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When you enter input data at the keyboard, you must be sure that each data value is appropriate for the data type of the variable in the input statement. |
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| Data Type of Variable In an >> Operation | Valid Input Data | | char | A single printable character other than a blank | | int | An int literal constant, optionally preceded by a sign | | float | An int or float literal constant (possibly in scientific, E, notion), optionally preceded by a sign |
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Notice that when you input a number into a float variable, the input value doesn't have to have a decimal point. The integer value is automatically coerced to a float value. Any other mismatches, such as trying to input a float value into an int variable or a char value into a float variable, can lead to unexpected and sometimes serious results. Later in this chapter we discuss what might happen. |
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When looking for the next input value in the stream, the >> operator skips any leading whitespace characters. Whitespace characters are blanks and certain nonprintable characters like the character that marks the end of a line. (We talk about this end-of-line character in the next section.) After skipping any whitespace characters, the >> operator proceeds to extract the desired data value from the input stream. If this data value is a char value, input stops as soon as a single character is input. If the data value is int or float, input of the number stops at the first character that is inappropriate for the data type, such as a whitespace character. Here are some examples, where i, j, and k are int variables, ch is a char variable, and x is a float variable: |
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| Statement | Data | Contents After Input | | 1. cin >> i ; | 32 | i = 32 | | 2. cin >> i >> j ; | 4 60 | i = 4, j = 60 | | 3. cin >> i >> ch >> x; | 25 A 16.9 | i = 25, ch = A, x = 16.9 | | 4. cin >> i >> ch >> x; | 25 | | | A | | | 16.9: | i = 25, ch = A, x = 16.9 | | 5. cin >> i >> ch >> x; | 25A16.9 | i = 25, ch = A, x = 16.9 | | 6. cin >> i >> j >> x; | 12 8 |  |
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i = 12, j = 8 (Computer waits for a third number) |
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| | 7. cin >> i >> x; | 46 32.4 15 | |
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i = 46, x = 32.4 (15 is held for later input) |
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