Question

我想知道这段代码的作用：

var something: String = "Hi"

if something = "Hello world!" {
    // Will this be executed?
}

它会分配给something变量并执行if正文吗？或者它是否仅为if正文设置该变量的值，在外面它不会改变？或者它与nil有什么关系？

Answer 1

分配不是返回布尔值的表达式，因此不能在DatasetGroup * make_array(){ DatasetGroup *dg_array; // Allocate space for a few groups dg_array = (DatasetGroup *) malloc(sizeof(DatasetGroup) * INCREMENT); return dg_array; } void add_group_to_array(DatasetGroup *dg_array, ...){ // Add a datasetgroup DatasetGroup *dg = new_DatasetGroup(...); // groupCount - 1 as the count is incremented when the group is created, so will always be one ahead of the array index we want to assign to dg_array[groupCount - 1] = dg; if (groupCount % INCREMENT == 0) { //Grow the array dg_array = realloc(dg_array, sizeof(DatasetGroup) * (groupCount + INCREMENT)); } }这样的内部使用。所以这不会编译。

（虽然你会得到一个误导性的编译器消息）

assignment in if failing to compile

Answer 2

此模式仅适用于可能失败的分配 - 也就是说，如果您正在分配返回Optional值的表达式的结果。在这种情况下，您使用的是if let，而不只是if。

Answer 3

我们不能对if条件使用赋值运算符，如果让假设你正在使用可选类型

，那么你会吗？
以下是一些可帮助您明确区分赋值运算符的运算符

salaries= load 'salaries' using PigStorage(',') As (gender, age,salary,zip); salaries= load 'salaries' using PigStorage(',') As (gender:chararray,age:int,salary:double,zip:long); salaries=load 'salaries' using PigStorage(',') as (gender:chararray,details:bag{b(age:int,salary:double,zip:long)}); highsal= filter salaries by salary > 75000; dump highsal salbyage= group salaries by age; describe salbyage; salbyage= group salaries All; salgrp= group salaries by $3; A= foreach salaries generate age,salary; describe A; salaries= load 'salaries.txt' using PigStorage(',') as (gender:chararray,age:int,salary:double,zip:int); vivek@ubuntu:~/Applications/Hadoop_program/pip$ pig -x mapreduce p.pig 15/09/24 03:16:32 INFO pig.ExecTypeProvider: Trying ExecType : LOCAL 15/09/24 03:16:32 INFO pig.ExecTypeProvider: Trying ExecType : MAPREDUCE 15/09/24 03:16:32 INFO pig.ExecTypeProvider: Picked MAPREDUCE as the ExecType 2015-09-24 03:16:32,990 [main] INFO org.apache.pig.Main - Apache Pig version 0.14.0 (r1640057) compiled Nov 16 2014, 18:02:05 2015-09-24 03:16:32,991 [main] INFO org.apache.pig.Main - Logging error messages to: /home/vivek/Applications/Hadoop_program/pip/pig_1443089792987.log 2015-09-24 03:16:38,966 [main] WARN org.apache.hadoop.util.NativeCodeLoader - Unable to load native-hadoop library for your platform... using builtin-java classes where applicable 2015-09-24 03:16:41,232 [main] INFO org.apache.pig.impl.util.Utils - Default bootup file /home/vivek/.pigbootup not found 2015-09-24 03:16:42,869 [main] INFO org.apache.hadoop.conf.Configuration.deprecation - mapred.job.tracker is deprecated. Instead, use mapreduce.jobtracker.address 2015-09-24 03:16:42,870 [main] INFO org.apache.hadoop.conf.Configuration.deprecation - fs.default.name is deprecated. Instead, use fs.defaultFS 2015-09-24 03:16:42,870 [main] INFO org.apache.pig.backend.hadoop.executionengine.HExecutionEngine - Connecting to hadoop file system at: hdfs://localhost:9000 2015-09-24 03:16:45,436 [main] ERROR org.apache.pig.tools.grunt.Grunt - ERROR 1000: Error during parsing. Encountered " <PATH> "salaries=load "" at line 7, column 1. Was expecting one of: <EOF> "cat" ... "clear" ... "fs" ... "sh" ... "cd" ... "cp" ... "copyFromLocal" ... "copyToLocal" ... "dump" ... "\\d" ... "describe" ... "\\de" ... "aliases" ... "explain" ... "\\e" ... "help" ... "history" ... "kill" ... "ls" ... "mv" ... "mkdir" ... "pwd" ... "quit" ... "\\q" ... "register" ... "rm" ... "rmf" ... "set" ... "illustrate" ... "\\i" ... "run" ... "exec" ... "scriptDone" ... "" ... "" ... <EOL> ... ";" ... Details at logfile: /home/vivek/Applications/Hadoop_program/pip/pig_1443089792987.log 2015-09-24 03:16:45,554 [main] INFO org.apache.pig.Main - Pig script completed in 13 seconds and 48 milliseconds (13048 ms) vivek@ubuntu:~/Applications/Hadoop_program/pip$

内部条件下的快速分配

3 个答案: