将具有数学表达式的字符串转换为float [英] Convert a string with mathematical expressions into a float

问题描述

文本字段包含数学表达式： 12 + 45-6 是否可以将此字符串转换为数字？

A text field contains mathematical expressions like: 12+45-6 Is possible to convert this string to a number?

我们可以使用谓词设施吗？

Can we use the predicate facility?

NSString *foo = @"((a*b)/c+(d/5))+15";
// dummy predicate that contains our expression
NSPredicate *pred = [NSPredicate predicateWithFormat:[foo stringByAppendingString:@" == 42"]];
NSExpression *exp = [pred leftExpression];
NSNumber *result = [exp expressionValueWithObject:[NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:4], @"a",[NSNumber numberWithInt:7], @"b",[NSNumber numberWithInt:2], @"c",[NSNumber numberWithInt:20], @"d",nil]context:nil];
NSLog(@"%@", result); // logs "33"<br/>

我也发现这 https://github.com/unixpickle/ANExpressionParser

推荐答案

't依赖外部库是首先将字符串转换为一个标记序列，然后使用分流算法链接到简单的堆栈机器上。

One solution that doesn't rely on external libraries is to first convert the string into a sequence of tokens, then evaluate the expression using the shunting-yard algorithm chained onto a simple stack machine.

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