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    <p>DDMathParser author here...</p> <p>To expand on <a href="https://stackoverflow.com/a/16585241/115730">Jonathan's answer</a>, here's how you could do it entirely in DDMathParser. However, to parse the string as-is, you'll need to do two things.</p> <p>First, you'll need to create an <code>if</code> function:</p> <pre><code>DDMathEvaluator *evaluator = [DDMathEvaluator sharedMathEvaluator]; [evaluator registerFunction:^DDExpression *(NSArray *args, NSDictionary *vars, DDMathEvaluator *eval, NSError *__autoreleasing *error) { if ([args count] == 3) { DDExpression *condition = [args objectAtIndex:0]; DDExpression *resultExpression = nil; NSNumber *conditionValue = [condition evaluateWithSubstitutions:vars evaluator:eval error:error]; if ([conditionValue boolValue] == YES) { resultExpression = [args objectAtIndex:1]; } else { resultExpression = [args objectAtIndex:2]; } NSNumber *result = [resultExpression evaluateWithSubstitutions:vars evaluator:eval error:error]; return [DDExpression numberExpressionWithNumber:result]; } return nil; } forName:@"if"]; </code></pre> <p>This creates the <code>if()</code> function, which takes three parameters. Depending on how the first parameter evaluates, it either evaluates to the result of the second or third parameter.</p> <p>The other thing you'll need to do is tell the evaluator what <code>height</code> and <code>weight</code> mean. Since they don't start with a <code>$</code> character, they get interpreted as functions, and not variables. If they started with a <code>$</code>, then it would be as simple as evaluating it like this:</p> <pre><code>NSString *expression = @"if(($height &gt; 0), ($weight+ 2 ), ( $weight-1 ) )"; NSDictionary *variables = @{@"height" : @42, @"weight" : @13}; NSNumber *result = [expression evaluateWithSubstitutions:variables evaluator:evaluator error:nil]; </code></pre> <p>However, since they <em>don't</em> start with a <code>$</code>, they're functions, which means you need to tell the evaluator what the functions evaluate to. You could do this by creating functions for both <code>height</code> and <code>weight</code>, just like you did for <code>if</code>:</p> <pre><code>[evaluator registerFunction:^DDExpression *(NSArray *args, NSDictionary *vars, DDMathEvaluator *eval, NSError **error) { return [DDExpression numberExpressionWithNumber:@42]; } forName:@"height"]; </code></pre> <p>Alternatively, you could make it a bit more dynamic and use the <code>functionResolver</code> block of <code>DDMathEvaluator</code>, which is a block that returns a block (woooooo) and would look like this:</p> <pre><code>NSDictionary *values = @{@"height": @42, @"weight": @13}; [evaluator setFunctionResolver:^DDMathFunction(NSString *name) { DDMathFunction f = ^(NSArray *args, NSDictionary *vars, DDMathEvaluator *eval, NSError **error) { NSNumber *n = [values objectForKey:name]; if (!n) { n = @0; } return [DDExpression numberExpressionWithNumber:n]; }; return f; }]; </code></pre> <p>With those two pieces in place (registering <code>if</code> and providing the values of <code>height</code> and <code>weight</code>), you can do:</p> <pre><code>NSString *expression = @"if((height &gt; 0), (weight+ 2 ), ( weight-1 ) )"; NSNumber *result = [expression evaluateWithSubstitutions:nil evaluator:evaluator error:nil]; </code></pre> <p>... and get back the proper result of <code>@15</code>.</p> <p>(I have plans to make <code>DDMathParser</code> allow unknown functions to fall back to provided variable values, but I haven't quite finished it yet)</p>
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    1. POhow to solve a conditional equation in iOS
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    1. COI like the start of the answer! :-)
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    2. COHow to approach in case an equation has a function call Ex: `if(xyz==0, if(abc<0, 270, 90), atan(xyz/abc)*180/_pi)`?
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    3. CO@Amar as long as you define what `_pi`, `xyz`, and `abc` all are, that'll work just fine. `atan` is a built-in function. (But if you called it `pi` instead of `_pi`, then you wouldn't need to define it. [`pi` is also a built-in function](https://github.com/davedelong/DDMathParser/wiki/Built-in-Functions).
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