Question

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strange output in comparision of float with float literal
Comparison of float and double variables

I have a test with double and float in C, but I cannot explain why.

    float x = 3.4F;
    if(x==3.4)
        printf("true\n");
    else printf("false\n");
    double y = 3.4;
    if (y==3.4)
        printf("true\n");
    else printf("false\n");

The result will be False and True. Please explain for me please.

Answer 1

No guarantee that the result will be false and true, but the basic idea is pretty simple: 3.4 has type double. When you assign it to a float, it’ll get rounded. When you compare, that rounded number will be promoted back to a double, not necessarily the same double as 3.4.

In the second case, everything’s double throughout.