How the President of Pakistan is elected under the 1973 constitution? What are his process and function?

The President

The President is the constitutional head of the state. 
He is a symbol of unity of Pakistan. He is elected by the Provincial Assemblies as well as the National Assemblies and is a binding force between the federation and the federating units.

The President is constitutionally required to act on the advice of the Prime Minister Cabinet, but such advice ought not to be leaded with a perception of dominance and veto power. If the President counsels the Prime Minister or the Cabinet, his counseling is entitled to weight. These are some of the norms of constitutional jurisprudence for successful operation of the constitutional prescription.  
The functioning of a parliament Democracy postulates the existence of a constitutional head who becomes the guardian of the constitution in so far as it provides for the control of power relation between the executive and the legislature. 
The President who becomes, by virtue of his office as the constitutional head, the symbol of a country’s national prides of its dignity, of its honor, is required to play as to represent the unity of the Republic. 
The President and the Prime Minister have defined roles under the constitution domain unhindered and uninterrupted by each other.

No doubt, the President as the symbol of unity of federation occupies a neutral position in the constitution and in that capacity he is entitled to highest respect and regard by all the functionaries of the state. But it is equally important that in order to protect and preserve the dignity of this high office and this neutral image under the constitution the President must keep along from all political imbroglio.