6669,6670,6671,6672,6673,6674,6675,6676,6677,6678,6679,6680,6681,6682,6683,6684,6685,6686,6687,6688,6689,6690,6691,6692,6693,6694,6695,6696,6697,6698,6699,6700,6701,6702,6703,6704,6705,6706,6707,6708,6709,6710,6711,6712,6713,6714,6715,6716,6717,6718,6719,6720,6721,6722,6723,6724,6725,6726,6727,6728,6729,6730,6731,6732,6733,6734,6735,6736,6737,6738,6739,6740,6741,6742,6743,6744,6745,6746,6747,6748,6749,6750,6751,6752,6753,6754,6755,6756,6757,6758,6759,6760,6761,6762,6763,6764,6765,6766,6767,6768,6769,6770,6771,6772,6773,6774,6775,6776,6777,6778,6779,6780,6781,6782,6783,6784,6785,6786,6787,6788,6789,6790,6791,6792,6793,6794,6795,6796,6797,6798,6799,6800,6801,6802,6803,6804,6805,6806,6807,6808,6809,6810,6811,6812,6813,6814,6815,6816,6817,6818,6819,6820,6821,6822,6823,6824,6825,6826,6827,6828,6829,6830,6831,6832,6833,6834,6835,6836,6837,6838,6839,6840,6841,6842,6843,6844,6845,6846,6847,6848,6849,6850,6851,6852,6853,6854,6855,6856,6857,6858,6859,6860,6861,6862,6863,6864,6865,6866,6867,6868,6869,6870,6871,6872,6873,6874,6875,6876,6877,6878,6879,6880,6881,6882,6883,6884,6885,6886,6887,6888,6889,6890,6891,6892,6893,6894,6895,6896,6897,6898,6899,6900,6901,6902,6903,6904,6905,6906,6907,6908,6909,6910,6911,6912,6913,6914,6915,6916,6917,6918,6919,6920,6921,6922,6923,6924,6925,6926,6927,6928,6929,6930,6931,6932,6933,6934,6935,6936,6937,6938,6939,6940,6941,6942,6943,6944,6945,6946,6947,6948,6949,6950,6951,6952,6953,6954,6955,6956,6957,6958,6959,6960,6961,6962,6963,6964,6965,6966,6967,6968,6969,6970,6971,6972,6973,6974,6975,6976,6977,6978,6979,6980,6981,6982,6983,6984,6985,6986,6987,6988,6989,6990,6991,6992,6993,6994,6995,6996,6997,6998,6999,7000,7001,7002,7003,7004,7005,7006,7007,7008,7009,7010,7011,7012,7013,7014,7015,7016,7017,7018,7019,7020,7021,7022,7023,7024,7025,7026,7027,7028,7029,7030,7031,7032,7033,7034,7035,7036,7037,7038,7039,7040,7041,7042,7043,7044,7045,7046,7047,7048,7049,7050,7051,7052,7053,7054,7055,7056,7057,7058,7059,7060,7061,7062,7063,7064,7065,7066,7067,7068,7069,7070,7071,7072,7073,7074,7075,7076,7077,7078,7079,7080,7081,7082,7083,7084,7085,7086,7087,7088,7089,7090,7091,7092,7Interrupted.
The Sieve of Eratosthenes is a technique for computing prime numbers that starts with a list of sorted numbers and moves across the list eliminating multiples of each prime as it is discovered. This is illustrated by the following video:Prelude> [2,4,6,8,10] [2,4,6,8,10] Prelude> [2,4,6,8,10] !! 4 10 Prelude> [42..] !! 100 142 Prelude> tail([42..]) !! 100 143
The following is a Haskell function that uses infinite lists and lazy evaluation to computes prime numbers with the Sieve of Eratosthenes (Source: Literate Programs*).
Prelude> let sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0] Prelude> let primes = sieve [2..] Prelude> primes !! 0 2 Prelude> primes !! 5 13 Prelude> primes [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,307,311,313,317,331,337,347,349,353,359,367,373,379,383,389,397,401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,491,499,503,509,521,523,541,547,557,563,569,571,577,587,593,599,601,607,613,617,619,631,641,643,647,653,659,661,673,677,683,691,701,709,719,727,733,739,743,751,757,761,769,773,787,797,809,811,821,823,827,829,839,853,857,859,863,877,881,883,887,907,911,919,929,937,941,947,953,967,971,977,983,991,997,1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,1153,1163,1171,1181,1187,1193,1201,1213,1217,1223,1229,1231,1237,1249,1259,1277,1279,1283,1289,1291,1297,1301,1303,1307,1319,1321,1327,1361,1367,1373,1381,1399,1409,1423,1427,1429,1433,1439,1447,1451,1453,1459,1471,1481,1483,1487,1489,1493,1499,1511,1523,1531,1543,1549,1553,1559,1567,1571,1579,1583,1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,1663,1667,1669,1693,1697,1699,1709,1721,1723,1733,1741,1747,1753,1759,1777,1783,1787,1789,1801,1811,1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,1901,1907,1913,1931,1933,1949,1951,1973,1979,1987,1993,1997,1999,2003,2011,2017,2027,2029,2039,2053,2063,2069,2081,2083,2087,2089,2099,2111,2113,2129,2131,2137,2141,2143,2153,2161,2179,2203,2207,2213,2221,2237,2239,2243,2251,2267,2269,2273,2281,2287,2293,2297,2309,2311,2333,2339,2341,2347,2351,2357,2371,2377,2381,2383,2389,2393,2399,2411,Interrupted.
Some languages, including Scheme (a dialect of Lisp distinct from Common Lisp) include constructs that allow programmers to explicitly delay evaluation of individual expressions, essentially allowing lazy evaluation on an expression-by-expression basis.
* Sieve of Eratosthenes (Haskell)C++ and Perl are a couple of languages which allow conditional targets. While Java supports the ternary conditional, it does not support conditional targets. Consider this C++ program:
#include <iostream>
int main(){
int flag = 0;
int x = 100;
int y = 100;
(flag ? x : y) = 0;
printf("X: %d, Y: %d\n", x, y);
}
--------------------------------------------
timberlake {~/cse305-ta-sp10/assign} > g++ CondTar.cpp
timberlake {~/cse305-ta-sp10/assign} > ./a.out
X: 100, Y: 0
(flag ? x : y) = 0; is equivalent to saying:
if(flag) x = 0; else y = 0;
x •= y;
means the same as x = x • y; for most operators,
•.
i = j = k assigns the value of
k to j and then to i.
In Perl the value of the assignment operation is the l-value of the left-hand side, even though assignment is still right-associative. The l-value is dereferenced to its r-value if the assignment expression is on a right-hand side, but if it's on a left-hand side, the l-value may be stored into. Here's an example, based on L. Wall, T. Christiansen & J. Orwant, Programming Perl, (Sebastopol, CA: O'Reilly) p. 25.
#! /util/bin/perl $x = $y = 3; $temp = 100; ($temp *= 9/5) += 32; print "x = $x, y = $y, temp = $temp\n"; ----------------------------------------- <timberlake:Test:1:42> perl assignment.perl x = 3, y = 3, temp = 212
In Python, assignment statements like i = j = k, are
allowed, even though a Python assignment statement is
not an expression at all:
Allowing an assignment statement to be an expression can lead to programmer error and difficult to read expressions, as in this C++ program:<wasat:~:1:108> python Python 2.2.1 (#2, Jul 19 2002, 09:50:59) [C] on sunos5 Type "help", "copyright", "credits" or "license" for more information. >>> x = y = 3 >>> x 3 >>> y 3 >>> print(None) None >>> print(z=7) File "", line 1 print(z=7) ^ SyntaxError: invalid syntax
#include <iostream>
int main(){
int x = 5;
if(x = 2) printf("here\n"); // "==" mistakenly typed as "="
printf("x = %d\n", x);
x = 5;
int y = 2;
int z = 4;
int a = 0;
printf("x = %d, y = %d, z = %d, a = %d\n", x, y, z ,a);
int q = x - (a = y * z) / 3; // difficult to read
printf("x = %d, y = %d, z = %d, a = %d, q = %d\n", x, y, z ,a, q);
}
------------------------------------------------------------------
timberlake {~/cse305-ta-sp10/assign} > g++ CAssign.cpp
timberlake {~/cse305-ta-sp10/assign} > ./a.out
here
x = 2
x = 5, y = 2, z = 4, a = 0
x = 5, y = 2, z = 4, a = 8, q = 3
++ and
--. The tricky issue is the postfix versions. In Java,
it is clear that the semantics of x++ in any context is
x fetch x x fetch 1 + store pop. Here's an example:
public class UnaryAsngmnt {
public static void main (String[] args) {
int x=3, y = 0;
System.out.println("x = " + x);
System.out.println("y = " + y);
y = x + x++;
System.out.println();
System.out.println("y = x + x++;");
System.out.println("x = " + x);
System.out.println("y = " + y);
x = x++;
System.out.println();
System.out.println("x = x++;");
System.out.println("x = " + x);
} // end of main ()
}// UnaryAsngmnt
--------------------------------------------------
<cirrus:Programs:1:124> javac UnaryAsngmnt.java
<cirrus:Programs:1:125> java UnaryAsngmnt
x = 3
y = 0
y = x + x++;
x = 4
y = 6
x = x++;
x = 4
In C the tricky uses of these operators are officially undefined since the order of evaluation of operands is unspecified. Let's see this in two C compilers. In cc:
#include <stdio.h>
int main() {
int x=3, y=0;
printf("x = %d, y = %d\n", x, y);
y = x + x++;
printf("\ny = x + x++;\n");
printf("x = %d, y = %d\n", x, y);
x = x++;
printf("\nx = x++;\n");
printf("x = %d, y = %d\n", x, y);
x = x++ + x;
printf("\nx = x++ + x;\n");
printf("x = %d, y = %d\n", x, y);
x = x++ + x++;
printf("\nx = x++ + x++;\n");
printf("x = %d, y = %d\n", x, y);
return 0;
}
---------------------------------------------
<wasat:Programs:2:183> cc unaryAsngmnt.c -o unaryAsngmnt.out
<wasat:Programs:2:184> unaryAsngmnt.out
x = 3, y = 0
y = x + x++;
x = 4, y = 6
x = x++;
x = 5, y = 6
x = x++ + x;
x = 11, y = 6
x = x++ + x++;
x = 24, y = 6
<wasat:Programs:2:185> gcc -Wall unaryAsngmnt.c -o unaryAsngmnt.out unaryAsngmnt.c: In function `main': unaryAsngmnt.c:8: warning: operation on `x' may be undefined unaryAsngmnt.c:12: warning: operation on `x' may be undefined unaryAsngmnt.c:16: warning: operation on `x' may be undefined unaryAsngmnt.c:20: warning: operation on `x' may be undefined unaryAsngmnt.c:20: warning: operation on `x' may be undefined <wasat:Programs:2:186> unaryAsngmnt.out x = 3, y = 0 y = x + x++; x = 4, y = 6 x = x++; x = 5, y = 6 x = x++ + x; x = 6, y = 6 x = x++ + x++; x = 7, y = 6
public class AsngmntOrder {
public static void main (String[] args) {
int i=0, a[] = {5, 5, 5};
System.out.println("i = " + i);
System.out.println("a = [" + a[0]
+ ", " + a[1]
+ ", " + a[2] + "]");
a[i] = i++;
System.out.println();
System.out.println("a[i] = i++;");
System.out.println("i = " + i);
System.out.println("a = [" + a[0]
+ ", " + a[1]
+ ", " + a[2] + "]");
a[i++] = i;
System.out.println();
System.out.println("a[i++] = i;");
System.out.println("i = " + i);
System.out.println("a = [" + a[0]
+ ", " + a[1]
+ ", " + a[2] + "]");
} // end of main ()
}// AsngmntOrder
------------------------------------------------
<cirrus:Programs:1:129> javac AsngmntOrder.java
<cirrus:Programs:1:130> java AsngmntOrder
i = 0
a = [5, 5, 5]
a[i] = i++;
i = 1
a = [0, 5, 5]
a[i++] = i;
i = 2
a = [0, 2, 5]
C leaves this case officially undefined,
This paragraph renders undefined statement expressions
such as
i = ++i + 1;
a[i++] = i; |
while allowing
i = i + 1;
a[i] = i; [ISO CStandard, Sect. 6.5(#58)]
x = y = ... = <exp> lets you
assign the same value to multiple variables, but usually the
assignment of multiple values to multiple variables requires multiple
assignment statements in sequential order.
Common Lisp's assignment "statement" allows multiple values to be stored in multiple variables in sequential order:
There is also a parallel version that allows multiple values to be stored in multiple variables in parallel order. This allows for the swapping of two variables without using an explicit temporary:cl-user(17): (setf x 1 y 2 z 3 x z) 3 cl-user(18): x 3 cl-user(19): y 2 cl-user(20): z 3
There's also a "respectively" version, that is useful, as we'll see later, when a function returns multiple values:cl-user(21): x 3 cl-user(22): y 2 cl-user(23): (psetf x y y x) nil cl-user(24): x 2 cl-user(25): y 3
cl-user(1): (setf x 2 y 3) 3 cl-user(2): x 2 cl-user(3): y 3 cl-user(4): (multiple-value-setq (x y) (values y x)) 3 cl-user(5): x 3 cl-user(6): y 2 cl-user(7):
Several other languages, including Perl, Python, and Ruby, allow assignment statements with several variables in the LHS and several expressions in the RHS. Assignment of experssions to respective variables is done in parallel. For example, in Python:
Such an assignment statement is treated as having a tuple of variables on the LHS and a tuple of expressions on the RHS:<timberlake:Test:1:31> python Python 2.6.4 (r264:75706, Dec 21 2009, 12:37:31) [GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> x,y = 3,5 >>> x 3 >>> y 5 >>> x,y = y,x >>> x 5 >>> y 3
This can be done with lists as well as tuples:>>> (x,y,z) = (3,3+3,3*3) >>> x 3 >>> y 6 >>> z 9
>>> [(let1,name1), (let2,name2), (let3,name3)] = [("a","alfa"), ("b","brave"), ("c","charlie")]
>>> print let1, let2, let3
a b c
>>> print name1, name2, name3
alfa brave charlie
In Python, the number of variables on the LHS must be the same as the number of expressions on the RHS:>>> (x,3,y) = (2,3,5) File "", line 1 SyntaxError: can't assign to literal >>> (x,y,z) = (2,u,5) Traceback (most recent call last): File " ", line 1, in NameError: name 'u' is not defined
Ruby, however, allows this, It ignores the extra RHS values, and assigns the extra LHS variables the value>>> x,y,z = 2,3 Traceback (most recent call last): File "", line 1, in ValueError: need more than 2 values to unpack >>> x,y = 2,3,4 Traceback (most recent call last): File " ", line 1, in ValueError: too many values to unpack
nil:
Ruby's splat operator packs or unpacks RHS values:<timberlake:Test:1:37> irb irb(main):001:0> x,y = 1,2,3 => [1, 2, 3] irb(main):002:0> x => 1 irb(main):003:0> y => 2 irb(main):004:0> x,y,z = 1,2 => [1, 2] irb(main):005:0> x => 1 irb(main):006:0> y => 2 irb(main):007:0> z => nil
Still, only variables may appear on the LHS, and no unbound variables may appear on the RHS:irb(main):018:0> x, *y, z = 1,2,3,4,5 => [1, 2, 3, 4, 5] irb(main):019:0> x => 1 irb(main):020:0> y => [2, 3, 4] irb(main):021:0> z => 5 irb(main):022:0> w,x,y,z = [1], *[2,3], *[4,5] => [[1], 2, 3, 4, 5] irb(main):023:0> w => [1] irb(main):024:0> x => 2 irb(main):025:0> y => 3 irb(main):026:0> z => 4
In Haskell and Erlang, the parallel assignment is actually done by pattern matching, so values, as well as variables, may appear on the LHS, but will fail if they do not match. In Haskell,
Using the list construction operator ("<timberlake:~:1:26> ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> let [x,3,y] = [2+1, 6/2, 3*3] Prelude> x 3.0 Prelude> y 9.0 Prelude> let [u,6,v] = [2,3,5] Prelude> u *** Exception::1:4-20: Irrefutable pattern failed for pattern [u, 6, v] Prelude> v *** Exception: :1:4-20: Irrefutable pattern failed for pattern [u, 6, v]
:") in Haskell, we
can use pattern matching to deconstruct a list:
We can also do this in Erlang, where "Prelude> let x:xs = [2,4..10] Prelude> x 2 Prelude> xs [4,6,8,10]
|" is the list
construction operator:
However, unbound variables still aren't allowed on the RHS of Haskell or Erlang assignment statements:<timberlake:~:1:28> erl Erlang R13B03 (erts-5.7.4) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false] Eshell V5.7.4 (abort with ^G) 1> [X|Xs] = [2,4,6.8,10]. [2,4,6.8,10] 2> X. 2 3> Xs. [4,6.8,10]
<timberlake:~:1:29> erl Erlang R13B03 (erts-5.7.4) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false] Eshell V5.7.4 (abort with ^G) 1> [X,3,Y] = [1,Z,4]. * 1: variable 'Z' is unbound
Prolog has the most sophisticated pattern matching, allowing variables on both sides of the operator:
<timberlake:~:1:30> prolog
SICStus 4.0.5 (x86_64-linux-glibc2.3): Thu Feb 12 09:48:30 CET 2009
Licensed to SP4cse.buffalo.edu
| ?- [X,3,Y] = [1,Z,4].
X = 1,
Y = 4,
Z = 3 ?
yes
| ?- [X,3,Y] = [1,Z,4], format("X=~d, Y=~d, Z=~d~n", [X,Y,Z]).
X=1, Y=4, Z=3
X = 1,
Y = 4,
Z = 3 ?
yes
Variable binding by pattern matching will be even more useful when we use it between formal parameters and actual arguments of functions.| ?- [U,V,9] = [X,6,X]. U = 9, V = 6, X = 9 ? yes
