Simple String Equation

I have 2 string, say textfield.text and [textArray objectAtIndex:i] They contain the same string (and tested by NSLog).

//this returns a false

if (textfield.text == [textArray objectAtIndex:i])

//this returns a true

[textArray replaceObjectA开发者_JS百科tIndex:i withObject:textfield.text];
if (textfield.text == [textArray objectAtIndex:i])

//this is also true

if ([textfield.text isEqualToString:[textArray objectAtIndex:i]])

Why is the first one returning a false?

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When you use an == on pointers like NSString * it is comparing memory addresses, not comparing the value of strings.

Your first example:

if (textfield.text == [textArray objectAtIndex:i])

Is comparing two different memory address. Since they are not the same memory address, the answer is false.

Your second example:

[textArray replaceObjectAtIndex:i withObject:textfield.text];
if (textfield.text == [textArray objectAtIndex:i])

Here you have assigned the memory address of textfield.text into [textArray objectAtIndex:i] thus making them the same memory address. Therefore, the memory addresses are the same so the result is true.

Your last example:

if ([textfield.text isEqualToString:[textArray objectAtIndex:i]])

Is the correct way to evaluate two strings regardless of memory addresses because isEqualToString compares the value of the strings and not their memory addresses.

Hope this helps.

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The == operator tests if both references point to the same object, not if the strings are equal. Use isEqualToString: if you want to compare the string contents.

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if (textfield.text == [textArray objectAtIndex:i])

is an equality (not logical equality sorry). You can't use that for strings.

if ([textfield.text isEqualToString:[textArray objectAtIndex:i]])

However isEqualToString is the equivalent of the statement above. Specifically for strings.

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In the first example, have you added the object to the array yet??

As a side note, you cannot compare your NSStrings with ==. You should use isEqualToString: for that.